Hot-electron generation at direct-drive ignition-relevant plasma conditions at the National Ignition Facility
A. A. Solodov, M. J. Rosenberg, W. Seka, J. F. Myatt, M. Hohenberger, R. Epstein, C. Stöeckl, R. W. Short, S. P. Regan, P. Michel, T. Chapman, R. K. Follett, J. P. Palastro, D. H. Froula, P. B. Radha, J. D. Moody, V. N. Goncharov
Abstract
Laser–plasma interaction instabilities can be detrimental for direct-drive inertial confinement fusion by generating high-energy electrons that preheat the target. An experimental platform has been developed and fielded on the National Ignition Facility to investigate hot-electron production from laser–plasma instabilities at direct-drive ignition-relevant conditions. The radiation-hydrodynamic code DRACO has been used to design planar-target experiments that generate plasma and interaction conditions comparable to direct-drive ignition designs: IL ∼ 1015 W/cm2, Te > 3 keV, and density-gradient scale lengths of Ln ∼ 600 μm in the quarter-critical density region. The hot-electron properties were inferred by comparing the experimentally observed hard x-ray spectra to Monte Carlo simulations of hard x-ray emission from hot electrons depositing energy in the target. Hot-electron temperatures of ∼40 keV to 60 keV and the fraction of laser energy converted to hot electrons of ∼0.5% to 5% were inferred in plastic targets for laser intensities at the quarter-critical density surface of (∼4 to 14) × 1014 W/cm2. The use of silicon ablators was found to mitigate the hot-electron preheat by increasing the threshold laser intensity for hot-electron generation from ∼3.5 × 1014 W/cm2 in plastic to ∼6 × 1014 W/cm2 in silicon. The overall hot-electron production is also reduced in silicon ablators when the intensity threshold is exceeded.