Plasma plume expansion with pulsed electron neutralization

Abstract

Electrons neutralizing the ion beam from a gridded ion source are typically provided by an external cathode. This cathode emits a continuous current that ensures quasi-neutrality of the plume, and current balance of the ion source. A new type of neutralization scheme has recently been identified in the context of radio-frequency (RF) biased ion sources, where instead of a continuous electron current, the plume is neutralized by electron pulses emitted from the same plasma source as the ion beam itself. In contrast to conventional gridded ion sources, experiments have shown that pulsed neutralization produces hot electrons with a strongly anisotropic energy distribution in the plume. By making use of a two-dimensional particle-in-cell (PIC) simulation, we analyze the pulsed neutralization and plasma expansion to understand the fundamental plume physics in these systems, and perform a direct comparison with the expansion observed in typical DC systems. Electron trapping in the near-field plume region is found to be critical for ensuring quasi-neutrality, and the plume potential is observed to be higher than the downstream acceleration grid potential to prevent excessive electron backstreaming into the plasma source. This potential difference results in the formation of high-energy electron beams that generate collective plume oscillations with frequencies above the applied RF frequency. A detailed parametric study is performed to investigate the influence of the pulse frequency, emission current, and capacitance between the source and outer surrounding boundaries. In particular, the pulse frequency and emission current have a significant effect on the resulting plume potential, and the effectiveness of the resulting ion beam neutralization.