Thermionic Emission Enhanced by Ion Trapping Beyond the Space-Charge Current Limitation

Recent one-dimensional simulations of planar plasma sheaths revealed how surface electron emission and collisions cause transitions between classical, space-charge limited and inverse sheath regimes [1]. However, multidimensional effects of strongly emitting surfaces on plasmas with collisions remained unexplored by simulations. We developed a novel 2D-2V continuum kinetic code to study the sheath physics, current flow and potential distributions in various configurations including floating and biased emissive probes, filament discharges, and nonuniformly emitting surfaces. The simulations provide an improved understanding of the I-V traces of emissive probes and indicate that strongly emitting probes float above the plasma potential. We found that even small negatively biased cathodes such as filaments can restructure the plasma to an inverse mode where ions are globally confined. Also, we report a previously unrecognized process by which trapped ions in the virtual cathode around a small hot cathode can raise the current flow well beyond the maximum predicted by collisionless space-charge limited sheath models. [1] Johnson and Campanell, Plasma Phys. Rep. 45, 69 (2019)