2010 Annual Report

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The fiscal year ending September 2010 (FY10) concluded the third year of the third five-year renewal of Cooperative Agreement DE-FC52-08NA28302 with the U.S. Department of Energy (DOE). This annual report summarizes progress in inertial fusion research at the Laboratory for Laser Energetics (LLE) during the past fiscal year including work on the National Ignition Campaign (NIC). It also reports on LLE’s progress on laboratory basic science research; laser, optical materials, and advanced technology development; operation of OMEGA and OMEGA EP for the NIC and high-energy-density (HED) campaigns, the National Laser Users’ Facility (NLUF), and for other external users; and programs focusing on the education of high school, undergraduate, and graduate students during the year.

One of the principal missions of the University of Rochester’s Laboratory for Laser Energetics (LLE) is to conduct research in inertial confinement fusion (ICF) with particular emphasis on supporting the goal of achieving ignition on the National Ignition Facility (NIF). This program relies on the full use of the OMEGA 60-beam UV laser as well as the OMEGA EP high-energy, short-pulse laser system. During FY10, a record total of 1823 target shots were taken on the Omega Laser Facility. Within the NIC, LLE plays a lead role in the validation of the performance of cryogenic target implosions, essential to all forms of ICF ignition. LLE is responsible for a number of critical elements within the Integrated Experimental Teams (IET’s) supporting the demonstration of indirect-drive ignition on the NIF and is the lead laboratory for the validation of the polar-drive approach to ignition on the NIF. LLE has also developed, tested, and constructed a large number of diagnostics that are being used on the NIF for the NIC. During this past year, progress in the inertial fusion research program continued to be made in three principal areas: NIC experiments; development of diagnostics for experiments on OMEGA, OMEGA EP, and the NIF; and theoretical analysis and design efforts aimed at improving direct-drive–ignition capsule designs and advanced ignition concepts such as shock ignition.

LLE achieved a fuel areal density of 0.3 g/cm² on an OMEGA cryogenic-DT direct-drive capsule driven with a picket pulse at a shell adiabat α ~ 2. This represents the highest DT-fuel areal density achieved to date in direct-drive implosions, and the results are a crucial step in validating predictive capabilities of hydrodynamic codes used to design ignition capsules to be used on the NIF. A critical challenge of hot-spot–implosion design is controlling the generation of strong shocks while simultaneously accelerating the fuel shell to a high velocity (~3 × 10⁷ cm/s). To avoid preheating the fuel, only shocks with shock strength less than a few million atmospheres (Mbar) can be launched into the cryogenic fuel at the start of an implosion. To avoid, however, break up of the shell caused by Rayleigh–Taylor instabilities, the shell must be driven at pressures exceeding 100 Mbar. The targets that successfully demonstrated high fuel areal density on OMEGA were driven with multiple picket pulses that launched a sequence of shocks of increasing strength in the fuel [multiple-shock (MS) designs].

We report on the development of a microfluidics-based, on-chip, electric-field–actuated technique to fabricate cryogenic-foam ICF targets. The electrowetting-on-dielectric and dielectrophoresis effects make it possible to manipulate both conductive and dielectric droplets simultaneously on a substrate. Aqueous and non-aqueous liquid droplets precisely dispensed from two reservoirs on a microfluidic chip are transported and combined to form oil-in-water-in-air or water-in oil-in-air double-emulsion droplets. The dispensing reproducibility is studied as a function of a set of operation parameters. Conditions for spontaneous emulsification for double-emulsion formation are developed in terms of droplet surface energies. This technique has the potential of meeting the high-precision, high-rate, low-cost, compact, low-tritium inventory requirements of future inertial fusion energy reactor systems.

A critical instrument used to measure compressed fuel areal density (ρR) in highly compressed DT targets is the magnetic recoil spectrometer (MRS). The MRS was developed and implemented on OMEGA as well on NIF by a collaborative team comprised of MIT, LLE, and LLNL scientists and engineers. The MRS measures the absolute neutron spectrum in the range of 5 to 30 MeV. In high-density target implosions, the fusion neutrons scatter on the deuteron and triton ions of the compressed core and the resulting downshift in the neutron energy can be used to directly infer the core areal density. The OMEGA MRS was used to measure the fuel ρR of cryogenic-DT implosions demonstrating ρR ~ 0.3 g/cm². A version of the OMEGA MRS was installed on the NIF in FY10.

Another important diagnostic to characterize the density of highly compressed targets is x-ray radiography. The first radiographs of cryogenic implosions on OMEGA were obtained using short-pulse, K-shell emission-line backlighters driven by the OMEGA EP laser. Simulations show that radiography near peak compression is feasible. The backlighter composition in this set of experiments was chosen so that the emission lines occurred at energies where the opacity profiles of the imploded cores provide a measurable range of optical depth and the specific intensity of the backlighter is capable of overcoming the core self-emission. Simulations of the first measured implosion radiographs were used to assess the implosion performance at times in advance of peak compression. Radial mass distributions were obtained from the radiographs using Abel inversion and the known temperature and density dependence of the free–free opacity of the hydrogen shell. Radiography based on Compton scattering of hard backlight x rays is also being investigated on the Omega Facility as an alternative approach.

An LLE team partnered with Physikalisch Technische Bundesanstalt, Braunscheig, Germany, to develop a gated liquid-scintillator–based neutron detector to be used for fast-ignitor experiments and down-scattered neutron measurements. The detection of neutrons in such experiments is very challenging since it requires the neutron-detection system to recover within 50 to 500 ns from a high background signal many orders of magnitude stronger than the signal of interest. The liquid-scintillator–based detector uses a gated microchannel photomultiplier that suppresses the high background signal and an oxygen-enriched liquid scintillation material that eliminates the afterglow present in conventional plastic scintillators.

The performance of a charge-injection device in the high-energy–neutron environment of laser-fusion experiments has been studied. Charge-injection devices (CID’s) are being used to image x rays in laser-fusion experiments on the OMEGA Laser System, up to the maximum neutron yields generated (~10¹⁴ DT). The detectors are deployed in x-ray pinhole cameras and Kirkpatrick–Baez microscopes. The neutron fluences ranged from ~10⁷ to ~10⁹ neutrons/cm², and useful x-ray images were obtained even at the highest fluences. At the NIF, CID cameras are intended for use as a supporting means of recording x-ray images. The results of this work predict that x-ray images should be obtainable on the NIF at yields up to ~10¹⁵, depending on distance and shielding.

In ICF, a shell of cryogenic DT is accelerated inward by either direct laser irradiation or by x rays produced by heating a high-Z enclosure (a hohlraum). At the stagnation of this implosion, the compressed fuel is ignited by a central hot spot surrounded by a cold, dense shell. Ignition occurs when the alpha-particle heating of the hot spot exceeds all the energy losses. We developed a metric to measure progress toward ignition. This ignition condition is derived in terms of measurable parameters: the areal density, the ion temperature, and the ratio of measured neutron yield to the calculated clean, perfect implosion yield (YOC).

Radiative–hydrodynamic simulations of implosion experiments on the OMEGA Laser System show that energy transfer between crossing laser beams can significantly reduce laser absorption. A new quantitative model for crossed-beam energy transfer has been developed, allowing one to simulate the coupling of multiple beams in the expanding corona of implosion targets. Scattered-light and bang-time measurements show good agreement with predictions of this model when nonlocal thermal transport is used. The laser absorption can be increased by employing two-color light, which reduces the crossed-beam energy transfer.

A collaboration between scientists from LLE and the University of California, Berkeley, led to the development of a first-principles equation-of-state (FPEOS) table for deuterium using the path-integral Monte Carlo method. Accurate knowledge about the equation of state (EOS) of deuterium is critical to ICF. Low-adiabat ICF implosions routinely access strongly coupled and degenerate plasma conditions. The FPEOS table covers typical ICF fuel conditions at densities ranging from 0.002 g/cm³ to +1600 g/cm³ and temperatures of 1.35 eV to 5.5 keV. Discrepancies in internal energy and pressure have been found in strongly coupled and degenerate regimes with respect to SESAME EOS. Hydrodynamics simulations of cryogenic ICF implosions using the FPEOS table have indicated significant differences in peak density, areal density (ρR), and neutron yield relative to SESAME simulations. The FPEOS simulations result in better agreement of compression ρR with experiments.

The annual report includes articles on advanced technology development at LLE. One such report discusses the development and implementation of a grating inspection system for large-scale (1.4-m aperture) multilayer-dielectric gratings on the OMEGA EP short-pulse laser system. The grating inspection system (GIS) is fully integrated within the vacuum grating compressor and enables one to carry out inspections while the compressor chamber is under vacuum. Damage is detected by imaging scattered light from damage sites on the grating surface. Features as small as 250 µm can be identified with this system.

We also report on work on large-aperture plasma-assisted deposition of ICF laser coatings. Plasma-assisted electron-beam evaporation leads to changes in the crystallinity, density, and stresses of thin films. A dual-source plasma system was developed that provides stress control of large-aperture, high-fluence coatings used in vacuum for substrates 1 m in aperture.

Under the facility governance plan that was implemented in FY08 to formalize the scheduling of the Omega Facility as a National Nuclear Security Agency (NNSA) facility, Omega Facility shots are allocated by campaign. The majority (~65%) of the FY10 target shots were allocated to the NIC conducted by integrated teams from the national laboratories and LLE and to the high-energy-density campaigns conducted by teams led by scientists from the national laboratories.

During FY10 30% of the facility shots in were allocated to basic science experiments. Half of these shots were conducted for university basic science under the National Laser Users’ Facility (NLUF) Program, and the remaining shots were allotted to the Laboratory Basic Science (LBS) Program comprising peer-reviewed basic science experiments conducted by the national laboratories and LLE/FSC.

The Omega Facility is also being used for several campaigns by teams from the Commissariat à l’énergie atomique (CEA) of France, and the Atomics Weapons Establishment (AWE) of the United Kingdom. These programs are conducted on the facility on the basis of special agreements put in place by the DOE/NNSA and the participating institutions.

During FY10 facility users included 11 collaborative teams participating in the NLUF Program; 12 teams led by LLNL and LLE scientists participating in the LBS program; many collaborative teams from the national laboratories conducting experiments for the NIC; investigators from LLNL and LANL conducting experiments for high-energy-density–physics programs; and scientists and engineers from CEA and AWE.

In FY10, DOE issued a solicitation for NLUF grants for the period of FY11–FY12. A total of 15 proposals were submitted to DOE for the NLUF FY11/12 program. An independent DOE Technical Evaluation Panel reviewed the proposals and recommended that 11 proposals receive DOE funding and 31 days of shot time be allocated on OMEGA in each of FY11 and FY12.

FY10 was the second of a two-year period of performance for the NLUF projects approved for the FY09–FY10 funding and OMEGA shots. Eleven NLUF projects were allotted Omega Facility shot time and conducted a total of 197 target shots on the facility.

In FY10, LLE issued a solicitation for LBS proposals to be conducted in FY11. A total of 23 proposals were submitted. An independent review committee reviewed the proposals and recommended that 16 proposals receive 29 shot days on the Omega Laser Facility in FY11.

Eleven LBS projects were allotted Omega Facility shot time and conducted a total of 303 target shots on the facility in FY10.

During FY10, the Omega Laser Facility conducted a record total of 1823 target shots on the OMEGA (1343 target shots) and OMEGA EP (480 target shots) lasers. Nearly 46% of the total shots were taken for or in support of NIC. External users accounted for ~63% of the total Omega Facility target shots in FY10 (64.4% of OMEGA and 60.4% of OMEGA EP target shots).

Many modifications were made to the OMEGA laser to improve low-adiabat direct-drive cryogenic implosion performance. OMEGA conducted 38 DT spherical cryogenic target shots and 40 planar cryogenic target shots in support of shock-timing experiments. The OMEGA Availability and Experimental Effectiveness averages for FY10 were 93% and 94%, respectively.

OMEGA EP was operated extensively in FY10 for a variety of users. A total of 308 short-pulse IR shots were conducted. Of these, 232 target shots were taken into the OMEGA EP target chamber and 76 joint shots were taken into the OMEGA target chamber. The Availability and Experimental Effectiveness for OMEGA EP averaged 86% and 94%, respectively.

As the only major university participant in the National ICF Program, education continues to be an important mission for the Laboratory. Laboratory education programs span the range of high school to graduate education.

During the summer of 2010, 16 students from Rochester-area high schools participated in the Laboratory for Laser Energetics’ Summer High School Research Program. The goal of this program is to excite a group of high school students about careers in the areas of science and technology by exposing them to research in a state-of-the-art environment.

Two hundred and sixty-five high school students have now participated in the program since it began in 1989. This year’s students were selected from a record 80 applicants.

Approximately 31 undergraduate students participated in work or research projects at LLE this past year. Student projects include operational maintenance of the OMEGA Laser Facility; work in laser development, materials, and optical-thin-film–coating laboratories; computer programming; image processing; and diagnostics development. This is a unique opportunity for students, many of whom will go on to pursue a higher degree in the area in which they gained experience at the Laboratory.

Graduate students are using the OMEGA Facility as well as other LLE facilities for fusion and high-energy-density physics research and technology development activities. These students are making significant contributions to LLE’s research program. Twenty-five faculty from the five University academic departments collaborate with LLE scientists and engineers. Presently, 82 graduate students are involved in research projects at LLE, and LLE directly sponsors 38 students pursuing Ph.D. degrees via the NNSA-supported Frank Horton Fellowship Program in Laser Energetics. Their research includes theoretical and experimental plasma physics, high-energy-density physics, x-ray and atomic physics, nuclear fusion, ultrafast optoelectronics, high-power-laser development and applications, nonlinear optics, optical materials and optical fabrication technology, and target fabrication.

In addition, LLE directly funds research programs within the MIT Plasma Science and Fusion Center, the State University of New York (SUNY) at Geneseo, and the University of Wisconsin. These programs involve a total of approximately 6 graduate students, 25 to 30 undergraduate students, and 10 faculty members.