Non-Perturbative Characterization of Electron Drift in a Hollow Cathode Discharge

Hollow cathodes thermionically emit electrons in order to sustain and neutralize a variety of plasma propulsion devices, including gridded ion thrusters and Hall thrusters. A key efficiency loss in these types of devices is the voltage drop required for the cathode plume to couple electrically to the main ion beam which produces thrust. Historically, this cathode coupling voltage has exceeded theoretical calculations, termed "anomalous" resistivity. These non-classical forces are typically attributed to plasma wave behavior, in particular ion acoustic turbulence which propagates axially in the cathode plume. Particle-in-cell simulations and and nonlinear wave theory suggest that the initial electron Mach number has a strong impact on the growth and saturation properties of these waves. Therefore, in order to better understand the first-principles mechanisms which may enable predictive and self-consistent modeling of hollow cathode behavior, it is necessary to non-perturbatively measure the electron distribution function in this device. We implement an incoherent Thomson scattering diagnostic to obtain the spectrum of scattered laser light from electrons in the plasma, which can be related to the distribution of electron velocities and characterize transport.