An investigation into the glow discharge phase of an LaB6 heaterless hollow cathode

Abstract

Hollow cathodes typically operate through the use of low work function emitters to deliver thermionic current. To achieve high thermionic current the emitters require heating to around 1500 K for barium oxide cathodes and over 1900 K for lanthanum hexaboride cathodes. Conventionally a heater component is utilised to raise the emitter to the required thermionic temperatures for ignition, however this has drawbacks: firstly additional mass and volume for the heater component is required, secondly there are reliability issues due to thermal cycling and high temperature variation, and finally there are long ignition times, up to 10 minutes, due to indirect heating of the insert. Thus replacing the heater component with a simpler and potentially faster ignition system will be highly advantageous. Conventional hollow cathodes can be cold started, though this leads to high voltages combined with unacceptable mass flow rates (order of magnitude higher).We are investigating an alternative approach to ignition by developing dedicated heaterless hollow cathodes (HHC) that meet the internal pressures required at nominal mass flow rates. In which the emitter heating is driven by a discharge between the keeper and the emitter. This method allows for direct heating of the emitter, lowering the overall HHC ignition time to as low as 2 seconds, without requiring additional components. Though to date HHC’s have only demonstrated lifetimes of hundreds of hours. This is primarily due to the absence of thermionic emission during the breakdown stage, such that higher breakdown potentials are used compared with conventional ignition. Hence the sputter erosion yields can be higher due to the higher energy ion bombardment and in addition cathodic spots can form through ignition, due to over powering, thus causing high localised erosion. This study investigates a novel power switching sequence to ignite the heaterless hollow cathode, which can enable repeatable ignition at relatively low voltages (<500V) and flow rates (<20 sccm), thus resulting in low erosion. This is achieved though adapting the voltage and current though through ignition to understand their influence on repeatability and erosion. This is examined through an experimental campaign conducted on the 20A heaterless hollow cathode under development at the University of Southampton. Results have shown that discharge stability can be increased by limiting current though the use of electrical ballasts due to the plasmas negative resistance characteristics observed. Erosion analysis is being conducted though the following diagnostics: scanning electron microscope for erosion detection, spectroscopy for species identification and periodic mass measurements for erosion quantification