Interpenetration and Kinetic Mix in Weakly Collisional, Fully-ionized Plasma Jets

Laser-driven "inverted corona" fusion targets have attracted interest as a low-convergence neutron source and as a platform for studying kinetic physics. These targets consist of a fuel layer lined along the interior surface of a hollow or gas-filled plastic hohlraum. The plasma streams generated in vacuum targets are initially nearly collisionless as they converge, leading to wide interaction length scales and long interaction time scales as the jets interpenetrate. With the inclusion of a low-density gas fill, ejected particles from the shell can pass far into the gas before colliding, leading to significant mixing across the gas-shell interface. Such interactions are difficult to accurately model using standard hydrodynamic simulations, which assume high collisionality. Instead we model the system using a kinetic-ion, fluid-electron hybrid particle-in-cell (PIC) approach. Simulations demonstrate significant kinetic effects (interpenetration, beam-beam fusion, and weakly collisional electrostatic shocks) that are mediated by collisional processes and can be tuned by changing the initial fill pressure of the gas. Using two-dimensional simulations we also investigate compression symmetry, hotspot velocity, and directional differences in neutron spectra, as well as make comparison with x-ray emission images.