Simulation of cathode plasma expansion in magnetically-insulated transmission lines

We describe a novel algorithm for the generation of cathode plasma expansion in particle-in-cell codes, and have applied the algorithm to investigate cathode plasma expansion in magnetically-insulated transmission lines (MITLs) in the particle-in-cell code LSP. The steady-state MITL electron current is modeled by a fully-kinetic electron species. Neutral particles are then desorbed from the cathode surface at a fixed rate and are allowed to fragment into electron-ion pairs as they propagate over a short distance, generally one or two cell widths, normal to the surface. These electron-ion pairs, modeled as fluid species, form the seed cathode plasma. Energetic plasma electron particles can be converted to kinetic electrons to resupply the electron flux at the plasma edge (the “effective” cathode). Using this model, we compare results for the time evolution of the cathode plasma and MITL electron flow with a simplified (isothermal) ambipolar diffusion model. We find good agreement between the two approaches for the time evolution of the coupled system of cathode plasma and MITL electrons.