• hollow cathode propulsion kink hall

Hollow cathodes have been used in the electric propulsion since 1970s, but significant progress has been made recently in performing these devices; lanthanum hexaboride (LaB6) hollow cathodes have been widely qualified for the use in flight thruster, such as Hall and ion thrusters. The next generation of high power electric thrusters in particular requires hollow cathode to present longer life and high discharge current level up to 300 A. This category of high current hollow cathodes has been widely tested, and it presents various fluctuations of the exterior plasma region, that can generate coherent plasma mode affecting the cathode performance and life.
This thesis focuses on the investigation of such hollow cathode plasma oscillations, in particular those associated with a rotational behaviour of the external plasma column. To investigate this goal, a dedicated data analysis routine has been elaborated to characterize the nature of the rotational oscillations that appears in the presence of an external magnetic field. This analysis has been performed on sets of experimental data, representing the operations of a high current LaB6 hollow cathode, that have been provided by the Jet Propulsion Laboratory research group.
The analysis procedure included: a spectral analysis, aimed at identify not only frequency contents but also typical mode wavenumbers, a particular phase averaging technique developed to characterized the plasma oscillations and a modal analysis adapted to the video images of the discharge plasma plume.
The results obtained from the analysis of the available data demonstrated that these oscillations are related to an electromagnetic mode, that is always present when an external magnetic field is applied. In particular this mode has been found to have typical features of magnetohydrodynamic (MHD) helical kink instability, with fundamental frequency f~55 kHz and wavenumbers m=1 and kz~50 m^-1. In addition the phase averaging technique allowed to identified a likely relation between this electromagnetic mode and the production of dangerous energetic ions.
Since the nature of the oscillations has been associated to electromagnetic mode, it follows that the mode occurrence and main properties could be predicted by ideal MHD theory.