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Completed TRL 3 (started at 2, targeting 3)
Hollow cathodes are a vital component for the ignition and plume neutralization of Hall effect thrusters (HETs). Traditional HETs are known to exhibit significant erosion along the channel walls of the thruster through ion bombardment. Magnetically shielded HETs have nearly eliminated this short term erosion mechanism; however, a new long term azimuthal erosion pattern has been measured on the inner front pole cover. Literature has suggested that ions stemming from both the cathode and the beam through charge exchange collisions are the cause for this new erosion pattern. However, the physical mechanism for this erosion and growth of ions from the cathode is unclear. The proposed research will investigate the high energy ions generated specifically at the cathode through observing the turbulence in a partially magnetized hollow cathode plume. In lone cathode experiments without an applied magnetic field, the cathode is known to operate under a quiescent "spot" mode and an oscillatory "plume" mode. Plume mode has shown to increase the keeper erosion of cathodes. It has been suggested that the predator prey oscillations in the plume coupled with ohmic heating due to ion acoustic turbulence (IAT) is the cause for the spot to plume mode transition. However, when electrons are magnetized, as they are in HET plumes, the cathode appears to operate in an entirely different mode, deviating from the traditional spot or plume mode. Further, with the application of the external magnetic field, azimuthally travelling high energy ions appear. The proposed research will investigate IAT and the azimuthally drifting ions with a high speed ion saturation probe array for a wave dispersion analysis (WDP), a high speed Langmuir probe (HSLP), and a high speed retarding potential analyzer (HSRPA). The dispersion analysis will be performed using a method defined by Beall for two ion saturation probes. Reconstructive Fourier transfer techniques will be used to resolve the spatial turbulent plasma parameters collected at the probes. An understanding of the turbulent plasma waves in hollow cathodes will provide insight in long-term erosion patterns observed in HETs. Further analysis with full thruster operation will show how the thruster environment affects the measured turbulence. The knowledge gained from this research will greatly contribute to the understanding of HET life-limiting factors.
The model developed by the proposed research aims to describe the instability that is suggested to cause the production of these high energy ions. The knowledge gained from this research will greatly contribute to the understanding of Hall effect thruster life-limiting factors.
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