Wear Mechanisms in Electron Sources for Ion Propulsion, II: Discharge Hollow Cathode

TLDR

Wear of the keeper electrode in discharge hollow cathodes limits ion propulsion for long‑duration missions, and a quantitative model of its erosion has remained elusive despite proposed mechanisms. A two‑dimensional partially ionized gas model was applied to two long‑duration 30‑cm ion thruster life tests, with extensive comparisons between simulated plasma properties and measured erosion patterns. The study shows that near‑plume plasma oscillations increase plasma resistivity and ion impact energy, thereby accelerating keeper erosion.

Abstract

The wear of the keeper electrode in discharge hollow cathodes is a major impediment to the implementation of ion propulsion onboard long-duration space science missions. The development of a predictive theoretical model for hollow cathode keeper life has long been sought, but its realization has been hindered by the complexities associated with the physics of the partially ionized gas and the associated erosion mechanisms in these devices. Thus, although several wear mechanisms have been hypothesized, a quantitative explanation of life test erosion profiles has remained incomplete. A two-dimensional model of the partially ionized gas in a discharge cathode has been developed and applied to understand the mechanisms that drove the erosion of the keeper in two long-duration life tests of a 30-cm ion thruster. An extensive set of comparisons between predictions by the numerical simulations and measurements of the plasma properties and of the erosion patterns is presented. It is found that the near-plume plasma oscillations, predicted by theory and observed by experiment, effectively enhance the resistivity of the plasma as well as the energy of ions striking the keeper.