|Michael S McDonald
Highly fuel efficient spacecraft propulsion typically relies on plasma propulsion systems such as Hall and gridded ion thrusters. In both devices, one of the most failure-prone components is the hollow cathode that ignites and neutralizes the discharge. These are thermionic emitters typically sourcing several to tens of amperes over emission areas of a few square centimeters, with work functions of 2-3 eV. As a result, state-of-the-art emitters operate in a temperature range of 1100-1700 C for typical use cases, driving thermal power losses due to radiation and conduction that reduce overall efficiency for higher power thrusters (100s-1000s watts) and preclude sub-100W thrusters almost entirely. We are interested in the maturation of new cathode architectures such as heaterless ignition, 3D printed geometries, large-area planar cathodes, and other techniques enabled by new materials such as low-temperature (<1000 C or work function <2 eV) thermionic emitters and field emitters.
Our research program includes experimental, theoretical and numerical modeling components, focused on fundamental physics of electron emission and applied physics of cathode plasma ignition/operation in cathode-only and full thruster environments. Of note are coupled thermal-plasma modeling of the emission/ignition process and plasma stability/coupling investigations of cathode operation in an external anode or thruster environment.
Current topics of particular interest are:
a) fundamental plasma physics investigations of plasma instabilities especially spot-plume mode transitions
b) development of predictive models for rapid ignition of a dense plasma via direct electrical breakdown in the absence of pre-heating a thermionic emitter
c) in-situ UHV fabrication and electron emission characterization of prospective thermionic and field emitting materials
Facilities available include high-vacuum test chambers and supporting equipment for hollow cathode operation and plasma diagnostic evaluation in representative space environments, dedicated ultra-high vacuum material characterization facilities, a wide variety of plasma diagnostic tools, and access to DoD HPC simulation resources.