LLE Review 126

Review 126


This volume of the LLE Review, covering January–March 2011, features "Precision Equation-of-State Measurements on NIF Ablator Materials from 1 to 12 Mbar Using Laser-Driven Shock Waves." The article reports on results to characterize the high-pressure behavior of germanium-doped glow-discharge polymer (Ge-GDP) ablators used for NIF ignition targets. Robust ignition simulations require knowledge of the ablator equation-of-state (EOS) and this study details the first EOS measurements on GDP and Ge-GDP films. The experiments used laser-driven shock waves on OMEGA to provide impedance-matching (IM) conditions with α-quartz as the standard material. The use of quartz as the IM standard along with a time-resolved VISAR diagnostic facilitated measurement of shock velocities to ~1% precision in transparent materials, which, in turn minimized measurement errors in GDP and Ge-GDP EOS results.

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

  • The refractive index of LiF was measured on OMEGA using ramp compression to 800 GPa. LiF was observed to remain transparent over a 30- to 800-GPa pressure range—the highest pressure under which a transparent insulator has ever been observed. Extrapolation of these results indicates that ramp-compressed LiF may remain transparent to >4000 GPa, making LiF a valuable window for extremely high pressure ramp-compression experiments.
  • Two-plasmon-decay instability is identified as a potential source of target preheat in OMEGA direct-drive experiments, and a physical model of electron heating was developed that relies on extended Zakharov simulations to predict the nonlinearly saturated Langmuir wave spectrum. Because of the relatively low areal density of the targets during the time of TPD instability, hot-electron recirculation and reheating are potentially important effects. These effects were modeled by using a particular form of boundary conditions on the test-particle trajectories. Adoption of these boundary conditions was shown to lead to increase in the computed hot-electron temperature by a factor of 3x.
  • Enhancement of the ion temperature and fusion yield has been observed in laser-driven magnetized inertial confinement fusion implosions at the Omega Laser Facility. A seed magnetic field of ~80-kG strength was embedded into spherical ICF targets imploded by the OMEGA laser in a polar-drive beam-pointing geometry. As a result of the target's hot-spot magnetization, the electron radial heat losses were suppressed and the observed ion temperature and neutron yield were enhanced by 15% and 30%, respectively. This data represents the first experimental verification of ICF target performance being enhanced by magnetizing the hot spot.
  • Nanosecond, 1053-nm optical pulses are amplified from 15 pJ to 240 nJ by a Yb-doped all-fiber regenerative amplifier (AFRA), achieving an overall gain of 42 dB. This is believed to be the highest AFRA output-pulse energy ever reported. The AFRA is an attractive candidate as a CPA seed source because of its high output-pulse energy in comparison to seed pulses commonly used in existing CPA systems.
  • The suppression of parasitic processes in noncollinear optical parametric amplifiers (NOPA) is investigated for walk-off and non-walk-off compensating configurations. Modeling shows the second-harmonic generation of the signal can reduce the NOPA output energy by 10%. Quantitative measurements on an ultra-broadband, few-cycle NOPA support these findings in the walk-off compensating case and the effect is reduced by an order of magnitude in the non-walk-off compensating case. A detailed phase-matching analysis for the most common nonlinear crystals is presented as a guide for designing NOPA systems.
  • The inclusion of alumina layers within the design of hafnia/silica optical coatings offers a film-stress compensation that prevents tensile film failure in dry-use environments. Hafnia/silica films deposited using electron-beam evaporation tend to exhibit high tensile stresses when used in vacuum or low-relative humidity environments resulting in film cracking or crazing. The inclusion of alumina layers within the film stack leads to a compressive overall film stress negating this failure mode. A film-stress model incorporating the stress of the individual materials and material thicknesses along with the interfacial film effects was developed to calculate the overall film stress when designing multilayer coatings using alumina, since this film stress (compressive) was measured to be very different than the alumina monolayer film stress (tensile).

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