Investigation of Plasma Mixing Processes in the Context of Indirect Drive Inertial Confinement Fusion

Abstract: In inertial confinement fusion (ICF), the dynamics of plasma mixing in hohlraums critically influence laser-plasma instabilities (LPI) and implosion performance. This study investigates the mixing of hohlraum ablated Au plasmas and filling C$5$H${12}$ plasmas using one-dimensional particle-in-cell (PIC) simulations. We find that ion-ion collisions slow the diffusion of ions, rendering Au ions sub-diffusive, while C and H ions remain super-diffusive. Due to their lower collisionality, H ions diffuse faster into Au regions than C ions, leading to a distinct separation between C and H ions at the interface. Although an electrostatic shock is still generated at the plasma interface in the presence of collisions, its electric field strength and propagation speed are notably reduced. To systematically explore plasma mixing in hohlraum environments, we evaluate the individual effects of incident laser irradiation, plasma flow, and inhomogeneous density profiles on ion mixing. We find that laser irradiation and plasma flow have a minor impact on ion mixing compared to diffusion-driven processes, while the inhomogeneous density profile restricts diffusion from low-density to high-density regions. By incorporating realistic hohlraum plasma conditions derived from radiation hydrodynamic models into the PIC simulations, we demonstrate that the diffusion of C and H ions continues to dominate ion mixing. Simple phenomenological fits are derived to describe the evolution of the mixing width in a hohlraum condition. Further theoretical calculations indicate that the penetration of H and C into Au plasmas suppresses stimulated Brillouin scattering (SBS) within the mixing layer. This finding underscores the importance of integrating ion mixing effects into LPI codes for more accurate modeling of ICF hohlraum dynamics.