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

William Riedel, Stanford University

Photo of William Riedel

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. These effects are detectable through neutron diagnostics. In particular, predictions of neutron yield scaling vs. gas pressure show excellent agreement with experimental data recorded at the OMEGA laser facility, suggesting that 1D kinetic mechanisms may be sufficient to capture the mix process.

Abstract Author(s): William Riedel, Nathan Meezan, Drew Higginson, Matthias Hohenberger, Mark Cappelli, Siegfried Glenzer