Low Frequency Oscillations in the Diverging Cusped-Field Thruster

thruster is markedly bi-modal, with a sharp transition to a visually distinct, low anode current mode at high applied voltages. Time resolved measurements of the anode current in the high current, low voltage mode revealed extreme oscillations at 3.5 kHz with the current nearly extinguishing during each period. Discharge current oscillations during high voltage, low current operation are minimal in contrast, though the dominant frequency remains the same. A high speed camera is used to discern the evolution of plasma luminosity through a typical cycle, with the geometry of the discharge chamber allowing shallow views of the ionization region to augment the typical axial imaging. Anode current is measured simultaneously and matched to representative cycles of image intensity inside the discharge chamber. The time resolved thruster dynamics are then compared for a range of operating conditions under each of the observed modes. Trends in oscillation frequency, magnitude, growth rates and decay rates are presented. A strong correlation between luminous plasma regions and magnetic cusp locations is observed.