Cathode-plume Instabilities Onset and Detection in Hollow Cathodes for Electric Thrusters

Three dominate plasma instabilities have been identified in the literature to exist under various conditions in the near cathode plume of hollow cathodes used in electric thrusters. These instabilities contribute strongly to the noise in the plasma discharge and the generation of energetic ions that affect thruster component life. The three instabilities are low frequency (<100 kHz) ionization instabilities (reminiscent of predator-prey instabilities in Hall thrusters), turbulent ion acoustic instabilities at higher frequencies (>200 kHz), and rotational instabilities that are found with an applied magnetic field similar to that found in the thruster and also operate at lower frequencies (<100 kHz). A set of emissive and Langmuir probes have been positioned in the near-plume region of a laboratory hollow cathode discharge and used to detect each of these instabilities and characterize their onset and mode competition or coupling. The amplitude and direction of the energetic ions generated during the presence of these instabilities is different depending on which instabilities are present. The instabilities produce a more gradual transition from the quiescent “spot” mode to the noisy “plume” mode that starts at higher flow rates than originally observed in ion thruster neutralizer cathodes, largely because the higher discharge currents and presence of applied magnetic fields in these experiments tend to drive multiple oscillations to exist at the same time with increasing amplitudes.