Hot electron preheat mitigation in ignition-scale direct-drive ICF implosions on the NIF

Hot electron preheat and its mitigation using mid-Z dopants has been quantified in ignition-scale polar direct-drive (PDD) implosion on the National Ignition Facility (NIF). Hard x-ray emission from buried Ge-doped layers was measured in NIF implosions of 2.3 mm CH shells at 730 and 850 kJ (intensities 1 to 1.25x10¹⁵ W/cm²), some of which contained Si dopant in the outer portion of the ablator. The Si-doped ablators were found to reduce stimulated Raman scattering (SRS) and consequently the hot electron energy deposition in the unablated shell by 30%, close to levels (~0.2% of laser energy) considered tolerable for direct drive. The extension of these experiments to ignition-relevant 3.0-mm, 1.3-MJ CH implosions and the extrapolation of these results to cryogenic DT designs will be discussed. These results provide a promising hot-electron preheat mitigation strategy that can expand the operable design space to higher intensities and allow for acceptable preheat levels in cryogenic DT implosions at intensities around 10¹⁵ W/cm². This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.