Anomalous hydrogen sound speed and the areal density problem in inertial fusion*

Systematically lower measured areal density compared to hydrodynamic simulations has been a long-standing issue in inertial confinement fusion (ICF), particularly for low adiabat (i.e., low shell entropy) implosions [D. E. Hinkel et al, High Energy Density Phys. 27, 100884 (2020)]. Experimental measurements reported anomalously low sound speed in shocked liquid deuterium [D. Fratanduono et al, Phys. Plasmas 26, 012710 (2019] compared to all theoretical models at a first-shock entropy relevant to recent ignition experiments. This talk will describe an avenue for more reliable ignition designs and higher ICF gain through better understanding of the relation between hydrogen sound speed, fuel compression, and ICF areal density.