Comparison of hollow cathode discharge plasma configurations

Abstract

Hollow cathodes are used in plasma contactor and electric propulsion devices to provide electrons for sustaining plasma discharges. Based on previous cathode life tests where erosion was observed on hollow cathode assembly components, it is desirable to understand the plasma flow field downstream of hollow cathodes. Plasma flow field measurements are presented herein for hollow cathode generated plasmas using both local and remotely located plasma diagnostics. Two cathode discharges are presented: 1) an open, no magnetic field configuration and 2) a setup simulating an ion thruster discharge chamber. In the open cathode configuration, large amplitude plasma potential oscillations, ranging from 20 V to 85 V within a 34 V discharge, were observed using a floating emissive probe. These oscillations were observed over a dc potential profile that contained a clear potential hill structure. A remotely located electrostatic analyzer (ESA) was used to measure the energy of ions produced within the plasma, and energies were detected that met and in some cases exceeded the peak plasma potentials detected by the emissive probe. In the prototype NSTAR-like discharge chamber configuration, plasma potentials from the emissive probe agreed with ion energies recorded by the remotely located ESA. A correlation model was used to compare the local and remote measurements in both cathode discharges. Nomenclature

Plasma oscillations in a 6-kW magnetically shielded Hall thruster

“…Low frequency mode (7)(8)(9)(10)(11)(12) From the observations in Sec. III, we summarize the characteristics of the fluctuations in the normalized light intensity at 7-12 kHz:…”

“…4,5 While the erosionless steady state that results from magnetic shielding modifications is now well-understood, the impact of the MS upgrade on the transients in the thruster remains unclear. This is an important, unresolved question in light of the effect oscillations have been proposed to have on processes such as cathode erosion [6][7][8] and anomalous transport. [9][10][11][12][13][14][15][16] Moreover, with new missions enabled by MS technology calling for higher specific impulse and power over longer timescales, 17,18 there is a need for high-fidelity models for thruster performance.…”

Ion acoustic turbulence in a 100-ALaB6hollow cathode

“…In spite of the success the authors found in using the IAT collisionality to capture a number of experimentally observed trends, Mikellides et al noted that the incorporation of this effect into their models was based on conjecture: There was no experimental confirmation of the IAT. To be sure, acoustic modes have been proposed to explain some of the salient trends in the observed frequency spectra of probes inserted into hollow cathode plumes [22,23], but the acoustic nature of these waves has never been directly measured. This lack of experimental confirmation brings a number of questions to mind about the role of the IAT in the operation of hollow cathodes: Does the IAT even exist in the plasma plume, and if so, how does it depend on operating conditions?…”

Investigation of Energetic Ions in a 100-A Hollow Cathode

“…3 Keeper erosion, on the other hand, is still considered anomalous. This is because keeper sputtering results from impact by ions from the cathode plume with energies that far exceed the cathode discharge voltage, [4][5][6][7][8][9][10][11] yet there is no classical reason why such high energy ions should exist. The erosion from these anomalous ions is becoming an increasingly important concern in light of new NASA programs that are calling for higher power electric propulsion missions with extended duration.…”