Mitigation and scaling of hot electron preheat in direct-drive ICF implosions on the NIF and OMEGA*

Hot electron preheat and the resulting degradation of compression is a potential concern for direct-drive ICF ignition designs and must be studied at the appropriate scale and plasma conditions. Polar-direct-drive implosions at the National Ignition Facility (NIF) and OMEGA studied the scaling of preheat with capsule size and laser intensity and demonstrated partial mitigation of hot electron preheat. Hard x-ray emission from buried Ge-doped layers was measured on NIF to infer ~0.2% of laser energy deposited as hot-electron preheat in the inner ~80% of the unablated shell at an intensity of 10¹⁵ W/cm², varying by +50% for +25% excursions in intensity. This is close to the tolerable level of preheat in direct-drive ignition designs. A thin buried Si layer reduces preheat by a factor of ~2 and effectively suppresses preheat at intensities of 8x10¹⁴ W/cm². Hydrodynamically equivalent implosions on OMEGA at 3.4 times smaller scale and 40 times less laser energy show similar levels of preheat. This result supports the hydrodynamic scaling of ambient target implosions with respect to preheat. Extrapolation of preheat in ignition-scale cryogenic implosions will be discussed and shown to be near acceptable levels.