Hot-electron generation at direct-drive ignition-relevant plasma conditions at the National Ignition Facility

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: I-L similar to 10(15) W/cm(2), T-e > 3 keV, and density-gradient scale lengths of L-n similar to 600 mu m 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 similar to 40 keV to 60 keV and the fraction of laser energy converted to hot electrons of similar to 0.5% to 5% were inferred in plastic targets for laser intensities at the quarter-critical density surface of (similar to 4 to 14) x 10(14) W/cm(2). The use of silicon ablators was found to mitigate the hot-electron preheat by increasing the threshold laser intensity for hot-electron generation from similar to 3.5 x 10(14) W/cm(2) in plastic to similar to 6 x 10(14) W/cm(2) in silicon. The overall hot-electron production is also reduced in silicon ablators when the intensity threshold is exceeded.