2006 Year in ReviewJanuary, 2007
2006 was a year of significant accomplishments at the Laboratory for Laser Energetics. Here are some of our major achievements and milestones in the past year.
Ignition target designs rely on accurate control of both the main fuel entropy and the asymmetry growth due to the hydrodynamic instabilities during the implosion. The shock wave launched at the beginning of the foot pulse determines the shell entropy. In addition, the evolution of the shell asymmetry during the shock transit determines the initial conditions for the Rayleigh–Taylor (RT) instability.
A comparison of the experimental results and simulations of a shock-timing experiment is plotted as a function of time for the case of a 20-μm wavelength initial perturbation. The simulation with a thermal flux limiter of f = 0.1 is in good agreement with the experimental data. Such a large value of the flux limiter, however, is not consistent with the shock-timing measurements. To resolve this discrepancy, a new nonlocal transport model has been developed. When applied to the shock-breakout a simulation, the nonlocal model reproduces the experimental shock-velocity data very well [thin line in Fig. 1(a)].
The first direct-drive, ignition-scaled, cryogenic target containing tritium was imploded on the OMEGA laser in February 2006. The target contained 0.06% tritium by atom fraction. This implosion was the first in a series of planned experiments that led to LLE scientists imploding two fully β-layered DT capsules during the week of 27 March 2006. The tritium fraction in each capsule was 13.5%. Both capsules were layered without external IR radiation, confirming earlier estimates that a tritium fraction of ~10% would be sufficient for β-layering to occur. This is the first time that a β-layered DT target was used in a laser-driven implosion. Figure 2 shows a typical shadowgraph of one of the targets.
Low-adiabat, high-contrast pulse shapes are required for OMEGA ignition-scaled cryogenic target experiments. Such pulse shapes are typically characterized by a narrow picket pulse on top of a low-intensity foot pulse followed by a high-intensity drive pulse. The new front end on OMEGA—the integrated front-end source (IFES)—is a highly stable optical pulse-generation system based on the fiber amplification of an optical signal that is temporally carved from a continuous-wave fiber laser. The use of fiber-optic lasers and amplifiers and waveguide temporal modulators makes IFES ideally suited to producing reliable, stable pulse shapes. Recent experiments on OMEGA have required ~100:1 contrast-ratio pulse shapes.
A year-long project to upgrade the ASBO diagnostic was completed in April 2006. Initially installed in 2000, this diagnostic proved valuable for equation of state (EOS) and shock-timing experiments for many years. An upgrade was deemed necessary to enable high-precision measurements and ease of operation. Using the existing system as a baseline, a new optical layout was conceived that used two Rochester Optical Streak System (ROSS) streak cameras (Fig. 8) as detectors for the two VISAR channels. The result is an outstanding optical device that provides excellent optical performance and smooth operation using the accurately calibrated ROSS cameras.
Sixty-one students, all children of LLE employees and between 8 and 14 years of age, came to the Lab for the fifth annual Kids@Work Day. They participated in a structured morning of lab tours and hands-on, optics experiences.
Absorption in OMEGA implosion experiments is routinely measured using scattered-light calorimetry in the two full-aperture backscatter stations (FABS25 and 30) (Fig. 6). The measured and predicted absorption fractions of many different pulse shapes and targets with strongly varying absorption fractions are plotted in this figure. In several shots the LILAC simulations were carried out using various ways of modeling thermal electron transport, resulting in a spread of predicted absorption fractions as indicated in the solid black squares in Fig. 6.
For several years, the Commissariat à l’Énergie Atomique of France (CEA/DIF) has been developing HRXI, a high-resolution, time-resolved, x-ray imaging diagnostic. Recently, HRXI was implemented and tested for the first time on OMEGA. HRXI combines two state-of-the-art x-ray technologies: a high-resolution x-ray microscope and a high-speed x-ray streak camera. The resulting instrument achieves a spatial and temporal resolution of ~5 μm and ~30 ps, respectively.
The Rochester Optical Streak System (ROSS) was developed by LLE to meet the high-speed data collection needs of the Laboratory’s laser-fusion experiments program on the OMEGA facility. The ROSS camera can record transient events with a time resolution better than 5 picoseconds. The ROSS system employs a patented automatic self-calibration technique that achieves 1% measurement accuracy. Eight prototype ROSS systems have operated on OMEGA experiments for more than five years, accumulating more than a half million streak measurements with better than 99.9% reliability.
Cryogenic DT Target Milestone
LLE researchers have successfully imploded ignition-scaled targets containing frozen DT fusion fuel with a very high quality inner ice surface. The quality of the inner ice surface of these implosions met the requirements for direct-drive ignition target designs that will eventually be used on the National Ignition Facility. The 3-D ice characterization, based on multiple shadowgraphs, indicated an inner-ice surface roughness as low as 0.72-μm rms at DT’s triple point, i.e., the temperature and pressure at which gas, liquid, and solid coexist in thermodynamic equilibrium. This is well within the smoothness required to complete the National Ignition Campaign (NIC) level 2 milestone for an inner ice roughness of 1-μm rms.
National Ignition Campaign Hohlraum Energetics Milestone Achieved on OMEGA
Scientists at LLE and Lawrence Livermore National Laboratory completed a National Ignition Campaign milestone on 17 August dealing with hohlraum energetics experiments with elliptical phase plates. A set of 43 elliptical phase plates (E-IDI300) were designed and manufactured for these experiments. Seven scale-1, thin-walled, Au hohlraums were irradiated with 40 beams smoothed with E-IDI-300 phase plates. Several gas fills were investigated. High-Z dopants were introduced into the gas fills to reduce hard x-ray production and laser scattering levels. The hohlraum energetics were measured for a 13.5-kJ-shaped laser pulse (PS26) with the following diagnostics: DANTE, full aperture backscatter, near backscatter imaging, gated hard x-ray imaging, gated soft x-ray imaging, and the hard-x-ray detector. As shown in Fig. 10, the peak radiation temperature Tr inferred from the measured levels of the x-ray flux increased by 17 eV when the laser beams were smoothed with phase plates. The improved coupling is a consequence of reduced laser scattering losses.
X-Ray Absorption Measurements of Areal Density in Cryogenic DT Implosions
The achievement of high fuel areal density is required for ignition target designs as this enables the fuel to capture charged fusion products, boosting the temperature and leading to ignition. Methods for measuring areal density are usually based on nuclear particles. However, the areal density can also be determined from the absorption of continuum x-rays emitted by the central hot spot. The resulting spectrum is measured by a pinhole-array spectrometer that records several hundred images, each at a slightly different photon energy. Figure 11 shows an example of the emitted spectrum from an OMEGA implosion of a cryogenic–deuterium target. The spectrum calculated by the 1-D code LILAC is also shown, as well as the calculated spectrum that would have been emitted in the absence of absorption (“zero” opacity). The difference between the two curves is due to absorption by the cold shell and can be used to infer its areal density.
K-Shell Spectroscopy of High-Intensity Laser-Solid Interactions
A detailed K-shell spectroscopy study of ultrahigh intensity laser–solid interactions is under way at the LLE multitertawatt (MTW) high-intensity-laser facility. This study will determine the conversion efficiency of laser energy into suprathermal electrons and the subsequent line emission characteristics over a wide range of laser pulse durations, focused laser intensities, and target compositions. Figure 12 shows the ratio of the total energy in Kα photons to the laser energy plotted as a function of laser intensity. The solid and dashed curves correspond to theoretical suprathermal electron refluxing and nonrefluxing models, respectively, for conversion efficiencies in the 1% to 40% range. Experimental data from MTW and PW are in good agreement with the refluxing model.
To meet the demands of its OMEGA EP laser, LLE added four solid-state, high voltage, large-aperture Pockels-cell drivers in 2006. Fabrication and assembly of the drivers took 10 months. Diligent checking and testing of all components resulted in the finished driver assemblies working the first time power was applied. Activation of these drivers on OMEGA/EP laser beam lines is scheduled for January of 2007. Building these drivers in-house provided LLE engineers with important technological skills and knowledge that can be applied to future high voltage, high current pulse applications utilizing solid-state switching technology.