LLE Review 140

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Highlights

This volume of the LLE Review, covering July–September 2014, features “Gigabar Spherical Shock Generation on the OMEGA Laser.” This article reports on the first experimental demonstration of the capability to launch shocks of several-hundred Mbars in spherical targets—a milestone for shock ignition. Using the temporal delay between the launching of the strong shock at the outer surface of the spherical target and the time when the shock converges at the center, the shock-launching pressure can be inferred using radiation–hydrodynamic simulations. Peak ablation pressures exceeding 300 Mbar are inferred at absorbed laser intensities of ~3 × 10¹⁵ W/cm². The shock strength is shown to be significantly enhanced by the coupling of suprathermal electrons with a total converted energy of up to 8% of the incident laser energy. At the end of the laser pulse, the shock pressure is estimated to exceed ~1 Gbar because of convergence effects.

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

• Direct-drive–ignition designs with mid-Z ablators are presented. Ablator materials of moderate atomic number Z reduce the detrimental effects of laser–plasma instabilities in direct-drive implosions. To validate the physics of moderate-Z ablator materials for ignition target designs on the National Ignition Facility (NIF), hydro-equivalent targets are designed using pure plastic, high-density carbon, and glass ablators. The hydrodynamic stability of these targets is investigated through 2-D single-mode and multimode simulations. The overall stability of these targets to laser imprint perturbations and low-mode asymmetries allows for the development of high-gain target designs using uniform illumination. Designs using polar-drive illumination are developed within the NIF Laser System specifications.
• Channeling of multikilojoule high-intensity laser beams in an inhomogeneous plasma is demonstrated. Experiments have been performed that investigate the transport of high-intensity (>10¹⁸ W/cm²) laser light through a millimeter-sized, inhomogenous, kilojoule, laser-produced plasma up to overcritical density. The experiments showed that 100-ps infrared pulses with a peak intensity of ~1 × 10¹⁹ W/cm² produced a channel to plasma densities beyond critical, while 10-ps pulses with the same energy but higher intensity did not propagate as far. The plasma cavity forms in less than 100 ps, using a 20-TW laser pulse, and advances at a velocity of ~3 µm/ps, consistent with a ponderomotive hole-boring model.
• Dependence of tritium release on temperature and water vapor from stainless steel is studied. In general, increasing either the sample temperature or the relative humidity causes an increased quantity of tritium to be removed. Increasing the temperature to 300°C in a dry gas stream results in a significant release of tritium and is therefore an effective means for reducing the tritium inventory in steel. For humid purges at 30°C, a sixfold increase in humidity results in a tenfold increase in the peak outgassing rate. Increasing the relative humidity from 0% to 20% when the sample temperature is 100°C causes a significant increase in the tritium outgassing rate.
• R. D. Petrasso’s (Plasma Science and Fusion Center, MIT) report on the Sixth Omega Laser Users Group Workshop is presented.
• This volume concludes with a summary of LLE’s Summer High School Research Program, the FY14 Laser Facility Report, and the National Laser Users’ Facility and External Users’ Programs.