Investigation of high current heaterless hollow cathode ignition

Abstract

The development of long life high powered hollow cathodes is of importance to meet the demand of increasingly powerful Gridded Ion Engines and Hall Effect Thrusters. High current (≥30 A) cathodes typically operate with a LaB6 emitter which operates at higher temperatures than a BaOW emitter, thus posing a significant challenge for heater reliability. The heater component commonly used to raise the emitter to thermionic temperatures, has inherent reliability issues from thermal fatigue caused by the thermal cycling with large temperature variations. A self heating hollow cathode allows for potentially higher reliability through the design simplicity of removing the heater component. This also results in significant cost savings, as well as savings in the mass, volume, ignition time and ignition power.
To investigate heaterless ignition, a novel high current heaterless hollow cathode (HHC) has been designed, constructed, and tested. Critically this system for the first time, controls the discharge current rise and attachment through heaterless ignition, to maintain a diffusive heating discharge that raises the emitter to the thermionic temperatures, without discharge localisation that can lead to high erosion and melting. The developed system successfully demonstrated operation up to 30 A, achieving proof of concept. This system also overcomes the need for excessive ignition voltages or propellant pulsing, with a reduced keeper orifice that enables ignition with 15 minutes; additionally, the system requires as little as 1/6^th of the ignition energy compared to that of conventional ignitions.
The novel HHC’s performance was characterised for the breakdown, heating, transition and thermionic phases of ignition, with operation tested in Xe, Ar and Kr. Thermocouples placed along the emitter axis provided emitter thermal profile trends through ignition and optical pyrometry has allowed measurements of the emitter tip temperature of an HHC for the first time. The internal cathode-keeper plasma has been investigated using optical emission spectroscopy to determine the plasma electron density.

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