Influence of hard x-rays on the ignition threshold in layered inertial fusion capsules*

Hot spot ignition within layered capsules is theoretically challenging due to the complex interplay between the dynamically forming hot spot and the dense surrounding fuel.  To derive the ignition threshold, researchers typically assume that the conductive portion of the heat flux leaving the hot spot is balanced by an inflow of enthalpy.  Recent theory and simulations have demonstrated this approximation breaks down as the dense fuel is heated, resulting in ablative inflows into the hot spot up to ~4x larger than previous estimates [1].  In the new theory, the net influence of mass ablation on the ignition threshold is regulated by a dimensionless parameter that depends sensitively upon the temperature of the dense fuel.  Here, we demonstrate that the M-band emission within the hohlraum is a key heating mechanism for the dense fuel, and thus has an unexpected influence on the ignition threshold.  By modestly increasing the high-Z dopant within the ablator, simulations predict the dense fuel remains cooler and ignition occurs at significantly lower hot spot temperature.  Such designs may offer a path to higher yields, while also probing the complex interaction between the igniting hot spot and the dense fuel.

[1] Daughton et al, Phys. Plasmas 30, 012704 (2023);  https://doi.org/10.1063/5.0129561