High Current Hollow Cathodes for High Power Electric Thrusters

Abstract: Higher power Hall thrusters continue to be developed for deep space robotic and human/cargo missions that require higher thrust and specific impulse than available in current thrusters. As the power increases, so does the need for higher current hollow cathode electron sources to drive them. The present generation of Hall thrusters in the 4.5 to 12.5 kW range require hollow cathodes capable of less than 30 A of discharge current with lifetimes of the order of 20 kh. The hollow cathodes intended for the next generation high power thrusters must reliably provide hundreds of amperes of discharge current with high reliability and longer life. NASA successfully demonstrated the XR-100 nested Hall thruster at power levels up to 100 kW, which was driven by a JPL lanthanum hexaboride (LaB6) hollow cathode called the “X3 cathode” operating at 250 A of discharge current. This hollow cathode is the third in a series of JPL LaB6 cathodes that first produced up to 40 A for the H6 thruster, up to 100 A for 20 kW Hall thrusters, and finally this state-of-the-art “X3 cathode” that has been operated at discharge currents of 25-350A in the JPL cathode test facility. The fourth generation of this LaB6 hollow cathode technology has been built and tested at JPL at up to 500 A of discharge current. Extending the discharge current capability to this level presented many challenges. The hollow cathode configuration required to obtain these high discharge currents is described with discussions about thermionic insert design, cathode design to provide sufficient plasma penetration into the insert region to avoid space charge limitations on the thermionic emission, near cathode plume gas injection techniques, heater and thermal considerations, and cathode life.