Determining acceptable limits of fast-electron preheat in direct-drive-ignition–scale target designs

In direct-drive-ignition designs, preheat by fast electrons created by the two-plasmon-decay or stimulated Raman instabilities can increase the adiabat in the fuel layer and reduce compression and neutron yields. Since eliminating the preheat entirely is a major challenge, it is necessary to understand the levels of preheat that preclude ignition in a direct-drive target. Two 1-D ignition-scale target designs serve as the basis for examining the effects of synthetically increasing the levels of fast electrons using the 1-D radiation-hydrodynamic code LILAC, which include two models of fast-electron transport. The first is an ignition design adapted from a 2-D polar-direct-drive design for the National Ignition Facility. The second is a variant of the first with identical dimensions and compositions but using a laser pulse that generates stronger shocks and a higher fuel adiabat. This more stable design approaches ignition and achieves yield multiplication as a result of alpha heating. The designs are then re-optimized to recover performance. The igniting design, when fast-electron transport was modeled with diffusion, was found to tolerate 50% more fast-electron preheat of the cold (sub-50eV) deuterium-tritium (DT) ice layer when the laser pulse was optimized using the optimizer Telios. When a straight-line fast-electron transport model was used, the effects of optimization were negligible. For the subignition design, an increase of over a factor of at least 3 in the tolerable level of fast-electron preheat was obtained for both transport models.