Observation of rotating magnetohydrodynamic modes in the plume of a high-current hollow cathode

High-current hollow cathodes are widely used in electric propulsion as well as for laboratory plasma generation applications. The plasma region just outside of high-current cathodes is characterized by the presence of many fluctuations, which can develop coherent plasma structures affecting the operation and life of the cathode. The properties of plasma oscillations have been investigated in the exterior region of a high-current hollow cathode operating at 25-150A of discharge current with an applied axial magnetic field. Electrostatic and magnetic probes were used to measure the electromagnetic fluctuations, and correlation analysis between each of the probes signals provides the spatiotemporal characterization of the generated waves. The results of this investigation reveal the presence of a coherent magnetohydrodynamic (MHD) azimuthal mode with a fundamental frequency of 58kHz. This mode has features of a helical kink instability with azimuthal wave number m 1 and axial wavenumber k z = 50 m - 1 that is readily observed in high-speed Fast-Cam images. The occurrence of this mode, the frequency, and the wavenumber at onset are found to be predicted well by the ideal MHD theory, considering the boundary condition of a non-line-tying cylindrical anode. Investigation of the MHD mode properties at different discharge currents and mass flow rate shows additional features that could qualitatively agree with resistive MHD modes. In particular, when the plasma has a finite conductivity associated with anomalous resistivity in the cathode plume, a gradual emergence of modes at relatively low discharge currents are observed.

Automated summary

This study investigated the plasma oscillations in the exterior region of a high-current hollow cathode operating at 25-150A, revealing the presence of a coherent magnetohydrodynamic (MHD) azimuthal mode with a fundamental frequency of 58kHz, characterized by helical kink instability. The frequency, wavenumber, and onset of this mode were well predicted by ideal MHD theory, considering the boundary condition of a non-line-tying cylindrical anode. Additional features of resistive MHD modes were observed at relatively low discharge currents, indicating the presence of anomalous resistivity in the cathode plume.