Studying plasma inter-diffusion in a high-energy-density plasma transitioning from a kinetic to hydrodynamic-like regime in shock-driven implosions at OMEGA*

Obtaining a fundamental understanding of particle and energy transport

near an interface is essential for advancing high-energy-density-plasma

science and for accurately modeling Inertial Confinement Fusion (ICF)

implosions. Hot-spot ignition designs require a cold-dense shell to

compress a hot-diffuse core, and inter-diffusion across this boundary

plays a significant role in the transport of particles and energy.

Previous work studied the mix between the shell and hot spot, specifically

examining the yield degradation. Radiation-hydrodynamic simulations, which

used empirically-tuned ion-kinetic models were able to explain the data

set by varying the amount of inter-diffusion in an ad-hoc fashion. This

work advances previous studies by constraining an ion-kinetic modeling of

plasma inter-diffusion using a suite of data from new x-ray and nuclear

diagnostics. A set of D3He gas-filled, thin-glass, low-convergence

implosions were conducted at OMEGA with initial gas-fill densities ranging

from 0.4 to 2.9 mg/cc, which changed the inter-diffusion coefficient at

the glass-D3He-gas interface. The observed x-ray yield was tripled by

transitioning from hydrodynamic-like to kinetic regime. The x-ray emission

transitioned from a shell-like to center-like emission source as the

initial gas-fill pressure decreased and the plasma transitioned into a

kinetic regime, predominantly caused by a 2× increase if hot-spot mass due

to enhanced inter-diffusion. While a binary diffusion model underestimated

the level of mixing and enhancement due to kinetic effects, ion

Fokker-Planck simulations were able to capture the resulting data. The

nuclear-yield degradation was also captured by the kinetic simulations as

the inter-diffusion increased, which indicates that kinetic effects must

be considered in the mixing process in addition to the hydrodynamic

picture of collisional inter-diffusion.