LLE Review

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Review 76

Highlights

LLE Review Volume 76 begins with reports on two new subsystems in the OMEGA laser facility: (1) a highly stable, diode-pumped Nd:YLF master oscillator and (2) a highly stable negative-feedback controlled regenerative amplifier. Another system development is the expansion of the pulse-shape bandwidth of OMEGA's driver line from approximately 3 GHz to over 5 GHz. Calculations of near-field intensity modulations in high-intensity laser beams due to self- and cross-phase modulation of OMEGA laser beams emerging from KDP polarization-smoothing wedges have shown that intensity modulations are not expected to be a significant source of intensity modulation. An x-ray radiographic system for measuring mass modulations in planar laser-driven targets has been characterized to measure perturbations imprinted on targets by laser nonuniformity. Observations of stimulated Raman scattering (SRS) have been at odds with theoretical predictions but now have a viable explanation in terms of calculated collisionless damping rates found to be much slower for plasma waves confined within small-radius filaments. The Summer High School Research Program Report, the Laser Facility Report, and the National Laser Users' Facility News conclude this volume.

Review 76 Editor

Review 76
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Review 77

Highlights

LLE Review 77 begins with two articles addressing issues applicable to direct-drive ICF on the National Ignition Facility (NIF): laser–plasma interactions and laser-irradiation uniformity. The experimental results of the first article indicate the parametric instabilities—stimulated Raman scattering and stimulated Brillouin scattering—are not likely to have a significant impact on target performance at the peak of the NIF direct-drive laser pulse. Laser-irradiation uniformity on OMEGA and the NIF is investigated in the second article, and improvements in irradiation uniformity planned for the OMEGA laser during 1999, which will reduce the rms nonuniformity to less than 1% when the intensity is averaged over 300 ps, are outlined. A novel charged-particle diagnostic has been developed that performs simultaneous rR measurements of the fuel, shell, and ablator regions of a compressed ICF target [which consists of an inner DT-fuel region, a plastic (CH) shell, and an ablator (CD)] by measuring the knock-on deuteron spectrum. Stress-inhibited laser-driven crack propagation and stress-delayed damage-initiation experiments in fused silica at 351 nm revealed a 70% increase in the damage initiation threshold when a modest amount of mechanical stress was applied to the optic. An analytic theory of the ablative Richtmyer–Meshkov instability shows that the main stabilizing mechanism of the ablation-front perturbations is the dynamic overpressure of the blowoff plasma with respect to the target material. Two articles outside of the scope of ICF research conclude this volume: (1) a triplet state of rose bengal—a dye used in photodynamic therapy—has been characterized; and (2) single-picosecond switching of a high-temperature-superconductor Josephson junction was observed.

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Review 78

Highlights

LLE Review 78 begins with two articles concerning issues relevant to 2-D SSD laser-beam smoothing on OMEGA. In the first article the design of an efficient, bulk-phase modulator operating at approximately 10.5 GHz is presented, which can produce substantial phase-modulated bandwidth with modest microwave drive power. This modulator is the cornerstone of the 1-THz UV bandwidth operation planned for OMEGA this year. In the second article a recently developed code—Waasese—that simulates the collective behavior of the optical components in the SSD driver line is described. Additional research highlights presented in this issued include:

Compression of shell material to areal densities of ~60 to 130 mg/cm2 was observed in a 60-beam implosion experiment of hollow-shell targets;
this experiment also surveyed target performance based on laser-irradiation uniformity and laser pulse shape. Two charged-particle magnetic spectrometers, which LLE has developed in collaboration with MIT and LLNL, were used to perform simultaneous measurements of the fuel areal density, shell areal density, and fuel temperature of D3He-filled imploding capsules.
The modeling of an aperture-coupled-stripline (ACSL), electrical-waveform generator is described, which produces an optical seed pulse for OMEGA and is critically related to the on-target pulse shape. (4) A measurement technique that enables the complete characterization of electronic devices having any dynamic temporal and spectral frequency response is presented, such as the photoconductive microwave switches on OMEGA's pulse-shaping system.
Damage to OMEGA's stage-C-input, C-output, D-input, E-input, and F-input fused-silica, spatial-filter lenses is examined, and safe operational damage criteria are outlined.

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Review 78
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Review 79

Highlights

The first two articles in LLE Review 79 address issues related to direct-drive cryogenic targets for OMEGA and the NIF. The first article describes an all-DT target design that is predicted to achieve ignition under the direct-drive irradiation conditions that will be available on the NIF. Using the NIF to carefully construct a two-shock compression is predicted to result in modest gain. The second article discusses cryogenic-target characterization for OMEGA implosion experiments. A novel optical technique is shown to resolve target perturbations at relevant levels of the uniformity expected on the inner ice surface of cryogenic DT targets. Additional articles in this issue cover the following:

A new concept of using an embedded titanium layer in spherical targets to determine the areal density of the compressed shell is introduced.
Techniques to characterize the laser-irradiation nonuniformity on OMEGA using time-integrated UV equivalent-target-plane imaging are presented.
Growth and imprinting characteristics of planar targets irradiated under the variety of laser beam-smoothing effects available on OMEGA are discussed.
Additional external applied stress is shown to increase optical damage thresholds for the initiation of cracks in fused silica and BK7 glasses.
Trapping high-energy (10-keV) electrons in a novel single-beam ponderomotive optical trap is shown to open the path to a myriad of future experiments that can use near-field phase control to create specifically tailored trapping conditions.

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Review 80

Highlights

LLE Review 80 covers a broad range of research topics, beginning with two theoretical analysis articles. The first feature article explains how laser nonuniformity can generate mass perturbations through ``laser imprint." Modulated laser illumination is shown to impact the predicted stability of velocity and acceleration of material during the early portion of the irradiation; thermal smoothing and mass ablation are shown to mitigate the perturbations. Simulated NIF direct-drive cryogenic designs with 1-THz SSD remain intact during the implosion. The second feature article distills self-focused filament stability in laser-formed plasmas to a tractable prediction of stability over a variety of size and shape filaments. This issue also presents articles on the following topics:

Broadband SSD implementation on OMEGA: architecture and integration issues
Temporal pulse shape's effect on laser imprinting and beam smoothing: experiments and simulations
Laser regenerative amplifier signal-to-noise experiments and modeling, covering sources of noise and system optimization
Optical fabrication developments that highlight the extension of magnetorheological fluid (MRF) techniques to crystalline optical materials.

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Review 81

Highlights

LLE Review 81 includes a broad range of topics that are currently being studied at LLE. The feature article examines the conditions under which direct-drive ignition capsules ignite. The authors identify inner-surface roughness of the DT-ice and laser nonuniformities as the principal seeds of instabilities that can potentially quench ignition and place constraints on these quantities by requiring a target gain of at least 10. Other articles in this issue include

A report on the initial performance of the high-pressure deuterium- and tritium-filling portion of the OMEGA Cryogenic Target Handling System.
A numerical study on the principal sources of target nonuniformities for a cryogenic target when placed in a layering sphere.
A description of target detection and shroud pull-sequencing aspects of cryogenic operations on OMEGA.
An investigation of laser prepulse levels on OMEGA.
A report on the design of dyes for improved contrast in Liquid Crystal Point Diffraction Interferometers.

Review 81 Editor

Review 81
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Review 82

Highlights

This volume of the LLE Review, covering the period January-March 2000, includes a broad range of topics being studied at LLE. The feature article is a report on OMEGA cryogenic target designs for the soon-to-be-commissioned OMEGA Cryogenic Target Handling System. The authors show that these targets are energy scaled from the NIF ignition designs and have similar 1-D behavior and stability properties.

Additional research highlights reported in this issue are

A numerical study of a novel laser-pulse modification for laser-imprint reduction in OMEGA cryogenic capsules.
A report on experimentally inferred fast-electron preheat in direct-drive targets due to laser irradiation.
A description of ongoing experimental and theoretical work relating to holographic grating design and fabrication.
A report on simulations of laser smoothing in the absence of smoothing by spectral dispersion (SSD).
A description of simulations of capsule implosions in tetrahedral hohlraum experiments carried out on OMEGA in a collaboration between LLE and Los Alamos.
An article on measurements of the nanohardness of magnetic and nonmagnetic particles used in the magnetorheological finishing (MRF) process.

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Review 83

Highlights

This volume of the LLE Review, covering the period from April to June 2000, includes articles on OMEGA diagnostics and experiments as well as research related to the manufacturing and characterization of optical elements used in the laser system. The feature article describes the implementation of electronic cameras to record time-integrated x-ray images on OMEGA.

Additional research highlights reported in this issue are:

An experimental study of the spatial structure of temperature and density in compressed laser targets on OMEGA.
A new diagnostic technique, demonstrated on OMEGA, for obtaining particle spectra from imploded targets.
An interferometric technique for characterizing OMEGA optical elements.
A study of the mechanisms of glass polishing in the magnetorheological finishing (MRF) technique.

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Review 83
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Review 84

Highlights

This volume of the LLE Review, covering the period from April to June 2000, includes articles on direct-drive experiments performed with the enhanced beam uniformity now available on the OMEGA laser system (1-THz SSD with polarization smoothing). The feature article (cover and p. 181) concerns calculated performance of directly driven capsules on the National Ignition Facility (NIF).

Additional research highlights reported in this issue include

an experimental study of the effect of beam smoothing and pulse shape on imprinting
an experimental study of direct-drive implosions of gas-fusion-fuel-filled capsules on OMEGA performed with 1-THz SSD, polarization smoothing, and high-quality power balance
a study of secondary-neutron-yield measurement using current-mode detectors, and
a description of a technique for measuring positional variations in target areal density using filtered x-ray framing cameras.

Review 84 Editor

Review 84
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Review 85

Highlights

This volume of the LLE Review, covering October-December 2000, features an article (cover and p. 1) that reports on the development of a theoretical model for the deceleration phase of an imploding inertial fusion capsule. Other research highlights presented in this issue include:

An experimental study of the effects of shock heating on the stability of direct-drive inertial
fusion capsules.
A report on a new model for material behavior under compression with specific application to
fused silica.
A description of the design and performance of a new selectable-streak-rate streak-camera deflection ramp generator.
A report on the development of a UV fiber-optic beam delivery system for the OMEGA 60-beam laser pulse shape characterization system.
A report on the fabrication and characterization of a simple-to-manufacture and simple-to-operate NbN hot-electron photodetector with picosecond response time.
A description of the preliminary design for the National Ignition Facility's (NIF's) 2-D SSD beam-smoothing system. LLE has the lead role in defining direct-drive system requirements for the NIF and preparing a preliminary 2-D SSD system design.

Review 85 Editor

Review 85
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Review 86

Highlights

LLE Review 86, covering January-March 2001, features an article (cover and p. 47) on the characterization of direct-drive implosion core conditions using time-resolved Ar K-shell spectroscopy. Electron densities in excess of 2.5 × 10²⁴ cm^-3 and electron temperatures ~2.5 keV were measured in these experiments. This represents the highest combination of electron temperature and density measured for these types of implosions in laser-driven inertial fusion experiments.

Additional research reported in this volume:

Studies of the implosions of direct-drive, DT-gas-filled polymer capsules using nuclear diagnostics. A comprehensive array of traditional neutron measurements as well as charged-particle diagnostics were used to compare the performance of capsules irradiated with full beam smoothing versus implosions of similar targets carried out with reduced beam smoothing.
The development of a measurement-based static model of the stagnated core and fuel-pusher mix. Excellent agreement with a suite of neutron and charged-particle diagnostics is obtained through this model.
The implementation of a high-resolution neutron imaging system on OMEGA by scientists from the Commissariat à L'Énergie Atomique (CEA) of France, Los Alamos National Laboratory (LANL), and LLE. This system is based on penumbral imaging and has an ultimate spatial resolution of ~13 µm for OMEGA implosions.
An analysis of the beam-smoothing performance of ultrafast picket-fence pulses for direct-drive targets on the NIF.
A report on a test of the feasibility of using extended x-ray absorption fine structure (EXAFS) spectra to characterize the properties of shocked solid materials.
A theoretical model for analyzing crack-load micro-indentation data in tetragonal crystals with particular application to KDP.

Review 86 Editor

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Review 87

Highlights

LLE Review 87, covering April-June 2001, features an article on the 60-beam streak camera system used on the OMEGA laser system. This article (cover and p. 109) describes the system and focuses on the hardware and software calibration techniques that maximize its utility. The system can diagnose each of the beams on every target shot and can measure beam energies with 8% accuracy, and timing at 7 ps rms. Beam-to-beam power variations of less than 5% can be detected.

Additional highlights of research presented in this issue include the following:

The evolution of shell modulations near the point of peak compression in spherical, direct-drive implosions is examined using both modeling and shot data. The effect of two different levels of beam smoothing is described. Both the model and the experiment show that modulations in the shell areal density decrease during compression and increase during decompression.
The first multibeam laser-plasma interaction experiments with a critical density surface present at all times are reported. These plasma conditions are tailored to resemble future direct-drive-ignition laser fusion implosions on the NIF. The results show strong evidence of electromagnetic (EM) wave seeding of SBS backscatter as well as evidence of strongly driven, common, symmetrically located ion waves. The expected SBS scattering levels for NIF direct-drive ignition experiments are well below 1%. This gives confidence that good direct-drive target performance will be achieved.
The main aspects of nonequilibrium hot-electron phenomena in superconducting films are reviewed. Various theoretical models developed to describe the hot-electron effect are presented. The article describes a number of radiation-sensing devices that have been fabricated and tested and demonstrate significantly improved performance over conventional implementations
The issues associated with determining the minimum drive energy needed to achieve ignition in inertial confinement fusion implosions are explored on a theoretical basis. A new model that consistently incorporates two competitive scaling approaches is developed. Topics covered include hot-spot dynamics, two approaches to shell modeling, derivations of ignition scaling, and verification of initial assumptions. Good agreement with other published results is shown.

Review 87 Editor

Review 87
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Review 88

Highlights

LLE Review 88, covering July through September 2001, begins with an article that describes a simple, low-cost, wide-dynamic-range, neutron bang time (NBT) detector that is currently installed on LLE's OMEGA laser. It is able to measure the neutron bang time of DD- and DT-filled ICF implosion capsules at neutron yields between 10⁷ and 10¹¹ with an absolute timing accuracy of better than 100 ps.

Additional research reported in this volume:

The current substrate cleaning and handling methods used in the application of high reflectance optical coatings are so effective that it is necessary to test large parts in order to achieve statistically meaningful assessments. LLE's Optical Manufacturing group to used equipment designed by LLNL to test new coating designs test on full-sized NIF substrates.
Calculations show that the use of CH foam shells that are soaked with DT fuel offers improvements in implosion stability and higher neutron yield in comparison to more conventional DT only designs.
Concepts for the creation of ultrafast I/O interfaces suitable for applying digital superconducting electronics to produce ultrafast telecommunication routers are reviewed.
A parametric study of vapor deposition polymerization techniques for producing polyimide shells for inertial confinement fusion (ICF) targets is reported. The production rate, yield, and reproducibility of the process were optimized.
This volume concludes with a reports on LLE's Summer High School Research Program, The FY01 Laser Facility Report, and the National Laser Users' Facility News.

Review 88 Editor

Review 88
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Review 89

Highlights

LLE Review 89, covering October to December 2001, begins with an article describing a project that was initiated by LLE's staff photographer when he began taking visible light photographs of imploding OMEGA targets for aesthetic rather than scientific reasons. These beautiful images were used to communicate LLE's mission to the general public. A closer examination of the images revealed a one-to-one correspondence between the bright spots in the image and each of the 60 laser beams. The intensity of the bright spots has been related to refraction and absorption in the plasma surrounding the imploding target. These photographs are now proving to be the basis of a new laser—plasma interaction diagnostic.

Additional research presented in this volume includes the following:

An analytical model of the nonlinear bubble evolution of single-mode, classical Rayleigh-Taylor (RT) instability at arbitrary Atwood numbers is described. The model follows the continuous evolution of bubbles from the early exponential growth to the nonlinear regime when the bubble velocity saturates.
A reduced-autocorrelation phase plate, calculated with a novel algorithm, for OMEGA and the National Ignition Facility improves the time-averages rms laser nonuniformity. The reduced autocorrelation phase design shifts the speckle energy up into the higher spatial frequencies where smoothing by spectral dispersion (SSD) and thermal smoothing in the target corona are most efficient.
LLE's Tritium Recovery System is used to clean up the various exhaust streams and to control tritium activity in the gloveboxes. This system uses the best-available technologies to extract tritium from inert gas streams in the elemental form and reduces the volumes of effluent to the maximum extent practical.
A SiO₂-thin-film system with absorbing gold nanoparticles was used to study the connection between the pulsed-laser energy absorption process and film damage morphology. The initial absorption was confined to the nanoscale defect. Energy absorbed by the defect quickly heats the surrounding matrix, changing it from a transparent to an absorbing media, which creates a positive-feedback mechanism that leads to crater formation.
The time-resolved dynamics of the superconducting-to-resistive transition in dc-based epitaxial YBa₂Cu₃O_7—x (YBCO) microbridges, excited by nanosecond-long current pulses, leads to resistive switching. The experimental dynamics were in agreement with the Geier and Schön theory, modified to include the dc bias.
A series of thin, hydrogenated amorphous carbon films have been deposited using the saddle-field deposition configuration. These films are a precursor to depositing tritiated films. Smooth, low-porosity films up to 15 µm thick and with densities up to 2 g/cc have been grown

Review 89 Editor

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Review 90

Highlights

This volume of the LLE Review, covering January to March 2002, features "First Results from Cryogenic Target Implosions on OMEGA," which describes initial results from direct-drive spherical cryogenic target implosions on the 60-beam OMEGA laser system. These experiments are part of the scientific base leading to direct-drive ignition implosions planned for the National Ignition Facility (NIF). Results shown include neutron yield, secondary-neutron and proton yields, the time of peak neutron emission, and both time-integrated and time-resolved x-ray images of the imploding core. The experimental values are compared with 1-D numerical simulations. The target with an ice-layer nonuniformity of σ_rms = 9 microns showed 30% of the 1-D predicted neutron yield. These initial results are encouraging for future cryogenic implosions on OMEGA and the NIF.

Additional highlights of research in this issue include the following

Experiments performed on OMEGA as part of the Stockpile Stewardship Program, which measure the equation of state of carbonized resorcinol foam, a porous material, have been modeled. Using the impedance -matching method, the foam Hugoniot was calculated from the well-know equation of state of aluminum and from measured shock speeds over the range of 100 kbar to 2 Mbar.
Drive lasers, with know, single-mode modulations, produce nonuniform shocks that propagate into CH targets. An optical probe beam is used to measure the arrival of these modulated shocks at various surfaces in the target. Experiments at moderate laser intensities (less than or approximately equal to 10¹³ W/cm²) exhibit behavior that is predicted by hydrocodes and simple scaling laws. This technique may be used to observe various dynamic effects in laser-produced plasmas and shock-wave propagation.
The time dependence of electron thermal flux inhibition in direct-drive laser implosions has been calculated. The nonlocal electron thermal conduction (in direct-drive CH target implosions driven with square pulses) calculated by one-dimensional Fokker-Planck solver has been combined with the hydrodynamic code. The results show that the electron thermal flux inhibition at the critical surface is time dependent, confirming that a larger flux limiter ms be used for shorter-duration pulses. Also, the growth of the Rayleigh-Taylor instability for short-wavelength perturbations is shown to be smaller due to the longer density scale length.
Precision spectral sculpting of broadband FM pulses amplified in a narrowband medium has been demonstrated. Amplification of broadband frequency-modulated (FM) pulses in high-efficiency materials such as Yb⁺³:SFAP results in significant gain narrowing, leading to reduced on-target bandwidths for beam smoothing and to FM-to-AM conversion. These effects were compensated for by applying precision spectral sculpting, with both amplitude and phase shaping, before amplifying the broadband FM pulses in narrowband gain media. The spectral sculpting, for centerline small-signal gains of 10⁴, produced amplified pulses that have both sufficient bandwidths for on-target beam smoothing and temporal profiles that have not potentially damaging amplitude modulation.
Polymer cholesteric liquid crystal flakes suspended in a fluid with non-negligible conductivity can exhibit motion in the presence of an ac electric field. The platelets have a strong selective reflection, which is diminished or extinguished as the flakes move. Flake motion was seen within a specific frequency bandwidth in an electric field as low as 5 mV_rms/mm.
The response time of a novel, freestanding LT-GaAs photoconductive switch has been measured. The switch was formed by patterning a 1-mm-thick layer of a single-crystal LT-GaAs into a 5-micron by 15-micron bar. The bar was separated from its GaAs substrate and placed across a gold coplanar transmission line deposited on a Si wafer. The switch was excited with 110-fs-wide optical pulses, and its photoresponse was measured with an elecro-optic sampling system. Using 810-optical radiation, 470-fs-wide electrical transients (640-GHz bandwidth) were recorded.

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Review 91

Highlights

This volume of the LLE Review, covering April to June 2002, features "Modeling Laser-Plasma Interaction Physics Under Direct-Drive Inertial Confinement Fusion Conditions" by J. Myatt, A. V. Maximov, and R. W. Short (p. 93). They report of pF3D, a parallel, three-dimensional laser-plasma interaction (LPI) code developed at LLNL for modeling indirect-drive plasmas, which recently has been modified for use under direct-drive conditions. Unlike indirect drive, modeling direct drive requires simulation of inhomogeneous supersonic flows and density profiles that include at critical surface. The treatment of the critical surface is particularly problematic in codes employing the paraxial approximation for the light waves. Myatt et al. describe the first results of the modified code: realistic simulations motivated by long-scale-length exploding-foil experiments conducted on LLE's 30-kJ, 351-nm, 60-beam OMEGA laser system and intended to represent future NIF direct-drive conditions.

Additional highlights of research in this issue are

A. V. Okishev, D. Battaglia, I. A. Begishev, and J. D. Zuegel (p. 103) have developed a new highly stable, diode-pumped, cavity-dumped, compact Nd:YLF regenerative amplifier (regen) of continuously shaped nanosecond pulses with a gain of ~10⁹ for the front-end laser system of OMEGA. High output energy, long-term energy and temporal pulse shape stability, and high-quality beam profile have been demonstrated. Reliability, simplicity, modular design, and compactness are key features of the new diode-pumped regenerative amplifier.
An experiment recently completed by J. P. Knauer and V. N. Goncharov (p. 108) has tested the ability of a direct-drive ICF laser pulse shape to vary the adiabat within a target shell. A picket pulse was added to a pulse shape designed to implode a cryogenic shell of D₂ with a ratio α of internal pressure to Fermi-degenerate pressure of 5. The effect of a picket is to strengthen the shock in the outer portion of the shell so that the ablation interface has a large α and the fuel maintains its α = 5, resulting in increased stability and improved capsule performance.
F. J. Marshall, J. A. Delettrez, R. L. Keck, J. H. Kelly, P. B. Radha, and L. J. Waxer (p. 116) describe implosion experiments with enhanced beam balance. Marshall et al. have implemented a new technique that determines the beam peak intensities at target chamber center on a full-power target shot by simultaneously measuring the x-ray flux produced by all 60 beams seen separated on a 4-mm-diam, Au-coated spherical target. Up to nine x-ray pinhole camera images are electronically recorded per shot from which beam-to-beam variations in peak intensity are determined, taking into account view angle and x-ray conversion efficiency. The observed variations are then used to correct the beam energies to produce a more-uniform irradiation. The authors present the results of implosion experiments with enhanced beam balance and comparisons to experiments with standard beam balance.
W. T. Shmayda, Y. Cao, and J. A. Szpunar (p. 125) present the effects of textures on hydrogen diffusion in nickel. Deuterium and tritium—isotopes of hydrogen—are the primary fuels for inertial confinement fusion (ICF), so determining and controlling their rate of diffusion through containment materials are important to the design of ICF facilities. When polycrystalline metals have texture, the preferential orientation of the metals affects hydrogen absorption and diffusion. Hydrogen permeation results show that there are significant differences among the three main textures of nickel membranes. Plating current density has a strong influence on texture development of nickel deposits. The texture of deposits can be easily manipulated by controlling plating conditions. In the experiments performed by Shmayda et al., textured Ni membranes were prepared using electrodeposition, and the effects of fabrications on their diffusion rates were determined.
A great deal of interest has been generated by the discovery of superconductivity in a hexagonal magnesium borides not only because of MgB₂'s high critical temperature and current density but also its lower anisotropy, larger coherence length, and higher transparency of grain boundaries to current flow. R. Sobolewski, P. Kús, A. Plecenik, L. Satrapinsky, and Y. Xu (p. 130) have for the first time fabricated MgB₂ superconducting films on flexible substrates. They describe their process, by which these films could be deposited on large-area foils (up to 400 cm²) and, after processing, cut into any shapes (e.g., stripes) with scissors or bent multiple times, without any observed degradation of their superconducting properties.
V. S. Smalyuk, P. B. Radha, J. A. Delettrez, V. Yu. Glebov, V. N. Goncharov, D. D. Meyerhofer, S. P. Regan, S. Roberts, T. C. Sangster, J. M. Soures, and C. Stoeckl (p. 133) have inferred the growth of target areal density near peak compression in direct-drive spherical target implosions with 14.7-Mev deuterium-helium ³ (D³He) proton spectroscopy on the OMEGA laser system. The target areal density grows by a factor of ~8 during the time of neutron production (~400 ps) before reaching 123±16 mg cm^-2 at peak compression in an implosion of a 20-μm-thick plastic CH target filled with 4 atm of D³ He fuel.
J. R. Zurita-Sanchez and L. Novotny (p. 139) have performed a theoretical investigation of a semiconductor quantum dot interacting with a strongly localized optical field, as encountered in high-resolution, near-field optical microscopy. The strong gradients of these localized fields suggest that the higher-order multipolar interaction will affect the standard electric dipole transition rates and selection rules. For a semiconductor quantum dot in the strong confinement limit, Zurita-Sanchez and Novotny have calculated the interband electric quadrupole absorption rate and the associated selection rules, finding that the electric quadrupole absorption rate is comparable with the absorption rate calculated in the electric dipole approximation.

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This volume of the LLE Review, covering July to September 2002, features an investigation of the radial structure of shell modulations near peak compression of spherical implosions. The structure of shell modulations at peak compression of implosions is measured using absorption of titanium-doped layers placed at various distances from the inner surface of 20-μm-thick plastic shells filled with D³He gas. Results show that the peak-compression, time-integrated areal-density modulations are higher at the inner shell surface, which is unstable during the acceleration phase of an implosion, than in the central part of the shell. The outer surface of the shell, which is unstable during the acceleration phase of an implosion, has a modulation level comparable to that of the inner shell surface.

Additional highlights of research presented in this issue includethe following:

Measurements of the neutron emission from inertial confinement fusion (ICF) implosions are providing important information about target performance that can be compared directly with numerical models. For room-temperature target experiments on OMEGA, the neutron temporal diagnostic (NTD), originally developed at Lawrence Livermore National Laboratory (LLNL), measures the neutron burn history with high resolution and good timing accuracy. Because the NTD is mechanically incompatible with cryogenic target experiments because of the standoff required to remain clear of the OMEGA Cryogenic Target Handling System (CTHS), a new cryogenic-compatible neutron temporal diagnostic (cryoNTD), which provides high-resolution neutron emission measurements for cryogenic implosions, has been designed for LLE's standard ten-inch manipulator (TIM) diagnostic inserters.
The yield of tertiary neutrons with energies greater than 20 MeV has been proposed as a method to determine the areal mass density of ICF targets. The use of carbon activations as a suitable measurement technique is discussed because of its high reaction threshold and the availability of high-purity samples.
The development of polyimide shells suitable for ICF cryogenic experiments on OMEGA is described along with the associated mechanical properties needed to define the processing conditions for operating the OMEGACTHS.
A linear model of anomalous stimulated Raman scattering from electron-acoustic (EA) waves in laser-produced plasmas is presented.
A 65(±5)-ps time delay in the onset of a resistive-state formation in 10-nm-thick, 200-nm-wide NbN superconducting stripes exposed to single photons has been measured. This delay in the photoresponse decreased down to zero when the stripe was irradiated by multi-photon (classical) optical pulses. The NbN structures were kept at 4.2 K, well below the material's critical temperature and were illuminated by 100-fs-wide optical pulses. This time-delay phenomenon is explained within the framework of a model based on photon-induced generation of a hotspot in the superconducting stripe and, subsequent, supercurrent-assisted resistive-state formation across the entire stripecross-section.
Reports on LLE's Summer High School Research Program, the FY02 Laser Facility Report, and the National Users' Facility News.

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This volume of the LLE Review, covering October–December 2002, describes progress toward validating the predicted performance of direct-drive capsules that are hydrodynamically equivalent to the baseline direct-drive ignition design for the National Ignition Facility (NIF). These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the cryogenic fuel during the implosion, the laser power balance, and the single-beam nonuniformity. Near 1-D hydrocode performance is measured with a high-adiabat drive pulse on a capsule containing a 100-µm-thick layer of D₂ ice, and near 2-D hydrocode performance is measured on a similar capsule with a low-adiabat drive.

Additional highlights of research presented in this issue include the following:

The growth of inner-surface modulations near peak compression in D³He-gas-filled spherical targets is probed using differential imaging of titanium-doped layers placed at various distances from the inner surface of the shell. Time histories of shell temperature and density are measured with titanium K-shell absorption spectroscopy and the shell areal density is estimated using 14.7-MeV D³3He proton spectra. These experiments provide a better quantitative understanding of the evolution of inner-shell modulations that grow due to the Rayleigh–Taylor instability and Bell–Plesset convergence effects in the deceleration phase of a spherical direct-drive implosion.
Improved target performance in direct-drive implosions using adiabat shaping with a high-intensity picket in front of the main-drive pulse is described analytically. Experiments have demonstrated an improvement in target yields by a factor of up to 3 for the pulses with the picket compared to the pulses without the picket.
A high-conversion-efficiency optical parametric chirped-pulse amplification (OPCPA) system is demonstrated using a spatiotemporally shaped pump pulse to maximize the conversion efficiency of the OPCPA process. This system is a test bed for a similar OPCPA design that will be used for injection of a short-pulse, petawatt-class laser.
A new class of ultrafast, superconducting single-photon detectors for counting both visible and infrared photons is presented. Applications for these devices range from noncontact testing of semiconductor CMOS VLSI circuits to free-space quantum cryptography and communications.
The temporal response characteristics of fast metal–semiconductor–metal ultraviolet photodiodes fabricated on GaN are measured. These detectors are attractive because they are relatively easy to fabricate and have no response in the visible region of the spectrum.
Novel glassy liquid crystals with tunable spectral characteristics have been developed for photonic applications. The molecular design of photoresponsive systems that combine reversible spectral tunability with superior fatigue resistance and thermal stability is described.
Near-field Raman spectroscopy and imaging of single-walled carbon nanotubes (SWNT) with unprecedented spatial resolution of less than 30 nm is presented.

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This volume of the LLE Review, covering January–April 2003, features “Optimization of Deposition Uniformity for Large-Aperture NIF Substrates in a Planetary Rotation System.” For the National Ignition Facility (NIF), coating thickness nonuniformity must not exceed 0.5% peak-to-valley over a 0.85-m aperture. This article describes the design and performance of a thin-film-deposition system used to produce multilayer dielectric thin-film coatings with highly uniform thickness over a full NIF aperture.

Additional highlights of research presented in this issue include the following:

Multiple OMEGA laser beams are used to study the two-plasmon-decay instability, which is the predominant source of suprathermal electrons in direct-drive inertial confinement fusion experiments. It is shown that the total overlapped intensity governs the scaling of the suprathermal-electron generation regardless of the number of overlapped beams, in contrast to conventional theories that are based on the single-beam approximation.
The classical Rayleigh–Taylor instability of the interface separating two homogeneous inviscid fluid layers undergoing uniform acceleration is examined. The formulation presented makes a formal distinction between perturbation behavior under acceleration and perturbation behavior as modified by compression and by geometrical convergence of a cylindrical or spherical interface.
Laser-induced adiabat shaping in ICF spherical targets by a technique referred to as “relaxation” is described. In this approach, the density profile of the capsule’s shell is shaped using a weak prepulse followed by a main pulse with a high-intensity foot. Rayleigh–Taylor growth rates are reduced without significantly degrading 1-D capsule performance.
The operation of a single-photon source—a key hardware element of quantum information technologies—via photon antibunching in the fluorescence of single terrylene molecules embedded in a cholesteric liquid crystal host is demonstrated. Planar-aligned cholesteric layers provide a one-dimensional photonic band gap, allowing an enhancement of the source efficiency.
The properties of compressed titanium due to laser-launched shocks by use of extended x-ray absorption fine structure (EXAFS) is studied. Fitting an EXAFS model to the data indicates compression by a factor of 1.3, in agreement with shock-speed measurements and with hydrodynamic simulations. The rate of decay of the modulation with wave number is shown to include a significant contribution from static disorder, in addition to thermal vibration.
Permeation filling and cooling thin-walled (<3-µm) cryogenic capsules with deuterium– tritium fuel is a critical phase of operation for providing direct-drive targets. A model of the thermal conditions inside the permeation cell is described and used to quantify the forces on the capsule during the cooldown cycle. Results of cooldown cycles of OMEGA cryogenic targets agree well with the simulation, and a cooling program is devised whereby the time for a capsule to reach the frozen state is reduced by 30%.
A qualitative understanding of the greenhouse effect has long been available through models based on globally and time-averaged quantities. A simple 864-cell climatological model reproduces yearly average temperatures obtained earlier from one of these global models and predicts a locally distributed nonradiative flux when observed temperatures are employed as input data. The model is a useful stepping stone for learning about radiative energy transfer into and out of Earth’s atmosphere.

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This volume of the LLE Review, covering April–June 2003, features “Reduction of the Ablative Rayleigh–Taylor Growth Rate with Gaussian Picket Pulses.” This article reports on the seminal work to experimentally validate the reduction in the Rayleigh–Taylor (RT) growth rate using a prepulse, or picket, preceding the main laser-drive pulse in planar-target experiments. The experimental results showed that a high-intensity picket (~50% of the drive-pulse intensity) significantly reduced the RT growth rate for a 20-µm-wavelength surface perturbation but had no effect on the growth rates of longer-wavelength perturbations (30 and 60 µm). However, both the 20- and 30-µm-wavelength perturbations showed no appreciable growth rate with a prepulse intensity equal to the drive-pulse intensity. These results suggest that the acceleration-phase RT growth rates for short-wavelength, laser-induced imprint perturbations may be virtually eliminated in spherical implosions by modifying the drive pulse to include a high-intensity picket on the leading edge. This work will be applied to spherical implosions in the near future.

Additional highlights of research presented in this issue include the following:

An analytic analysis of laser-induced adiabat shaping in inertial fusion implosions shows that the adiabat profile between the ablation surface and the fuel–shell interface induced by a decaying shock follows a simple power law of the shell areal density. Significantly, the calculated profiles are nearly identical to those observed in 1-D hydrodynamic simulations. This similarity suggests that the calculated profiles can be used to quickly and easily design an optimal laser prepulse to maximize the adiabat ratio between the inner- and outer-shell surfaces, leading to improved hydrodynamic stability.
The development and application of a numerical model to systematically investigate the performance of an optical parametric chirped-pulse amplification (OPCPA) system are described. The model uses both Gaussian and super-Gaussian spatial and temporal pump laser pulse shapes and includes the effects of pump– signal spatial walk-off and spatiotemporal noise. The results of this numerical investigation show that good energy stability, good beam quality, and high overall conversion efficiency can be obtained by carefully designing the OPCPA configuration and optimizing the spatiotemporal profile of the pump laser.
The nonlinear propagation of light through a plasma near the critical density is described using a model that includes filamentation, forward stimulated Brillouin scattering (SBS), backward SBS, the reflection of light from the critical-density surface, and the absorption of light. Because the model incorporates nonparaxial propagation of light, it can describe the reflection of light from the critical-density surface and the propagation of crossing laser beams. The study shows that the model successfully describes experimentally observed features of scattered light and is well suited to describe the oblique incidence of laser beams on a critical-density surface.
Ultrafast voltage transients in optically thick YBCO superconducting microbridges driven into the resistive flux state by nanosecond-wide supercritical current pulses and synchronously excited by femtosecond optical pulses have been investigated. Using a flexible experimental setup, the dynamics of the YBCO photoresponse have been measured. These measurements demonstrate that a YBCO superconductor in the flux-flow state can operate as a GHz-rate, high-power, optically triggered switch for microwave-based telecommunication applications.
The fabrication and testing of ultrafast photodetectors fabricated on high-energy-nitrogen–implanted gallium-arsenide (GaAs) are described. By direct comparison, it is shown that this novel photodetector is significantly more sensitive than commercially available low-temperature GaAs photodevices used extensively in high-speed applications.
By incorporating tritium—the radioactive isotope of hydrogen—into standard hydrogenated amorphous silicon (a-Si:H) devices, it may be possible to establish a new family of devices in which the energy output of the tritium decay is integrated with the optoelectronic properties of a-Si:H (e.g., photovoltaics and active matrix displays). This article describes the fabrication process and shows unequivocally that tritium bonds stably in amorphous silicon.

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This volume of the LLE Review, covering July–September 2003, features “The Coherent Addition of Gratings for Pulse Compression in High-Energy Laser Systems.” The article describes the conceptual development and experimental demonstration of the coherent summation of multiple gratings to form a larger grating. The most-promising reflection-grating technology for short-pulse, high-energy petawatt-class laser systems utilizing chirped-pulse amplification (CPA) is a holographically formed grating combined with a multilayer dielectric (MLD) coating. The aperture size and damage threshold of such gratings determine the ultimate short-pulse energy capability of these laser systems. Current state-of-the-art gratings would limit a laser such as OMEGA EP to an energy of less than 1 kJ per beam. While it may be possible in the future to manufacture very large gratings, tiling the MLD gratings available today represents an extremely attractive alternative for the OMEGA EP. The key result presented in this article is the conclusive demonstration of subpicosecond pulse compression using tiled gratings. This is truly an enabling technology for the high-energy, short-pulse lasers planned for the coming decade.

Additional highlights of research presented in this issue include the following:

The feasibility of using the x-ray-drive beam configuration at the National Ignition Facility (NIF) to achieve direct-drive ignition is examined computationally. The baseline x-ray-drive beam configuration was designed to illuminate a vertically oriented hohlraum with beams arrayed symmetrically around the polar regions of the target chamber. The article shows that near symmetric direct-drive illumination could be achieved by repointing some of the polar beams toward the equator of the capsule and adjusting the beam-spot sizes and energies. This new drive concept is called polar direct drive (PDD). The article describes the current status of the theoretical work focusing, in particular, on the beam-pointing strategy. The long-term impact of this work within the national ICF program is potentially of great importance if ignition conditions can be achieved on the NIF using the PDD concept.
The latest experimental results on the equation of state (EOS) of hydrogen at pressures of a few megabars, temperatures of a few electron volts, and compressions of up to several times liquid density are presented. A better understanding of the hydrogen EOS is important for the accurate simulation of direct and indirect ignition target designs on the NIF. At present, there are several different models for the hydrogen EOS, and proven to be exceptionally difficult to measure experimental observables with sufficient accuracy to discriminate among the models. The experimental results reported here are based on a new re-shock technique that is more sensitive to differences between these EOS models.
A method to modulate both the phase and amplitude of a laser beam with a single-phase-only spatial light modulator (SLM) using a carrier spatial frequency and a spatial filter is described. It is shown that with this technique, dynamic corrections to a laser-beam profile are possible.
The implementation of a new diagnostic system for direct-drive-implosion studies on the OMEGA laser system is reported. The proton temporal diagnostic (PTD) was designed to measure the fusion reaction history in capsule implosions containing D³He fuel. By measuring the temporal emission history and final energy spectrum of this high-energy proton, it is possible to study the areal-density evolution of the shell during the shock and compressive burn phases of an implosion. Existing range-filter spectrometers routinely measure the high-energy proton spectra from both D₂ and D³He implosions. This data can now be combined with the temporal emission history of the new PTD to provide new constraints on the multidimensional hydrocodes used to understand implosion performance on OMEGA.
The use of conventional magnetorheological finishing (MRF) techniques to improve the surface finish and figure of several standard polymer optics is described in detail. Since these optics are generally soft with high linear expansion coefficients and poor thermal conductivities, they are typically used as manufactured even though, in some instances, it would be desirable to have much better surface finishes. It is shown that the rms surface roughness of four different optical polymers can be reduced significantly using MRF.
This volume concludes with a summary of LLE’s Summer High School Research Program, the FY03 Laser Facility Report, and the highlights of the National Laser Users’ Facility and External Users’ Programs.

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This volume of the LLE Review, covering October–December 2003, features “Direct-Drive Cryogenic Target Implosion Performance on OMEGA.” Layered and characterized cryogenic D₂ capsules have been imploded using high-contrast pulse shapes on the 60-beam OMEGA laser at the Laboratory for Laser Energetics. These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the fuel during the implosion, and the laser power balance. The goal is to demonstrate a high neutron-averaged fuel ρR with low angular variance using a scaled α ~ 3 ignition pulse shape driving a scaled all-DT ignition capsule. Results are reported with improvements over previous experiments in target layering and characterization and in laser pointing and target positioning on the OMEGA laser. These capsules have been imploded using up to 23 kJ of 351-nm laser light with an on-target energy imbalance of less than 2% rms, full beam smoothing (1-THz bandwidth, 2-D SSD, and polarization smoothing), and new, optimized, distributed phase plates. Pulse shapes include high-adiabat (α ~ 25) square pulses and low-adiabat (α < 5) shaped pulses. The data from neutron and charged-particle diagnostics, as well as static and time-resolved x-ray images of the imploding core, are compared with 1-D and 2-D numerical simulations. Scaling of target performance to a weighted quadrature of inner ice roughness at the end of the acceleration phase is investigated.

Additional highlights of research presented in this issue include the following:

The compressed-core, temperature-density profiles of a cryogenic deuterium (D₂) target are characterized using measured primary deuterium–deuterium (DD) and secondary deuterium–tritium (DT) yields, neutron-averaged ion temperature, and x-ray images at peak neutron production. In addition, the electron pressure and the areal density of the neutron production region are inferred to be 2.7±0.4 Gbar and ~10 mg/cm², respectively.
A laser-source–based, extended x-ray absorption fine structure (EXAFS) measurement is used to study the properties of laser-shocked metals on a nanosecond time scale. The ability to measure shock-induced temperatures of the order of 0.1 eV is essentially unique to EXAFS. EXAFS measurements of vanadium shocked to ~0.5 Mbar with a 3-ns laser pulse yield a compression and temperature in good agreement with hydrodynamic simulations and shock-speed measurements. In laser-shocked titanium at the same pressure, the EXAFS modulation damping is much higher than warranted by the increase in temperature. This is explained by the α-Ti to ω-Ti phase transformation known to occur around ~0.1 Mbar in the longer (µs) shocks obtained in gas-gun experiments. In the ω-Ti phase, the disparate neighbor distances cause a beating of the modulation frequencies and thus an increased damping. These results demonstrate that EXAFS measurements can be used to study nanosecond-scale shocks and phase transformations in metals.
Metal–semiconductor–metal ultraviolet photodiodes fabricated on GaN are tested in the picosecond regime with an electro-optic sampling system. The best performance of a device with a feature size of 1 µm showed a 1.4-ps rise time and 3.5-ps full width at half maximum, which represents the fastest ultraviolet GaN photodiode reported to date. The derived electron velocity in GaN was in good agreement with an independent photoexcitation measurement. A comparison with Monte Carlo simulation was made, and the slower impulse response observed in a device with a smaller feature size of 0.5 µm was discussed.
Photonic crystals offer great promise in a variety of applications in optoelectronics, from lasers to the creation of all-optical circuits for computing. The presented research project focuses on the creation of novel photonic crystals through the self-assembly of core-shell structured colloidal particles. Layer-by-layer electrostatic self-assembly was used to deposit polyelectrolyte shells around spherical colloidal particles. By exploiting electrostatic attraction, shells of controllable thickness were formed by alternating the deposition of positive- and negative-charged polyelectrolytes. The coated colloidal particles were deposited as thin films of hexagonally close-packed crystals onto glass slides. The crystalline films display a partial photonic band gap and preferentially reflect light of a wavelength dependent on the size of the particles making up the crystal. The chemical functional groups in the shell surrounding the colloidal particles offer a potential route to immobilize optically active species in the shell to enhance the photonic band gap of the crystal.
A ytterbium fiber laser mode-locked at its 280th harmonic, which corresponds to a repetition rate greater than 10 GHz, is considered. The laser produces linearly polarized, 2.6-ps chirped pulses with up to 38 mW of average output power. The mode-locked pulses are tunable over a 55-nm window centered on 1053 nm.
A series of microgrinding and polishing experiments are performed on glass-ceramics. Microgrinding includes deterministic microgrinding (fixed infeed rate) and loose-abrasive lapping (fixed pressure). Material mechanical properties (Young’s modulus, hardness, fracture toughness) and chemical properties (chemical susceptibility, or mass loss under chemical attack) are correlated with the quality of the resulting surface (surface microroughness and surface grinding-induced residual stresses). Deterministic microgrinding (at fixed infeed) and loose-abrasive microgrinding (at fixed pressure) are compared in terms of material removal rates and resulting surface quality.

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This volume of the LLE Review, covering January–March 2004, features “Performance of 1-THz-Bandwidth, 2-D Smoothing by Spectral Dispersion and Polarization Smoothing of High-Power, Solid-State Laser Beams.” Laser-beam smoothing achieved with 1-THz-bandwidth, two-dimensional smoothing by spectral dispersion and polarization smoothing on the 60-beam, 30-kJ, 351-nm OMEGA laser system is reported. These beam-smoothing techniques are directly applicable to direct-drive ignition target designs for the 192-beam, 1.8-MJ, 351-nm National Ignition Facility. Equivalent-target-plane images for constant-intensity laser pulses of varying duration were used to determine the smoothing. The properties of the phase plates, frequency modulators, and birefringent wedges were simulated and found to be in good agreement with the measurements.

Additional highlights of research presented in this issue include the following:

The contribution of the gradients in the laser-induced electric field to the current flow, heat flux, and electric stress tensor is studied in laser-produced plasmas. The transport coefficients, previously derived in the limit Z >> 1, are obtained for an arbitrary ion charge Z. It is shown that the ponderomotive terms significantly modify the thermal transport near the laser turning points and the critical surface.
The target areal-density (ρR) asymmetries are measured in OMEGA direct-drive spherical implosions. The rms variation (δρR)/(ρ) for a low-mode-number structure is approximately proportional to the rms variation of on-target laser intensity (δI )/(I) with an amplification factor of ~1/2(C_r–1), where C_r is the capsule convergence ratio. This result has critical implications for future work on the National Ignition Facility (NIF) as well as on OMEGA.
The stimulated Brillouin scattering (SBS) in one- and two-ion plasmas is described by using the ion-fluid and Poisson (IFP) equations with phenomenological damping terms and the light-wave equation. A computer code is tested by comparing numerical and analytical results in the linear limit. The code is used to compare effects of Landau damping, pump depletion, and ion-acoustic nonlinearities on the saturation of SBS in one- and two-ion plasmas. In the latter, SBS from fast and slow ion-acoustic waves is considered separately. SBS is simulated for hydrocarbon (CH) plasmas with parameters typical for experiments on OMEGA.
Measurements of the imprint efficiency in 20-μm-thick plastic foils driven by 351-nm laser light at an intensity ~2 x 10¹⁴ W/cm² are reported. The measured target spatial modulations were imprinted from spatial laser nonuniformities during laser-ablated plasma formation at the beginning of the drive. The laser modulations consisted of broadband nonuniformities from six beams incident at 23° to the target normal and single-mode perturbations from one beam incident at 48° to the target normal. The measurements were performed at a spatial wavelength of 60 μm with and without smoothing by spectral dispersion (SSD). The measured imprint efficiencies at 60-μm spatial wavelength were 2.5±0.2 μm for the beam with 48° angle of incidence and 3.0±0.3 μm for the beams with 23° angle of incidence. The SSD reduced modulations by a factor of ~2.5 at the same spatial wavelength.
An analytical model of the interaction of directed energetic electrons with a high-temperature hydrogenic plasma is presented. The randomizing effect of scattering off both plasma ions and electrons is treated from a unified point of view. For electron energies of less than 3 MeV, electron scattering is equally important. The net effect of multiple scattering is to reduce the penetration from 0.54 to 0.41 g/cm² for 1-MeV electrons in a 300-g/cm³ plasma at 5 keV. These considerations are relevant to “fast ignition” and to fuel preheat for inertial confinement fusion.
Shielding strategies are considered to optimize the signal-to-background ratio and to obtain high-quality x-ray spectra. The use of a single-photon–counting x-ray CCD camera as an x-ray spectrometer is a well-established technique in ultra-short-pulse laser experiments. In the single-photon–counting mode, the pixel value of each readout pixel is proportional to the energy deposited from the incident x-ray photon. For photons below 100 keV, a significant fraction of the events deposits all energy in a single pixel. A histogram of the pixel readout values gives a good approximation of the x-ray spectrum. This technique requires almost no alignment, but it is very sensitive to signal-to-background issues, especially in a high-energy petawatt environment.
The theory of the adiabat profile induced by a strong shock propagating through a relaxed density profile in inertial confinement fusion (ICF) capsules is developed. The relaxed profile is produced through a laser prepulse, while the adiabat-shaping shock is driven by the foot of the main laser pulse. The adiabat shape is calculated for the cases of intense, short prepulses and weak, long prepulses. The theoretical adiabat profiles accurately reproduce the simulation results to within a few-percent error. ICF capsules with a shaped adiabat are expected to benefit from improved hydrodynamic stability while maintaining the same one-dimensional performances as flat-adiabat shells.
Models for determining the areal density of hot fuel (ρR_hot) in compressed, D₂-filled capsules are investigated. Measurements from three classes of direct-drive implosions on OMEGA were combined with Monte Carlo simulations to assess the impact of mix and other factors on the determination of ρR_hot. The results of the Monte Carlo calculations were compared to predictions of simple, commonly used models that use ratios of either secondary D³He proton yields or secondary DT neutron yields to primary DD neutron yields to provide estimates of ρR_hot,p or ρR_hot,n, respectively, for ρR_hot.

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This volume of the LLE Review, covering April–June 2004, features “Multidimensional Simulations of Plastic-Shell Implosions on the OMEGA Laser.” The multidimensional hydrodynamic code DRACO is applied to studies of shell stability during the acceleration phase in the presence of nonuniform illumination and target roughness. Simulations show that for thick shells remaining integral during acceleration, the target yield is reduced by a combination of long-wavelength modes due to surface roughness and beam-to-beam imbalance as well as intermediate modes due to single-beam nonuniformities. Compared to 1-D predictions, the neutron-production rate for these shells truncates. Diminished yield for thin shells is mainly due to shell breakup at short-wavelength scales of the order to the in-flight shell thickness. DRACO simulation results are consistent with experimental observations.

Additional highlights of research presented in this issue include the following:

A comprehensive overview is given of the methodology of, and issues involved in, the preparation of deuterium-ice layers in OMEGA targets. The process of first forming and then smoothing the ice layer is governed by multiple parameters that, when optimally controlled, yield ice layers approaching a 1-μm-rms roughness in low-spatial-frequency modes.
A new, modular x-ray streaked imager has been developed and fielded that combines a four-mirror Kirkpatrick–Baez microscope with a high-current PJX streak tube. Performance optimized for use at 1.5 keV, this instrument provides better-than-5-μm spatial resolution over its central 200-μm field of view. When equipped with a mix of iridium-coated and multilayer mirrors, simultaneous images in distinct x-ray energy bands can be obtained.
Growing 25-mm-diam Nd:YLF rods with high optical-wavefront quality is extremely challenging. Bulk wavefront distortions of several microns are typically found in state-of-the-art rods. These have now been successfully compensated by magnetorheological single-surface correction. Large-aperture rods corrected in this manner render nearly diffraction-limited output-beam performance.
Several prototypes of NIF neutron-time-of-flight detectors have been developed and tested on OMEGA with DD and DT implosions. For low and moderate NIF neutron yields, systems comprising fast plastic scintillators and fast photomultiplier tubes have proven successful. For higher yields, a third type based on chemical-vapor-deposition (CVD) diamonds provides superior response. Taken together, these detectors cover a neutron yield range between 10⁹ and 10¹⁹.
Ultrafast current sensing has reached a new level of sensitivity and speed by taking advantage of the magneto-optic Faraday effect in CdMnTe single crystals. Based on pump–probe magneto-optic sampling measurements at 10 K, response times of a few hundred femtoseconds can be realized at a current sensitivity of ~0.1 mA. According to the wavelength dependence of this sensitivity, excitonic contributions enhance this material’s dynamic Verdet constant.
The intrinsic high steady-state peak electron velocity in GaN offers promise for fast UV detectors that is exploited in an interdigitized-finger, metal–semiconductor–metal photodiode design yielding 50-ps response times for subpicojoule optical-energy inputs. At higher input energies, space-charge screening leads to broadening of the electrical output pulse. These experimentally observed phenomena closely resemble whole-package simulations involving a single, adjustable parameter for external quantum efficiency.
Creating micrometer-scale structures through the use of electrometric stamps experienced a further refinement: controlled deformation of the stamp by overpressure enables pattern formation of nanoparticles or self-assembled monolayers on a scale length up to an order of magnitude smaller than the original stamp as well as patterns that do not exist in the original masters. As one example, magnetic ring and anti-ring structures are being fabricated for memory-device applications.

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This volume of LLE Review, covering July–September 2004, features “Shock Propagation in Deuterium–Tritium–Saturated Foam.” Testing the assumption of homogeneous mixing in fibrous foams saturated with cryogenic deuterium and tritium, shock passage in wetted-foam mixtures is simulated by the adaptive-mesh, two-dimensional hydrodynamic code AstroBEAR. For foam fibers of diameter ~1/10 µm and for relevant foam densities, the mixing length behind the shock is found to be of the order of microns. Transverse motion decays quickly such that, at the mixing region’s edge farthest from the shock, Rankin–Hugoniot jump conditions are obeyed to within a few percent and shock speeds are also within a few percent of their homogeneous values. Also considered is the question of feedthrough and the decay time of shock-front perturbations seeded by the foam. Because shock fronts are stable, perturbations rapidly decay once the shock has entered the homogeneous DT-ice layer, rendering mute the feedthrough concern. As a result, full-scale simulations of foam-target implosions may model the wetted foam as a homogeneous mixture

Additional highlights of research presented in this issue include the following:

A status update is provided on the OMEGA EP tiled-grating-compressor technology. Similar to the scheme known from astronomy where very large telescopes are constructed using an array of mirrors, the large gratings required for petawatt-class, pulsed compression can be assembled from multiple smaller gratings. For the first time, real-time, closed-loop, computer-controlled phasing of a multilayer-dielectric-grating triplet, using interferometric feedback via nanopositioners, is demonstrated and a transform-limited, far-field spot is achieved.
A method for simultaneously measuring beam position, shape, and relative intensity of all 60 of OMEGA’s beams is introduced by CID-recorded, focal-spot x-ray-image analysis. A 4-mm, Au-coated pointing surrogate is used as a target. This method offers pivotal input into efforts to improve target-illumination uniformity by improving beam pointing and reducing variations from average in beam intensities.
Chemical durability is an important issue for water-sensitive laser glasses. A survey of commercial glasses—both cast and continuously melted—explores their hazing tendency under high relative humidity as a function of different polishing and cleaning conditions. While for any of the glass types tested no unambiguous correlation exists between initial finished surface quality and quantifiable magnitude of humidity-driven degradation, aggressive aqueous cleaning does result in hazing: more so for pitch and pad–finished surfaces than for MRF-processed ones.
Single-molecule spectroscopy is used to determine the electronic and structural properties of individual, single-walled carbon nanotubes. Specifically, tip-enhanced, near-field Raman imaging and spectroscopy afford spatial resolution of 10 to 20 nm. Unlike most other molecules studied to date, single-walled nanotubes’ fluorescence intensity does not fluctuate. In addition, a nonuniform distribution of Raman bands is found along the tube axis.
Polymeric cholesteric liquid crystals reflect light of a specific wavelength and circular polarization state. When broken into small flakes and suspended in a dielectric host fluid, they can be collectively oriented by an external field as useful, for instance, in a display application. An account is given of experimental measurements as well as theoretical simulations of flake rotation in an ac field. As an underlying mechanism, the field coupling to an induced dipole moment is identified—a dipole that arises on the flake surface due to Maxwell–Wagner polarization.
This volume also includes a summary of the LLE Summer High School Research Program, the FY03 Laser Facility Report, and the National Laser Users’ Facility and External Users’ Programs.

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This volume of the LLE Review, covering October to December 2004, highlights the significance of shaped adiabats to inertial confinement fusion. Theory suggests that inertial confinement fusion (ICF) capsules compressed by shaped adiabats will exhibit improved hydrodynamic stability. Although the theoretical formulas for the adiabat profiles generated by the relaxation method of adiabat shaping have been previously derived, the formulas presented in this Review are simplified to power-law expressions. Experiments demonstrate that target stability improves when picket pulses are used to increase and shape the ablator adiabat. Rayleigh–Taylor (RT) growth of nonuniformities is suppressed in both planar and spherical targets with picket-pulse laser illumination. Two types of picket pulses—a “decaying-shock-wave picket” and a “relaxation” picket—are used to shape the adiabat in spherical targets. Planar growth measurements using a wide, intense picket to raise the adiabat of a CH foil show that the growth of short-wavelength perturbations is reduced, and even stabilized, by adjusting the intensity of the picket. Data from planar imprint experiments show that the imprint level is reduced when a picket is added and, for short wavelengths, is as effective as 1-D, 1.5-Å SSD. Results from relaxation-picket implosions show larger yields from fusion reactions when the picket drive is used.

Additional research developments presented in this issue include the following:

Crater formation in SiO₂ thin films containing artificial defects by UV-pulsed-laser irradiation depends on the lodging depth of the defects. At laser fluences close to the crater-formation threshold and for lodging depths of a few particle diameters, the dominating material-removal mechanism is melting and evaporation. For absorbing defects lodged deeper than ~10 particle diameters, a two-stage material-removal mechanism occurs. The process starts with the material melting within the narrow channel volume, and, upon temperature and pressure buildup, film fracture takes place.
Altering the fluid composition of magnetorheological (MR) fluids prepared with a variety of magnetic and nonmagnetic ingredients minimizes artifact formation on the surface of CVD ZnS flats and greatly improves the smoothing performance of magnetorheological finishing. A nanoalumina abrasive used with soft carbonyl iron and altered MR fluid chemistry yields surfaces with roughness that do not exceed 20 nm p–v and 2-nm rms after removing 2 µm of material. The formation of “orange peel” and the exposure of “pebble-like” structure inherent in ZnS from the CVD process are suppressed.
A 63-channel, high-resolution, ultraviolet (UV) spectrometer to check the tuning state of KDP triplers has been designed and tested. The spectrometer accepts an input energy of 1 µJ per channel, has a dispersion at the detector plane of 8.6 X 10^–2 picometers (pm)/µm, and has a spectral window of 2.4 nanometers (nm) at λ = 351 nm. The wavelength resolution varies from 2.5 pm at the center of the field of view to 6 pm at the edge.
The quantum efficiency (QE) and the noise equivalent power (NEP) of the latest-generation, nanostructuredNbN, superconducting, single-photon detectors (SSPD’s) operated at temperatures in the 2.0- to 4.2-K range in the wavelength range from 0.5 to 5.6 µm has been measured. The detectors are designed as 4-nm-thick, 100-nm-wide NbN meander-shaped stripes, patterned by electron-beam lithography. Their active area is 10 X 10 µm². The best-achieved QE at 2.0 K for 1.55-µm photons is 17%, and the QE for 1.3-µm infrared photons reaches its saturation value of ~30%. The SSPD NEP at 2.0 K is as low as 5 X 10–²¹ W/Hz–^1/2. These SSPD’s, operated at 2.0 K, significantly outperform their semiconducting counterparts. Together with their GHz counting rate and picosecond timing jitter, they are the devices-of-choice for practical quantum key distribution systems and quantum optical communications.
The first measurements of electron preheat in direct-drive laser implosions of cryogenic deuterium targets are reported. Preheat due to fast electrons generated by nonlinear laser–plasma interactions can reduce the gain in laser-imploded fusion targets. The preheat level is derived directly from the measured hard-x-ray spectrum. The fraction of the incident laser energy that preheats the deuterium fuel is found to be less than 0.1%, suggesting that the preheat will have a negligible impact on target performance.

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Review 102

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This volume of the LLE Review, covering January–March 2005, features the new “Saturn” target design concept for use in polar direct drive on National Ignition Facility (NIF) while the facility is in its initial, indirect-drive configuration. The concept differs from earlier polar-direct-drive designs by adding a low-Z ring around the capsule equator. Refraction in the plasma formed around this ring permits time-dependent tuning of the capsule drive uniformity. Proof-of-principle, polar-direct-drive (PDD) experiments on OMEGA using 40 repointed beams of the 60-beam OMEGA Laser System to approximate the NIF PDD configuration have been carried out. Backlit x-ray framing-camera images of D₂-filled spherical CH capsules show a characteristic nonuniformity pattern that is in close agreement with predictions. Saturn targets increase the drive on the equator, suggesting that highly symmetric PDD implosions may be possible with appropriate tuning. Two-dimensional simulations reproduced the approximately threefold reduction in yield found for the non-Saturn PDD capsules. Preliminary simulations for a NIF Saturn target design predict a high gain close to the 1-D prediction. These results increase the prospects of obtaining direct-drive ignition with the initial NIF configuration.

Additional research developments presented in this issue include the following:

Direct-drive, spherical, cryogenic, D₂-filled capsules have been illuminated using the 60-beam OMEGA Laser System. The targets are energy scaled from the baseline ignition design developed for NIF. Thin-walled (~4-µm), ~860-µm-diam deuterated (CD) polymer shells are permeation filled with D₂ gas and cooled to the triple point (~18.7 K). Cryogenic ice layers with a uniformity of ~2-µm rms are formed and maintained. The targets are imploded with high-contrast pulse shapes using full single-beam smoothing (1-THz bandwidth, two-dimensional smoothing by spectral dispersion with polarization smoothing) to study the effects of the acceleration- and deceleration-phase Rayleigh–Taylor growth on target performance. These experiments have produced fuel areal densities up to ~100 mg/cm², primary neutron yields
~4 x 10¹⁰, and secondary neutron yields 1% to 2% of the primary yield.
The interaction of directed energetic electrons with hydrogenic plasmas are analytically modeled from fundamental principles. The effects of stopping, straggling, and beam blooming are rigorously treated in a unified approach for the first time. Enhanced energy deposition, which occurs in the latter portion of beam penetration, is inextricably linked to straggling and beam blooming. Both effects asymptotically scale with the square root of the linear penetration. Eventually, they dominate over all other sources of beam divergence. Understanding their effects is critical for evaluating the requirements of fast ignition.
Direct-drive, plastic-shell implosions on OMEGA with a 1-ns square pulse have been simulated using the multidimensional hydrodynamic code DRACO. Yield degradation in “thin” shells is primarily caused by shell breakup during the acceleration phase because of short-wavelength perturbation growth, whereas “thick” shell performance is influenced primarily by long and intermediate modes. Simulation yields, temporal history of neutron production, areal densities, and x-ray images of the core compare well with experimental observations. Thin-shell neutron production history falls off less steeply than one-dimensional predictions because of shell breakup induced under compression and delayed stagnation. Thicker, more-stable shells show burn truncation due to instability-induced mass flow into the colder bubbles. Estimates of small-scale mix indicate that turbulent mixing does not influence primary neutron yields.
Effects of temporal density variation and spherical convergence on the nonlinear bubble evolution of single-mode, classical Rayleigh–Taylor instability are studied using an analytical model based on Layzer’s theory. When the temporal density variation is included, the bubble amplitude in the planar geometry asymptotes to a fixed value that depends on the Layzer bubble velocity, the fluid density, and a factor to account for the two- and three-dimensional geometries. The model can be applied to spherical geometries to predict the nonlinear bubble amplitude.

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This volume of the LLE Review, covering April—June 2005, features the design and optimization of targets for fast-ignition inertial confinement fusion relevant to direct-drive inertial fusion energy. It is shown that a 750-kJ laser can assemble fuel with V_I = 1.7 × 10⁷ cm/s, α = 0.7, ρ = 400 g/cc, ρR = 3 g/cm², and a hot-spot volume of less than 10% of the compressed core. If fully ignited, this fuel assembly can produce energy gains of 150.

Recent OMEGA experiments have studied the fuel assembly of gas-filled, cone-in-shell, fast-ignition targets. Using both fusion products and backlit images, an areal density of ~60-70 mg/cm² was inferred for the dense core assembly. The results are promising for successful integrated fast-ignition experiments on the OMEGA EP Facility, scheduled for completion in 2007.

Additional research developments presented in this issue include the following:

A high-performance "planar" Cryogenic Target Handling System has been added to LLE's OMEGA Laser Fusion Facility. The system has demonstrated a shot-to-shot cycle interval of less than two hours and has fielded more than 125 experiments using several distinct target types. An overview of the cryogenic capabilities at LLE and a comparison of operational requirements of LLE's spherical and planar cryogenic systems is given.
Nonlinear growth measurements of 3-D broadband nonuniformities near saturation using x-ray radiography in planar foils accelerated by laser light have been made. The initial target modulations were seeded by laser nonuniformities and later amplified during acceleration by the Rayleigh—Taylor instability.
The significant developments in tritium-capture technology that have occurred over the past two decades are described. The merits and drawbacks of the various technologies that have been developed for both air and inert gas streams are discussed.
An all-solid-state, diode-pumped Nd:YLF laser system has been developed and tested. It produces fiducial timing signals at three wavelengths (fundamental, second, and fourth harmonics) and will be used as a primary timing reference for the OMEGA facility diagnostics. Performance results of the new OMEGA fiducial laser are reported.
Extended x-ray absorption fine structure (EXAFS) measurements have been used to demonstrate the phase transformation from body-centered-cubic (bcc) to hexagonal-closely-packed (hcp) iron due to nanosecond, laser-generated shocks. This is a direct, atomic-level, and in-situ proof of shock-induced transformation in iron.

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Review 104

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This volume of the LLE Review, covering July through September 2005, features a report on backlit optical shadowgraphy, the primary diagnostic for D₂ ice layer characterization of cryogenic targets for the OMEGA Laser System. Measurement of the position of the most prominent rings, caused by the reflection and refraction of light in t he ice layer, in conjunction with model predictions allows constructions of a 3-D ice layer representation suitable for implosion modeling.

Another highlight is a description of velocity interferometry and optical self emission measurements from shock waves in polystyrene targets. The velocity histories, coalescence times, and transit times are unambiguously observed and are in good agreement with 1-D code predictions. The timing of multiple shock waves is crucial to the performance of inertial confinement fusion ignition targets.

Additional highlights of research presented in this issue include the following:

Designs set to achieve direct-drive ignition on the NIF at 1 MJ using the x-ray-drive beam configuration are examined. This approach, known as polar direct drive (PDD), achieves the required irradiation uniformity by repointing some of the beams toward the target equator.
Simulations of integrated fast-ignition experiments on the combined OMEGA/OMEGA EP Laser Systems with the multidimensional hydrodynamic code DRACO are presented. An OMEGA cryogenic DT target has been simulated in 2-D with and without nonuniformities. The neutron yield is predicted to be in excess of 10¹⁵ (compared to ~10¹⁴ without an ignitor beam).
The development of a proton emission imaging system that has been used to measure the nuclear burn regions in the cores of inertial confinement fusion implosions is described. The imaging technique relies on the penumbral imaging of 14.7-MeV D³He fusion protons.
A hot, T_e ~ 2- to 3-keV surface plasma in the interaction of a 0.7-ps petawatt laser beam with solid copper-foil targets at intensities >10²⁰ W/cm² has been observed. These temperatures were inferred from Cu, He_α, and Ly_α emission lines. Such lines have not previously been observed in interactions with ultrafast laser pulses.
A summary of LLE's Summer High School Research Program.
The FY05 Laser Facility Report.
The National Laser Users' Facility and External Users' Programs summary.

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Review 105

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This volume of the LLE Review, covering October–December 2005, features results of measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments. Radial profiles of nuclear burn in directly driven implosions have been systematically studied for the first time using a proton emission imaging system at the OMEGA Laser Facility. The system is sensitive to energetic 14.7-MeV protons from the fusion of deuterium (D) and 3-helium (³He). Clear relationships have been identified between variations in the size of the burn region and variations in such experimental parameters as capsule size, shell composition and thickness, gas-fill pressure, and laser energy.

Additional highlights of research presented in this issue include the following:

The Rayleigh–Taylor growth measurements of 3-D modulations in a nonlinear regime are presented. The measured modulation Fourier spectra and nonlinear growth velocities are in excellent agreement with Haan’s model. In real-space analysis, the bubble merger was quantified by a self-similar evolution of bubble size distributions, in agreement with the Alon–Oron–Shvarts theoretical predictions.
Results of studies of isotopic fractionation during solidification of H₂-HD-D₂ mixtures are reported. An isotopic fractionation reduces the efficiency of the fusion reaction in future cryogenic D-T targets. It is found that frozen H-D mixtures have spatial concentration gradients of the order 0.02 to 0.05 molecular fraction per millimeter, which points to little separation of isotopes.
Implications of hydrogen fractionation in ICF ignition target designs are discussed. A numerical investigation of the effects that fractionation has on hot-spot formation, ignition, and burn in ICF target designs indicates that small levels of fractionation (~10%) are acceptable for ignition performance on the NIF.
Polar-direct-drive (PDD) simulations and experiments on the OMEGA Laser System are described. Forty OMEGA beams arranged in six rings to emulate the NIF x-ray–drive configuration are used to perform direct-drive implosions of CH shells filled with D₂ gas. Simulations performed with DRACO code are in good agreement with experimental x-ray radiographs and show ignition with a gain of 20 and the development of a 40-µm-radius, 10-keV region with a neutron-averaged ρr of 1270 mg/cm² near stagnation.
Surface features of tungsten carbide composites processed by bound abrasive microgrinding and magnetorheological finishing (MRF) are analyzed. It was found that the peak-to-valley microroughness of the surface gives a measure of the deformed layer depth. MRF spots revealed the true depth of the grinding-induced deformed surface layer.

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Review 106

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This volume of the LLE Review, covering January–March 2006, features a description of the characteristics and performance of a high-gain, polarization-preserving, Yb-doped fiber amplifier for low-duty-cycle pulse amplification. The authors report on a high-gain, low-noise, double-pass ytterbium-doped amplifier for which amplified spontaneous emission (ASE) suppression techniques were utilized to fabricate a double-pass amplifier with the noise properties of a single-pass amplifier. A double-pass configuration allows for significantly higher gains to be obtained in a fiber amplifier than can be achieved in a single-pass configuration. Simulations based on a rate equation model were used to analyze the ASE and the impact of the suppression techniques. These techniques were implemented in an alignment-free, double-pass fiber amplifier with 26-dB gain at a wavelength 23 nm off the gain peak and a –48-dB noise floor, while amplifying linearly polarized optical pulses with a low duty cycle.

Additional highlights of research presented in this issue include the following:

Development of methods and techniques for the decontamination of metals and alloys from tritium is reported. The efficacy of tritium removal from stainless steel using four different approaches has been studied: thermal desorption in a dry inert gas purge, thermal desorption in a wet inert gas purge, thermal desorption in an inert gas purge containing hydrogen peroxide, and radio-frequency-driven argon plasma irradiation. This study indicates that reducing the activity in metals below 0.5 µCi/g is feasible without generating secondary active waste byproducts other than water.
A review of the basic concept of laser-driven ICF ignition is presented with emphasis on the direct-drive ignition target designing, requirements for the temporal shape of the laser pulse, and consideration of the stability issues.
The results on direct-drive implosions of targets filled with different mixtures of D₂ and ³He gas on the OMEGA Laser System are reported. At temperatures above a few electron volts, D₂ and ³He gasses are fully ionized and hydrodynamically-equivalent fuels with different ratios of D₂ and ³He can be chosen to have the same mass density, total particle density, and equation of state. Implosions with a 50/50 mixture of D:³He by atom consistently result in measured nuclear yields half of that anticipated by scaling from measured yields of implosions with pure D₂ and nearly pure ³He. This observation is seen over a wide range of experimental configurations, including targets with a variety of shell thicknesses and fill pressures, simultaneously for two different nuclear yields (D-D and D-³He), as well as for shock and compression yields. A number of possible mechanisms to cause the scaling are considered, but no dominant mechanism has been identified.
Deterministically polarized fluorescence from single emitters (dye molecules) is demonstrated for the first time. In this experiment a planar-aligned, nematic liquid-crystal host provides uniaxial alignment of single-dye molecules in a preferred direction. As a result, fluorescence of these single emitters is deterministically polarized (single-photon source) which allows one to consider such systems for applications in photonic quantum information.
The performance of the fiber-coupled single-photon detectors based on NbN superconducting nanostructures for practical quantum cryptography and photon-correlation studies is described. Several two-channel, single-photon detector systems based on two fiber-coupled superconducting single-photon detectors were built and characterized. The best device reached the system quantum efficiency of 0.3% in the 1540-nm telecommunication wavelength with a fiber-to-detector coupling factor of about 30%.
The results on design and synthesis of transition metal dithiolene near-IR dyes are presented. Transition metal complexes based on nickel, palladium, or platinum dithiolene cores show substantial promise for guest–host liquid crystal devices operating in the near- to mid-IR region. The authors show some specific application examples for these materials in LC electro-optical devices and discuss the most recent results in the computational modeling of physical and optical properties of this interesting class of organometallic optical materials.

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Review 107

Highlights

This volume of the LLE Review, covering April–June 2006, features a description of the characteristics and performance of the high–contrast plasma–electrode Pockels cell (PEPC). The authors report on the development of the OMEGA EP PEPC prototype and the demonstration of high-switching contrasts exceeding 500:1 throughout the clear aperture. The key to producing this level of performance has been the reduction of stress birefringence using circular windows. In addition to the more typical role of holding the pulse in the cavity for four passes, the PEPC will be used to provide isolation from target retroreflections. Most existing multipass high–energy laser systems use frequency conversion to direct second- or third-harmonic light onto the target. This is not the case for the short–pulse part of OMEGA EP; therefore, any light reflected by the target can experience gain in the unsaturated amplifiers as it propagates back up the system, posing a significant damage threat to the system.

Additional highlights of research presented in this issue include the following:

Results are presented for Kodak Biomax–MS film calibration to x rays using an e–beam–generated x–ray source, a crystal/multilayer monochromator, a film pack, and an absolutely calibrated x–ray photon detector. The results agree with predictions from a theoretical model presented in a companion article in this issue.
A response model for Kodak Biomax–MS film to x rays is presented. This detailed film characterization starts with simple mathematical models and extends them to T–grain film. This is the companion article for the experimental results described above.
Results are presented for a new high-yield bang time detector for the OMEGA laser. The time interval from the beginning of the laser pulse to the peak of neutron emission (bang time) is an important parameter in inertial confinement fusion experiments. The NTD streak camera currently deployed on OMEGA is saturated by neutron yields above 3 x 10¹³, whereas the latest OMEGA experiments and those planned for OMEGA EP are expected to produce neutron yields above 10¹⁴. This new detector will support these experiments and also high-yield experiments at the National Ignition Facility (NIF).
Issues associated with operating target diagnostics in a petawatt environment are discussed. Sensitive electronic devices are difficult to operate in petawatt laser–target interaction experiments because there are copious amounts of relativistic electrons, hard x rays, and other charged particles created by the experiments. This has serious consequences for the design and integration of diagnostics inside or close to the target chamber.
Results of simulation for gain apodization in highly doped distributed-feedback (DFB) fiber lasers are presented. DFB lasers can be designed with an internal grating structure to provide high output power (up to 60 mW), single frequency, single polarization, and high optical signal–to–noise ratio. The authors investigate the effects of gain apodization on threshold behavior along with the impact on output power and mode discrimination. Apodization of the longitudinal gain profile is found to lower the laser threshold by 21% without degrading mode discrimination.

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Review 108

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This volume of the LLE Review, covering July–September 2006, features a progress report on cryogenic DT and D₂ targets for inertial confinement fusion. The author reports on the development of cryogenic targets since the need was first recognized more than 30 years ago. Virtually all ignition target designs for the National Ignition Facility (NIF) are based on a spherical low-Z ablator containing a solid, cryogenic-fuel layer of deuterium and tritium. Techniques developed at LLE can produce targets with an inner ice surface that meets the surface-smoothness requirement for ignition (<1-µm rms in all modes). Significant progress with the characterization of such targets is also reviewed along with results from the most recent cryogenic implosions that simulations suggest have yielded values for ρR_peak as high as 190±20 mg/cm².

Additional highlights of research presented in this issue include the following:

Results are presented for a study of the displacement of cryogenic targets from target chamber center (TCC). While beam smoothing and power balancing can ensure highly uniform illumination at TCC, target displacements of 5 µm or more from TCC can unbalance the illumination enough to initiate hydrodynamic instability. Correlation between target vibration at TCC and the response characteristics obtained in their study indicate that the modes of the spider silk are primary cause of displacement.
A novel imaging technology for measuring E and B fields in laser-produced plasmas using monoenergetic proton radiography is presented. The generation of electromagnetic fields by the interaction of laser light with matter is a process of fundamental interest in high-energy-density physics. They present high-resolution, time-gated radiography images of a plastic foil driven by 10¹⁴ W/cm² laser that imply B fields of 0.5 MG and E fields of 1.5 x 10⁸ V/m. Furthermore, their measurements demonstrate the beneficial focal smoothing effects produced by distributed phase plates for substantially reducing medium-scale chaotic field structure.
An evaluation of cleaning methods for multilayer-dielectric (MLD) diffraction gratings is presented. MLD diffraction gratings are essential components for the OMEGA EP short-pulse, high-energy laser system, and as such they must have both high optical-diffraction efficiency and high laser-damage threshold. The authors report on a study of chemical processes for cleaning MLD gratings that identifies techniques for removing contaminants left during fabrication in a way that does not compromise a grating's efficiency or damage threshold.
A study into the design and analysis of binary beam shapers for high-power laser systems is presented. OMEGA EP uses square beams with high-order super-Gaussian profiles to maximize the fill factor of amplifiers without exceeding the damage fluence of the laser components. The spatially dependent gain of the amplifiers can be, to a large extent, precompensated by attenuating regions of the input beam according to the gain they receive in the amplifiers. The authors have applied an error diffusion algorithm to the design of binary beam shapers consisting of a nonuniform array of 10-µm-sq pixels, which can be produced using standard lithographic techniques on high damage- threshold, metal-on-glass substrates.

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Review 109

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This volume of the LLE Review, covering October–December 2006, features a progress report on laser absorption, mass ablation rate, and shock heating in direct-drive inertial confinement fusion. The authors report on experimental studies on the OMEGA Laser System, which are used to validate hydrodynamic simulations. A comprehensive set of measurements tracking the flow of energy from the laser to the target was conducted. Time-resolved measurements of laser absorption in the corona are performed on spherical implosion experiments. The mass ablation rate is inferred from time-resolved Ti K-shell spectroscopic measurements of nonaccelerating, solid CH spherical targets with a buried tracer layer of Ti. Shock heating is diagnosed in planar-CH-foil targets using noncollective spectrally resolved x-ray scattering and also in targets with a buried tracer layer of Al using time-resolved x-ray absorption spectroscopy. A detailed comparison of the experimental results and the simulations indicates that a time-dependent flux limiter in the thermal transport model is required to simulate the laser-absorption measurements.

Additional highlights of research presented in this issue include the following:

Nuclear measurements were conducted to study fuel–shell mix in inertial confinement fusion implosions on OMEGA. To probe the extent of mix, nuclear yields were measured from implosions of capsules containing a deuterated plastic (CD) layer and filled with pure ³He. D³He-proton spectral measurements have been used to constrain the amount of mix at shock time, to demonstrate that some of the fuel mixes with the CD layer, and that capsules with a higher initial fill density or thicker shell are less susceptible to the effects of mix.
Magnetic-field evolution and instabilities were investigated in laser-produced plasmas. Monoenergetic proton radiography was used to make the first measurements of a laser–plasma-generated magnetic (B) field structure and evolution over a time interval that is longer compared to the laser pulse duration. While a circular, long-pulse (1-ns), low-intensity (~10¹⁴ W/cm²) laser beam illuminates a plastic foil, a hemispherical plasma bubble forms and grows linearly, surrounded by a symmetric B field. After the laser turns off, the bubble continues to expand, but field strengths decay and the field structure around the edge becomes asymmetric through the resistive-interchange instability.
The performance of the 1-MJ, wetted-foam target design for the National Ignition Facility was numerically studied. Wetted-foam designs take advantage of the increased laser absorption provided by the higher-atomic-number elements in a target ablator composed of plastic foam saturated with deuterium–tritium. A stability analysis of a 1-MJ design was performed using the two-dimensional hydrodynamic code DRACO. A nonuniformity-budget analysis has been constructed and suggests that two-dimensional smoothing by spectral dispersion (SSD) is needed to reduce single-beam nonuniformities to levels sufficient for ignition to proceed.
Petawatt laser-generated hot electrons in mass-limited, solid-foil–target interactions at '"relativistic' laser intensities were modeled using copper targets and parameters motivated by recent experiments at the Rutherford Appleton Laboratory (RAL) Petawatt and 100-TW facilities. Electron refluxing allows a unique determination of the laser–electron conversion efficiency and a test with simulations. Implications of the results for fast-ignition experiments on OMEGA EP are considered.
Three-dimensional characterization of spherical cryogenic targets using ray-trace analysis of multiple shadowgraph views was developed. A 3-D ray-tracing model into the backlit optical shadowgraph analysis, which is the primary diagnostic for hydrogenic ice-layer characterization in cryogenic targets at LLE, was incorporated. The result is an improved self-consistent determination of the hydrogen/vapor surface structure for cryogenic targets up to mode numbers around l_max = 16.
Filamentation analysis in large-mode-area fiber lasers was performed. Starting from the paraxial wave equation, an analytic expression for filament thresholds in fiber lasers is derived. The occurrence of filamentation is determined by the larger of two thresholds—one of perturbative gain and one of spatial confinement. The threshold value is around a few megawatts.
A technique for enhanced-dynamic-range, single-shot measurement of nanosecond optical pulses by averaging of replicated pulses was developed. A dynamic-range enhancement of three bits is experimentally demonstrated and compared with conventional multi-shot averaging. This technique can be extended to yield an increase of up to seven bits of additional dynamic range over nominal oscilloscope performance.

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This volume of the LLE Review, covering January–March 2007, features a progress report on laser-driven magnetic flux compression for magneto-inertial fusion. The authors report on an experiment to explore the magneto-inertial fusion (MIF) approach to inertial confinement fusion (ICF). In MIF, a magnetized target would be directly irradiated by a laser to compress a preseeded magnetic flux to levels sufficient to inhibit thermal transport out of the hot spot. This approach could eventually lead to ignition of massive shells, imploded at low velocity. Higher gain than that possible with conventional ICF could be reached. In this initial OMEGA experiment, a compact magnetic pulse system is used to generate a >0.15-MG seed magnetic field in a cylindrical target that is compressed by 40 OMEGA beams. A proton deflectrometry technique is being developed to probe the magnetic field using 14.7-MeV protons generated by an independently targeted (with the 20 remaining beams) capsule filled with D³He gas.

Additional highlights of research presented in this issue include the following:

Hydrodynamic simulations of high-gain, fast-ignition targets were conducted, including one-dimensional simulations of the implosion and two-dimensional simulations of ignition by a collimated electron beam and burn propagation. These simulations are used to generate gain curves for fast-ignition, direct-drive inertial confinement fusion. It is found that realistic fast-ignition targets can be ignited by monoenergetic collimated electron beams with a radius of 20 μm, duration of 10 ps, and energy of 15 kJ. Simulations using ponderomotive temperature scaling for fast electrons and Gaussian laser pulses predict a minimum laser energy for ignition of 235 kJ (106 kJ) for the energy conversion efficiency from the laser to fast electrons of 0.3 (0.5) and a wavelength of 1.054 μm.
The time-resolved generation and detection of coherent acoustic phonons in very high quality bulk GaN single crystals were studied experimentally and theoretically, using a femtosecond, two-color, all-optical pump/probe technique.
The use of spots taken with magnetorheological finishing for estimating subsurface damage depth from deterministic microgrinding was demonstrated for three hard ceramics: aluminum oxynitride (Al₂₃O₂₇N₅), polycrystalline alumina (Al₂O₃), and chemical vapor–deposited silicon carbide (Si₄C).
The instrument-limited suppression of out-of-band amplified spontaneous emission in a Nd:YLF diode-pumped regenerative amplifier (DPRA) was demonstrated using a volume Bragg grating (VBG) as a spectrally selective reflective element. A VBG with 99.4% diffraction efficiency and 230-pm-FWHM reflection bandwidth produced a 43-pm FWHM output spectral width in an unseeded DPRA compared to 150-pm FWHM in the same DPRA with no VBG.
Beam-quality factor measurements for an amplified emission source based on ytterbium-doped, large-mode-area, multimode fiber were discussed. The measurements indicate that the beam-quality factor decreases until the gain becomes saturated. A model using spatially resolved gain and transverse-mode decomposition of the optical field shows that transverse spatial-hole burning is responsible for the observed behavior.
The first time-dependent nuclear measurements of turbulent mix in inertial confinement fusion were presented. Implosions of spherical deuterated-plastic shells filled with pure He gas require atomic scale mixing of the shell and gas for the D-³He nuclear reaction to proceed. The time necessary for Rayleigh– Taylor (RT) growth to induce mix delays the peak nuclear production time, compared to equivalent capsules filled with a D-³He mixture, by 75±30 ps, equal to half the nuclear burn duration. These observations indicate the likelihood of atomic mix at the tips of core-penetrating RT spikes.

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This volume of the LLE Review, covering July–September 2007, features the analysis and the experimental results of a study of pump-induced, temporal-contrast degradation in optical parametric chirped-pulse amplification (OPCPA). In OPCPA systems, the temporal fluctuations of the pump pulse are coupled to the spectrum of the chirped signal by the instantaneous parametric gain and lead to a reduction in the temporal contrast of the recompressed amplified signal. The authors derive equations describing the contrast degradation in an OPCPA system due to the pump amplified spontaneous emission. They also quantify the reduction of the contrast in the amplified pulse both analytically and via simulations for an OPCPA system. The placement of a Bragg grating in the regenerative amplifier produces a simple and an efficient pump-intensity reduction, demonstrating contrast improvements up to 30 dB.

Additional highlights of research presented in this issue include the following:

Observations of the collapse of strong convergent shocks at the center of spherical capsules filled with D₂ and ³He gas, which induces both D-D and D-³He nuclear production. Temporal and spectral measurements of products from both reactions verify data reliability and allow efficient and insightful alterations in ICF simulations.
Work on equation-of-state measurements in Ta₂O₅ aerogel. Highly porous samples of tantalum pentoxide (Ta₂O₅) aerogel were compressed from initial densities of 0.1, 0.15, and 0.25 g/cm³ by shock waves with strengths between 0.3 and 3 Mbar. A comparison of the compression measurements and an available high-energy-density equation-of-state (HED-EOS) model found that the model underestimates the level of compression achieved by shock loading below a Mbar. The thermal measurements also indicate less significant heating than models predict.
Discussion of EXAFS (extended x-ray absorption fine structure) measurements used to determine the temperature and compression in a vanadium sample quasi-isentropically compressed to pressures of up to ~0.75 Mbar. VISAR (velocity interferometer system for any reflector) measurements, with aluminum substituting for the vanadium, are used to calibrate the drive pressure. The experimental results obtained by EXAFS and by VISAR agree with each other and with the simulations of a hydrodynamic code. The role of a shield to protect the sample from impact heating and the role of radiation heating from the imploding target and the laser-absorption region are also studied.
Report on the effect of resonance absorption in OMEGA direct-drive designs and experiments. Simulations demonstrate an important contribution of the resonance absorption during both the short laser picket (~100 ps) and the first 200 to 300 ps in the long laser pulse. Planar reflection light experiments on OMEGA were conducted to validate the theoretical results.
Presention of the diagnosing of direct-drive, shock-heated, and compressed plastic planar foils with noncollective spectrally resolved x-ray scattering. Plastic (CH) and Br-doped CH foils were driven with six beams, having an overlapped intensity of ~1 × 10¹⁴ W/cm² and generating ~15-Mbar pressure in the foil. The uniformly compressed portion of the target was probed with 9.0-keV x rays from a Zn He_α backlighter created with 18 additional tightly focused beams.
An examination of the scattered x-ray spectra reveals that an upper limit of Z ~ 2 and T_e = 20 eV can be inferred, since low average ionizations (i.e., Z < 2) cannot be accurately diagnosed in this experiment.

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This volume of the LLE Review, covering October–December 2007, features the development of aperture tolerances for neutron-imaging systems in inertial confinement fusion. Neutron-imaging systems are being considered as an ignition diagnostic, which is vital to the inertial confinement fusion effort. Given the importance of these systems, a neutron-imaging design tool is being used to quantify the effects of aperture fabrication and alignment tolerances on reconstructed neutron images for inertial confinement fusion. The simulations indicate that alignment tolerances of more than 1 mrad would introduce measurable features in a reconstructed image for both pinholes and penumbral aperture systems. Simulated fabrication errors suggest that penumbral apertures are several times less sensitive to these errors than pinhole apertures.

Additional highlights of research presented in this issue include the following:

Discussion of hohlraum energetics and implosion-symmetry experiments conducted using laser beams arranged in three cones and smoothed with elliptical phase plates. A shift in symmetry was observed between vacuum and gas-filled hohlraums having identical beam pointing. The ratio of x-ray drive at the poles of the capsule relative to the waist increased for the gas-filled hohlraum.
Presentation of an improved measurement of preheat in cryogenic targets. A reformulated and more consistent analysis of preheat measurements is performed, and the sensitivity of the results to the assumptions made in the analysis is discussed. The results are applied to both cryogenic as well as to CH targets.
Work on two-dimensional particle-in-cell simulations show that laser channeling in millimeter-scale underdense plasmas is a highly nonlinear and dynamic process. This process involves laser self-focusing and filamentation on the electron time scale, ponderomotive plasma blowout in the filaments, eventual whole beam blowout that transversely launches high-mach-number shocks, longitudinal plasma snowplowing, laser hosing, and channel bifurcation and self-correction.
The development and the optimization of the cleaning process that removes a wide variety of organic (photoresist) materials, metals, and metal oxides, which commonly remain on the surface of multilayer dielectric (MLD) diffraction gratings. The removal of such contaminants, a number of which have a significant optical absorbance and can lead to laser-induced damage, is critical to the performance of the OMEGA EP Laser System.
Research on the shock ignition of thermonuclear fuel with high areal densities. In direct-drive inertial confinement fusion (ICF), the "ignitor" shock can be launched by a power spike at the end of the laser pulse. For targets with the same adiabat and implosion velocities, the laser energy required for ignition is significantly lower for shock-ignition ICF than for standard ICF.
This volume concludes with a summary of LLE's Summer High School Research Program, the FY07 Laser Facility Report, and the National Laser Users' Facility and External Users' Programs.

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This volume of the LLE Review, covering October–December 2007, features a report on target experiments using the Multi-Terawatt (MTW) Laser Facility to study the isochoric (constant volume) heating of solid-density targets by electrons produced from intense, short-pulse laser irradiation. Electron refluxing occurs due to target-sheath field effects and contains most of the fast electrons within the target volume. This efficiently heats the solid-density plasma through collisions. X-ray spectroscopic measurements indicate that laser energy couples to fast electrons with a conversion efficiency of ~20%. Bulk electron temperatures of at least 200 eV are inferred for the smallest mass targets.

Additional highlights of research presented in this issue include the following:

The design of a high-resolution optical transition-radiation diagnostic for fast-electron-transport studies using the MTW facility.
The performance of direct-drive cryogenic target implosions is analyzed.
Results of initial implosion experiments to study shock-ignition inertial confinement fusion target concepts are discussed.
Time-resolved absorption measurements in cryogenic as well as room-temperature, direct-drive OMEGA implosions are presented.
A report is included on monoergetic proton radiography of electromagnetic field and particle density distributions in ICF target implosions.

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This volume of the LLE Review, covering January–March 2008, features a report on the status of layering cryogenic DT and D₂ targets at LLE for inertial confinement fusion (ICF) targets. This critical effort achieves the important milestone of routinely providing cryogenic DT targets that meet the 1.0-µm (rms) OMEGA ice-quality-surface specification. The best D₂-ice layers produced so far (rms roughness of 1.1 µm) are approaching the quality typically achieved in DT targets. Efforts to improve the consistency of this process are reported along with investigations supporting the National Ignition Campaign (NIC) studying issues relevant to indirect-drive and direct-drive cryogenic targets.

Additional highlights of research presented in this issue include the following:

Simulations that show an improved laser speckle smoothing scheme that augments the current National Ignition Facility (NIF) 1-D SSD system by using multiple-FM modulators is sufficient for direct-drive-ignition targets and pulse shapes analyzed thus far, and may even allow reducing the bandwidth enough to eliminate the need for dual-tripler frequency conversion on the NIF.
Results of time-gated, monoenergetic proton radiography that provides unique measurements of implosion dynamics of spherical targets in direct-drive ICF.
Performance of a single-shot cross-correlator based on a pulse replicator with a demonstrated dynamic range higher than 60 dB over a temporal range larger than
200 ps.
A novel focal-spot diagnostic developed for OMEGA EP that will be used to characterize on-shot focal spots to support high-quality laser–matter interaction experiments.
A systematic study to improve the laser-damage resistance of multilayer high-reflector coatings for use at 351 nm on the OMEGA EP Laser System that lead to exceptional improvement over previous damage thresholds measured at this wavelength.

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This volume of the LLE Review, covering April–June 2008, features a report on the development and implementation of large-aperture, tiled-grating compressors for the OMEGA EP Laser System. These units are critical to temporally compress the laser pulses on OMEGA EP from nanosecond to subpicosecond durations. There are two large (1.5-m) grating compressors on the OMEGA EP laser. Each of these consists of four sets of tiled-grating assemblies. The article provides details on the techniques used for tiling individual tiled-grating assemblies and for optimizing the overall performance of the compressor. Both assemblies achieved subpicosecond-pulse duration without tiling-induced temporal degradation.

Additional highlights of research presented in this issue include the following:

A comprehensive scientific program is being pursued at LLE to investigate the fast-ignitor concept for inertial confinement fusion. The OMEGA EP laser, completed in April 2008, is key to this program. Two of the OMEGA EP beams can operate in short-pulse mode with up to 2.6 kJ at a 10-ps duration. These beams can be routed into either the OMEGA EP chamber or combined collinearly into the existing OMEGA target chamber for integrated fast-ignition experiments. Fuel-assembly experiments have already started on OMEGA and have achieved an areal density of ~200 mg/cm²—sufficient to stop MeV electrons produced by the short-pulse laser.
A focal-spot diagnostic for high-energy petawatt-class power lasers. Accurate measurements at full energy are demonstrated using high-resolution wavefront sensing, in combination with techniques to calibrate on-shot measurements with low-energy sample beams. Results are shown for full-energy activation shots on OMEGA EP.
Suprathermal electrons generated by the two-plasmon-decay instability, gas-filled hohlraums. The OMEGA experiments focused on studies of this instability in the exploding laser entrance hole window in gas-filled hohlraums. This instability results in the generation of high-energy electrons. A threshold laser intensity of 5 × 10¹⁴ W/cm² was measured. As a result of this research, the initial overlapped intensity laser incident on the entrance hole window of the ignition target for the NIF has been set below the measured intensity threshold to retain ignition margin by staggering the turn-on time of the inner and outer cones of beams.
Effectiveness of silicon as a laser shinethrough barrier for 351-nm laser light. Silicon (Si) has been found to be a promising candidate for a direct-drive cryogenic target shinethrough-barrier material. Several cryogenic targets have been coated with Si, permeation filled with either deuterium or deuterium–tritium, and subsequently layered and optically characterized. Experiments have shown that a 200-µm-thick coating of Si is sufficient to mitigate shinethrough in cryogenic targets.
Mitigation of self-pulsing in watt-level, dual-clad, ytterbium-doped fiber lasers. High-power, high-beam quality, stable cw fiber lasers are desired in sensing, ranging, telecommunications, and spectroscopy. Although high-output powers have been achieved in many high-power fiber-laser systems, self-pulsing often occurs under specific conditions and cause catastrophic damage to the fiber laser. Self-pulsations are caused by the dynamic interaction between the photon population and population inversion. The addition of a long section of passive fiber in the laser cavity makes gain recovery faster than the self-pulsation dynamics, allowing only stable continuous-wave lasing.
Resolving dark pulses from photon pulses in NbN superconducting single-photon detectors (SPD's). Some applications of SPD's include quantum communications, quantum key distributions, and satellite communications. A desirable feature of an ideal SPD is its photon-number-resolution (PNR) capability. InGaAs avalanche photodiodes work at telecommunications wavelengths and are commercially available; however, they suffer severe after-pulsing and require time gating, limiting their maximum count rate. A scheme is presented using an NbN superconducting SPD (SSPD) that uses a low-noise cryogenic high-electron mobility transistor and a high-load resistor directly integrated with the detector to achieve amplitude resolution of dark and photon counts. This scheme makes it possible to study the physical origin of dark counts in SSPD's and may enable both photon-number resolving and energy-resolving capabilities of the standard, meander-type SSPD.

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This volume of the LLE Review, covering July–September 2008, features a report on expressions for the yield of electron–positron pairs, their energy spectra, and production rates have been obtained in the interaction of multi-kJ pulses of high-intensity laser light interacting with solid targets. The Bethe–Heitler conversion of hard x-ray bremsstrahlung is shown to dominate over direct production (trident process). The yields and production rates have been optimized as a function of incident laser intensity, by the choice of target material and dimensions, indicating that up to 5 × 10¹¹ pairs can be produced on the OMEGA EP Laser System. The corresponding production rates are sufficiently high that the possibility of pair-plasma creation is shown to exist.

Additional highlights of research presented in this issue include the following:

Simulations describing neutron yields of direct-drive, low-adiabat (α ≅ 2 to 3) cryogenic D₂ target implosions on OMEGA have been systematically investigated using the two-dimensional (2-D) radiation hydrodynamics code DRACO. Low-mode (l × 12) perturbations, including initial target offset, ice-layer roughness, and laser-beam imbalance, were found to be the primary source of yield reduction in implosions for thin-shell (5-µm), low-α, cryogenic targets. These 2-D numerical results provide a reasonably good guide to understanding the yield degradation in direct-drive, low-adiabat, cryogenic, thin-shell-target implosions and track experimental results. Simulations including the effect of laser-beam nonuniformities indicate that high-l-mode perturbations caused by laser imprinting play a role in further degrading the neutron yield of thick-shell implosions.
A time-resolved Al 1s–2p absorption spectroscopy is used to diagnose direct-drive, shock-wave heating and compression of planar targets having nearly Fermi-degenerate plasma conditions (Te ~ 10 to 40 eV, ~3 to 11 g/cm³) on the Omega Laser System. The laser-ablation processes launch 10- to 70-Mbar shock waves into the CH/Al/CH target. The Al 1s–2p absorption spectra were analyzed using the atomic physics code PrismSPECT to infer Te and in the Al layer, assuming uniform plasma conditions during shock-wave heating, to determine when the heat front penetrated the Al layer. The predictions of simulated shock-wave heating and the timing of heat-front penetration are compared with the observations. The experimental results for a wide variety of laser-drive conditions and buried depths have shown that the LILAC predictions using f = 0.06 and the nonlocal model accurately model the shock-wave heating and timing of the heat-front penetration while the shock is transiting the target.
The ignition condition (Lawson criterion) for inertial confinement fusion can be cast in a form dependent on the only two measurable parameters of the compressed fuel assembly: the hot-spot ion temperature and the total areal density (ρR_tot) that includes the cold shell contribution. A marginal ignition curve is derived in the ρR_tot, plane and current implosion experiments are compared with the ignition curve. Such a criterion can be used to determine how surrogate D₂ and sub-ignited DT target implosions perform with respect to the one-dimensional ignition threshold.
Ultrafast THz-pulse time-domain spectroscopy (TDS) and femtosecond optical-pump THz-probe (OPTP) studies of Hg-Ba-Ca-Cu-O (HBCCO) high-temperature, superconducting thin films. The time-resolved OPTP spectroscopy experiments showed that the quasiparticle relaxation process exhibited an intrinsic single-picosecond dynamics with no phonon bottleneck, which is a unique feature among superconductors and makes the HBCCO material very promising for ultrafast radiation detector applications.
This volume concludes with a summary of LLE's Summer High School Research Program, the FY08 Laser Facility Report, and the National Lasers Users' Facility and External Users' Programs.

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This volume of the LLE Review, covering October–December 2008, features "Demonstration of the Shock-Timing Technique for Ignition Targets at the National Ignition Facility." In this article the authors report on a technique to measure the velocity and timing of shock waves in a capsule contained within hohlraum targets. This technique is critical for optimizing the drive profiles for high-performance inertial confinement fusion capsules, which are compressed by multiple precisely timed shock waves. The shock-timing technique was demonstrated on OMEGA using surrogate hohlraum targets heated to 180 eV and fitted with a re-entrant cone and quartz window to facilitate velocity measurements using velocity interferometry. Cryogenic experiments using targets filled with liquid deuterium further demonstrated the entire timing technique in a hohlraum environment. Direct-drive cryogenic targets with multiple spherical shocks were also used to validate this technique, including convergence effects at relevant pressures (velocities) and sizes. These results provide confidence that shock velocity and timing can be measured in NIF ignition targets, thereby optimizing these critical parameters.

Additional highlights of research presented in this issue include the following:

A review of recent progress in direct-drive, cryogenic implosions at the Omega Laser Facility. Ignition-relevant areal densities of ~200 mg/cm² in cryogenic D₂ implosions with peak laser-drive intensities of ~5 x 1014 W/cm² were previously reported. The laser intensity is being increased to ~10¹⁵ W/cm² to demonstrate ignition-relevant implosion velocities of 3 to 4 x 10⁷ cm/s, providing an understanding of the relevant target physics. Planar-target acceleration experiments show the importance of the nonlocal electron- thermal-transport effects for modeling the laser drive. Nonlocal, hot-electron preheat is observed to stabilize the Rayleigh–Taylor growth at the peak drive intensity of ~10¹⁵ W/cm². The shell preheat caused by the hot electrons generated by two-plasmon-decay (TPD) instability was reduced by using Si-doped ablators. The measured compressibility of planar plastic targets driven with high-compression, shaped pulses agrees well with 1-D simulations at these intensities. Shock mistiming has contributed to compression degradation of recent cryogenic implosions driven with continuous pulses. Multiple-picket (shock-wave) target designs make it possible for a more robust tuning of the shock-wave arrival times. Cryogenic implosions driven with double-picket pulses demonstrate improved compression performance at a peak drive intensity of ~10¹⁵ W/cm².
A new method for analyzing the spectrum of knock-on deuterons (KOd's) elastically scattered by primary DT neutrons, from which a fuel ρR can be inferred for values up to ~200 mg/cm². This new analysis method, which used Monte Carlo modeling of a cryogenic DT implosion, significantly improves the previous analysis method in two fundamental ways: First, it is not affected by significant spatial-yield variations, which degrade the diagnosis of fuel ρR (spatial-yield variations of about ±20% are typically observed), and secondly, it does not break down when the fuel ρR exceeds ~70 mg/cm².
A method from which the cold fuel layer density is inferred from framed x-ray radiographs of a laser-driven spherical implosion. The density distribution is determined by using Abel inversion to compute the radial distribution of the opacity κ from the observed optical depth τ. With the additional assumption of the mass of the remaining cold fuel, the absolute density distribution can be determined. This is demonstrated at the Omega Laser Facility with two x-ray backlighters of different mean energies that lead to the same inferred density distribution independent of backlighter energy.
Integrated simulations of implosion, electron transport, and heating for direct-drive fast-ignition targets. A thorough understanding of future integrated fast-ignition experiments combining compression and heating for high-density thermonuclear fuel requires hybrid (fluid + particle) simulations of the implosion and ignition process. Different spatial and temporal scales need to be resolved to model the entire fast-ignition experiment. The two-dimensional (2-D) axisymmetric hydrocode DRACO and the 2-D/three-dimensional hybrid-PIC code LSP have been integrated to simulate the implosion and heating of direct-drive, fast-ignition targets. DRACO includes the physics required to simulate compression, ignition, and burn of fast-ignition targets. LSP simulates the transport of hot electrons from their generation site to the dense fuel core where their energy is absorbed. The results from integrated simulations of cone-in-shell CD targets designed for fast-ignition experiments on the OMEGA-OMEGA EP Laser System are presented. Target heating and neutron yields are computed. The results from LSP simulations of electron transport in solid-density plastic targets are also presented. They confirm an increase in the electron-divergence angle with the laser intensity in the current experiments. The self-generated resistive magnetic field is found to collimate the hot-electron beam and increase the coupling efficiency of hot electrons with the target. Resistive filamentation of the hot-electron beam is also observed.
In-situ, simultaneous measurements of both drag and normal forces in magnetorheological optical finishing (MRF) using a spot-taking machine (STM) as a test bed to take MRF spots on stationary optical parts. The force measurements are carried out over the entire removal area, produced by the projected area of the MRF removal function/spot on the part surface, using a dual-force sensor. This approach experimentally addresses the mechanisms governing material removal in MRF for optical glasses in terms of the hydrodynamic pressure and shear stress, applied by the hydrodynamic flow of magnetorheological (MR) fluid at the gap between the part surface and the STM wheel. This work demonstrates that the volumetric removal rate shows a positive linear dependence on shear stress. Shear stress exhibits a positive linear dependence on a material figure of merit that depends on Young's modulus, fracture toughness, and hardness. A modified Preston's equation is proposed that will better estimate MRF material removal rate for optical glasses by incorporating mechanical properties, shear stress, and velocity.
This volume concludes with an article that proposes and experimentally validates the concept of effective Verdet constant to describe the Faraday rotation characteristics of optical fiber. The effective Verdet constant of light propagation in fiber includes contributions from the materials in both the core and the cladding. This article presents a measured Verdet constant in 25-wt% terbium-doped-core phosphate fiber to be –6.2±0.4 rad/Tm at a wavelength of 1053 nm, which is 6x larger than in silica fiber. The result agrees well with the Faraday rotation theory for optical fiber.

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This volume of the LLE Review, covering January–March 2009, features "Applied Plasma Spectroscopy: Laser-Fusion Experiments." In this article the authors highlight how high-energy-density plasmas created in laser-fusion experiments are diagnosed with x-ray spectroscopy. Hans Griem, considered the father of plasma spectroscopy, provided an excellent foundation for this research. He studied the effect of plasma particles, in particular the fast-moving free electrons, on the Stark-broadening of spectral line shapes in plasmas. Over the last three decades, x-ray spectroscopy has been used to record the remarkable progress made in inertial confinement fusion research. Four areas of x-ray spectroscopy for laser-fusion experiments are highlighted in this article: K_α emission spectroscopy to diagnose target preheat by suprathermal electrons, Stark-broadened K-shell emissions of mid-Z elements to diagnose compressed densities and temperatures of implosion cores, K- and L-shell absorption spectroscopy to diagnose the relatively cold imploding shell (the "piston") that does not emit x rays, and multispectral monochromatic imaging of implosions to diagnose core temperature and density profiles. The seminal research leading to the original x-ray-spectroscopy experiments in these areas is discussed and compared to current state-of-the-art measurements.

Additional highlights of research presented in this issue include the following:

A discussion of high-resolution coherent transition radiation (CTR) imaging for diagnosing electrons accelerated in laserÐsolid interactions with intensities of ~10¹⁹ W/cm². The CTR images indicate electron-beam filamentation and annular propagation. The beam temperature and half-angle divergence are inferred to be ~1.4 MeV and ~16°, respectively. Three-dimensional hybrid-particle-in-cell code simulations reproduce the details of the CTR images assuming an initial half-angle divergence of 56°. Self-generated resistive magnetic fields are responsible for the difference between the initial and measured divergence.
Recent experiments using proton backlighting of laser–foil interactions to provide a unique opportunity for studying magnetized plasma instabilities in laser-produced high-energy-density plasmas. Time-gated proton radiograph images indicate that the outer structure of a magnetic field entrained in a hemispherical plasma bubble becomes distinctly asymmetric after the laser turns off. It is shown that this asymmetry is a consequence of pressure-driven, resistive magnetohydrodynamic (MHD) interchange instabilities. In contrast to the predictions made by ideal MHD theory, the increasing plasma resistivity after laser turn-off allows for greater low-mode destabilization (mode number m > 1) from reduced stabilization by field-line bending. For laser-generated plasmas presented herein, a mode-number cutoff for stabilization of perturbations is found in the linear growth regime. The growth is measured and is found to be in reasonable agreement with model predictions.
An extension of the theory governing motion of polymer cholesteric liquid crystal flakes in the presence of ac electric fields by introducing the effect of gravity acting on flakes, an important term when the flake density differs from the density of the suspending host fluid. Gravity becomes the driving force for flake relaxation when the electric field is removed, and it is now possible to predict relaxation times. Experimental results are compared with predictions from the extended theoretical model.
A modeling method for the effect of microencapsulation on the electro-optical behavior of polymer cholesteric liquid crystal (PCLC) flakes suspended in a host fluid. Several microencapsulation configurations in an applied ac electric field are investigated using Comsol Multiphysics software in combination with an analytical model. The field acting on the flakes is significantly altered as various encapsulant materials and boundary conditions are explored. The modeling predicts that a test cell with multiple materials in the electric-field path can have a wide range of electro-optic responses in ac electric fields. Both theoretical predictions and experimental evidence show that for PCLC flake reorientation to occur as a result of Maxwell–Wagner polarization, a reasonably strong electric field must be present along with at least moderately dissimilar PCLC flake and host fluid material dielectric constants and conductivities. For materials with low dielectric constants, electrophoretic behavior is observed under dc drive conditions at high field strengths for all evaluated microencapsulation configurations. The modeling method is shown to be a useful predictive tool for developing switchable particle devices that use microencapsulated dielectric particles in a host fluid medium.
A technique by which the ponderomotive force, exerted on all dielectric liquids by a nonuniform electric field, can be used for the remote, voltage-controlled manipulation of 10- to 100-µl volumes of cryogenic liquids. This liquid dielectrophoretic (DEP) effect, imposed by specially designed electrodes, combines with capillarity to influence the hydrostatic equilibria of liquid deuterium. A simple, 1-D model accurately predicts the measured meniscus rise of D₂ against gravity for sufficiently wide, parallel electrodes. For narrow electrodes, where the sidewalls influence the equilibrium, a finite-element solution using the Surface Evolver software correctly predicts the behavior. A bifurcation phenomenon previously observed for room-temperature dielectrics is also observed in liquid deuterium. This effect could possibly be used in the future to meter cryogenic deuterium when fueling targets for laser fusion.
This volume concludes with a description of the spectral and output-power stability of a 3-µm-wavelength GaSb-based diode laser operated at room temperature. More than 50 mW of output power has been achieved at 14°C with high spectral and output-power stability. This diode laser has a direct application for layering cryogenic targets for inertial confinement fusion implosions on the OMEGA laser.

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This volume of the LLE Review, covering April–June 2009, features shock-ignition experiments on OMEGA that have been performed in spherical geometry. Shock ignition is a two-step inertial confinement fusion (ICF) concept in which a strong shock wave is launched at the end of the laser drive pulse to ignite the compressed core of a low-velocity implosion. Fuel assembly and ignition are separated by the two steps, relaxing the driver requirements and promising high gains. In the experiments described in this article, room-temperature plastic shells were compressed on a low adiabat by 40 beams of the 60-beam OMEGA Laser System. The remaining 20 beams were delayed and tightly focused onto the target to drive a strong shock into the compressed core. Near the inner shell surface this ignitor shock collides with the return shock driven by the hot-spot pressure and propagating outward through the shell. After the ignitor and return-shock collision, a third shock wave, resulting from the collision, propagates inward, leading to further compression of the hot spot. Good coupling of the shock-beam energy was observed in these experiments, leading up to an ~20× increase in neutron yield. The authors observed significant stimulated Raman backscattering of laser energy and resultant fast electrons that are actually beneficial for shock ignition as they augment the strong hydrodynamic shock.

Additional highlights of research presented in this issue include the following:

Laser-driven magnetic-flux compression in high-energy-density plasma experiments are described that demonstrated, for the first time, magnetic-field compression to many tens of megagauss (MG) in cylindrical implosions of inertial confinement fusion targets. The very high magnetic-flux compression was achieved using the ablative pressure of the OMEGA laser to drive a cylindrical shell at high-implosion velocity, trapping and compressing an embedded external field to tens of MG, high enough to magnetize the hot-spot plasma. Line-averaged magnetic fields between 30 and 40 MG were observed.
It is shown that monoenergetic proton radiography combined with Lorentz mapping can be used to uniquely detect and discriminate magnetic and electric fields. This unique detection of electromagnetic fields and identification of field type and strength as a function of position was used to determine the nature of self-generated fields in a novel experiment with laser-generated plasma bubbles on two sides of a plastic foil. The results provided absolute identification and measurement of a toroidal magnetic field around each bubble and determined that any electric field component parallel to the foil was below measurement uncertainties.
A spatially resolved spectral interferometry technique, known as S² imaging, is used to measure higher-order mode content of a large-mode-area amplifier at full power for the first time. The technique was adapted for the short-fiber amplifier at full power and revealed a small amount of a co-polarized LP11 mode. This mode's power relative to the fundamental LP01 mode depends on the alignment of the input signal.
Optical differentiation in a regenerative amplifier with temperature-tuned volume Bragg grating (VBG) as an intracavity spectral filter is demonstrated for the first time. A simple, reliable laser system that produces multimillijoule ~150-ps pulses without mode-locking using an RA with VBG as an optical differentiator is described.
Crack growth in brittle glass plates is examined using finite element modeling. Fracture is analyzed in terms of strength, fracture toughness, or slow crack growth and this article outlines a procedure that estimates the deepest-allowable surface flaw.
Finite element analysis is applied to ultrafast photoconductive switches of the metal–semiconductor–metal (MSM) type to explain why MSM devices with alloyed electrodes show improved photoresponse efficiency compared to devices with surface contact electrodes. The model is also used to predict improved responsivity, based on electrode spacing and antireflective coating.

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