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

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

Investigation of High Current Heaterless Hollow Cathode Ignition

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 1/6th 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.<br/

Vacuum current emission and initiation in an LaB6 hollow cathode

This paper presents the first investigation of pre-ignition currents and the ignition process in an LaB6 hollow cathode running on krypton propellant. Vacuum and pre-ignition currents are found to be consistent with space charge limited behaviour. A novel, low power ignition strategy with the potential to reduce insert and orifice erosion is also shown

Thermal profile of a lanthanum hexaboride heaterless hollow cathode

Higher power cathodes are increasingly utilizing LaB6 emitters due to their increased emission current density capabilities over barium oxide emitters, however, these emitters require higher operating temperatures which poses a significant challenge to maintain heater reliability. Hence, there is ongoing development of heaterless hollow cathodes that allow for potentially higher reliability through design simplicity of removing the heater component, and in addition, there is potential savings in mass, volume, ignition time and power. In this study a LaB6 heaterless hollow cathode with imbedded high temperature thermocouple instrumentation demonstrated reliably operation in diode mode from 5-8 A at xenon flow rates of 0.2-1.75 sccm. Thet emperature profile of the emitter is characterized through ignition, which lasts up to 50 seconds. In addition, pyrometer measurements in an open keeper configuration have shown the emitter surface temperature to be significantly lower than that predicted by the Richardson equation for LaB6 emitter, perhaps indicating a strong influence of the surface chemistry

Development of a high current heaterless hollow cathode

Higher power in-space propulsion systems are increasingly been developed and utilised due to payload increases and mission demands. This requires higher current cathodes to match this demand, as such cathodes are increasingly adopting lanthanum hexaboride (LaB6) emitters due to the higher current density operation capabilities. However, LaB6 cathodes operate at higher temperatures compared with traditional barium oxide cathodes, due to the material’s relatively higher work function. Hence, this poses higher challenges to the heater technologies, which raise convention HCs to thermionic temperature. Here we present the development of a heaterless hollow cathode (HHC) system, which self-heats within only tens of seconds before nominal operation, and enables currents up to 30 A. Such that the system effectively removes the burden of the heater subcomponent. Within this work, the breakdown, cold (heating) discharge and nominal performance of the HHC have been characterised. In addition, optical emission spectroscopy is utilised to assess the relative erosion rates

Characterisation of a 30 a heaterless hollow cathode

A novel high-current heaterless hollow cathode (HHC) has been designed, constructed, and tested, with the developed system successfully demonstrating operation up to 30 A. This system overcomes the need for excessive ignition voltages or propellant pulsing, by utilising a reduced keeper orifice that enables ignition with &lt;350 V, and nominal flow rates (&lt;15 sccm). The system has also demonstrated full ignition in 50 seconds compared with conventional ignition, which can require more than 10 minutes; additionally, the system requires as little as 1/6th of the ignition energy compared to that of conventional designs. The HHC’s performance was characterised with operation tested in Xe, Kr and Ar. Optical pyrometry has allowed measurements of the emitter tip temperature of a HHC for the first time. Furthermore, the internal cathode-keeper plasma has been investigated using optical emission spectroscopy to determine the plasma electron density

Development of a heaterless hollow cathode for miniature EP systems

A Heaterless Hollow Cathode (HHC) has been designed and developed to be integrated into thrusters for LEO and MEO satellites in the mass range of a few hundred kilograms. There are several missions for which a low power (100-500 W) and cost-effective EP system would be beneficial, including small telecom constellations in LEO, small earth observation LEO satellites and Mega Telecom constellations. The BaO-W HHC was tested in diode mode with xenon propellant over a wide range of operating parameters. Significantly, this HHC has demonstrated stable operation for a wide range of currents, 0.9 - 7.5 A, with flow rates below 10 sccm and an anode voltage less than 25 V. For instance, at 7.5 A the HHC established operation in spot mode with as low as 4 sccm and an anode voltage of 12.7 V. The HHC system has a compatible flange to allow for planned integrated coupling tests with the MSL GIE150, a 150 mm Ring Cusp Engine

Development of a low current dry neutralizer in a Pierce Gun configuration

One of the critical aspects of the development of next generation electric propulsion systems for new deep-space small satellites and CubeSats is represented by the neutralization system. Within the framework of the RIT3.5 Sub-Assembly Development, a dedicated Neutraliser, the RIT3.5 NTR, is being designed and developed. The RIT3.5 NTR is based on a Pierce electron gun configuration able to deliver an electron current of 10-50 mA at a max voltage bias &lt;400 V with a power consumption &lt;10 W.After a review of all main neutralization techniques and cathode technologies currently under development, dry neutralizers have been selected as the best solution for the NTR requirements. An electrostatic modelling parametric analysis, along with a thermal and structural analysis, have been developed to indicate performance trends and for guideline in the design of prototype system. Results of the review and the numerical investigation, as well as an overview of the forthcoming test campaign for the modelling validation, are presented and discussed in detail in the paper