Design and Operation of a Multiple-Cathode, High-Power, Rectangular Discharge Chamber

A high-power, rectangular discharge chamber is being designed by the University of Michigan for operation with multiple discharge cathode assemblies (DCAs). The multiple cathode approach attempts to increase thruster lifetime by operating three DCAs sequentially, possibly providing a threefold increase in discharge life. The baseline multiplecathode discharge chamber (MCDC) magnetic field topology is developed based on the NASA Evolutionary Xenon Thruster (NEXT) magnetic field. The selected MCDC magnetic field consists of permanent magnet rings, an electromagnet, and magnetic iron c-channels to augment the field. Experimental results are obtained by operating the MCDC with an ion collection grid (without beam extraction) in the University of Michigan Large Vacuum Test Facility. Operation of the MCDC with the active DCA located on centerline and offcenterline is accomplished, as well as operation with the dormant cathodes floating and connected to cathode common. Different magnetic field configurations are experimentally tested by adjusting the electromagnet current or adding the iron c-channels. Discharge stability is analyzed by measuring discharge voltage oscillations, and 13 button probes are placed on the ion collection grid to determine uniformity. MCDC grid-plane plasma properties, backplate electron current, and dormant cathode current and voltage characteristics are also monitored. A stable discharge is obtained for all operational configurations. Results indicate that the 0 A electromagnet configuration provides the best performance and flatness with optimum values of 194 W/A at 0.89 propellant efficiency and 0.55, respectively. Backplate electron current ratios indicate that the majority of the discharge current is deposited in the corners of the rectangular MCDC. Finally, operation of the dormant cathodes with propellant flow is suggested to reduce potential erosion of those units.