Simulation of cathode plasma expansion in magnetically insulated transmission lines

We describe a novel algorithm for the simulation of cathode plasmas in particle-in-cell (PIC) codes, and have applied the algorithm to investigate cathode plasma expansion in magnetically-insulated transmission lines (MITLs) using the PIC code LSP. The MITL electron current is modeled by a fully-kinetic electron species. Electron and ion macroparticles, both modeled as fluid species, form a plasma which is initially localized at the cathode surface. 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) diffusion model. Simulations in 1D show a slow diffusive expansion of the plasma from the cathode surface. But in multiple dimensions the plasma can expand much more rapidly due to anomalous diffusion caused by a flute instability. We also present some preliminary results of diode simulations with both cathode and anode plasmas modeled self-consistently.