Measurement of hot electron preheat during capsule implosions on the NIF with hard x-ray imaging

Hot electrons of energies between 170 and 250 keV can penetrate the capsule ablator and preheat the DT fuel in indirect-drive ICF implosions, reducing the final compressed fuel area density and ignition margin. We have fielded a high aspect ratio pinhole imager with 400 $\mu$m resolution, 0.9x magnification viewing through a Laser Entrance Hole to measure the $50 - 125$ keV hard x-ray Bremsstrahlung emission from hot electrons slowing in the capsule. The absolutely calibrated, time-integrating image plate detector allows inferring an upper limit of 150 J in hot electrons with E $>$ 170 keV impinging on the fusion capsule in a 1.3 MJ experiment with a 20 ns laser drive. Time-resolved, spatially integrated hard x-ray measurements confirm that these hot electrons are generated close to the end of the laser pulse. Based on measured hot-electron energy and time history, simulations predict a degradation of implosion performance by $<$ 10\% due to hot electron preheat.