Formation of Anomalously Energetic Ions in Hollow Cathode Plume by Charge Separation Instability

Abstract

Hollow cathodes are becoming the bottleneck of many electric propulsion systems, because of the sputtering and erosion on both cathodes and thrusters from the generation of anomalously energetic ions. So far, it is believed that energetic ions are formed by waves and instabilities always accompanied in cathode discharge, but there is no evidence yet that those proposed instabilities can lead to such high ion energies measured in experiments. In this work, a new mechanism of charge separation instability in hollow cathode plume is found via fully kinetic PIC simulations, which can easily produce energetic ions to the same level as measured in experiments.

Figures (4)

• Figure 1: A photo of a hollow cathode discharge Wang_2022 (left) and 2D-RZ simulation setup (right).
• Figure 2: The time evolution of the charge separation instability illustrated by the distributions of potential $\phi$, electron density $n_e$ ($m^{-3}$), ion density $n_i$ ($m^{-3}$), electron and ion $z$-$v_z$ phase space plots, at different time steps $t$.
• Figure 3: From the left to the right: the average density and energy over time; the anode current oscillation over time; the spectral plot in the MHz scale; and the spectral plot in the GHz scale.
• Figure 4: From left to right: ion current density $J_i$ over collector potential $\phi_c$ on $r=L_r$ and $z=L_z$ two planes; variation of the neutral atom density distribution $n_a$, where $\xi^2 = r^2+z^2$; $J_i$-$\phi_c$ on $r=L_r$ plane and the GHz spectral plots of cases varying $n_a$.