Characterization of standing plasma instabilities in a hollow cathode plume

Hollow cathodes are thermionic electron current sources commonly used in electric propulsion devices such as Hall thrusters. Although these cathodes have been used in space for decades, limited understanding of the fundamental plasma physics in the plume of the device has prevented the development of predictive numerical models. The low temperature plasma (3 eV) in the plume of the cathode is subject to a variety of plasma instabilities that dictate the operation of the device. For example, there exists a low frequency oscillation that is known to empirically onset at high current or low flow rate. This wave has been associated with the generation of ions with energies that exceed the discharge voltage, leading to erosion of the cathode and limiting its lifetime. Recent measurements of this plasma oscillation have indicated that a localized, standing instability generates structures that can be described as wave packets and solitons that propagate away from this source. A clear description of the source is currently unavailable. Presented is an experimental characterization of this standing wave and an analytical description of the onset criterion for this instability.