Design and testing of a high power electron cyclotron resonance neutralizer

Current neutralizer technology is unsuited for extremely long duration missions. To date, hollow cathode technology has been the primary neutralization method for electric propulsion missions. However, the operational lifetime of a hollow cathode is limited by depletion of the barium insert and erosion of the thin-walled cathode. Electron cyclotron resonance (ECR) neutralizers are a promising solution to the lifetime problem. Relying on the resonant coupling of particle motion around a static magnetic field and an incident microwave, an ECR neutralizer generates a plasma from which electrons can be extracted. The neutralizer is electrodeless and does not rely on a depletable insert, thereby eliminating the most common failure modes of the hollow cathode. In this work, a high power, waveguide-fed ECR neutralizer was designed and built, and initial tests conducted. The system serves as a proof-of-concept model and, as such, was designed to be easily modifiable for future design iterations and investigations. This paper reports on initial tests of the neutralizer in which it was found to perform as expected. The neutralizer was tested at background pressures between 5×10^-4 and 7×10^-3 torr. Plasma ignition was noted with a minimal input power of 40 W. In addition, both low and high modes of operation were noted, with the high mode initiating at around 300 W. An extraction current of 2.45 A was achieved with 550 W of input power at 0.7 mtorr. In addition, a maximum extraction current of 2 A was achieved at 850 W and 1 mtorr. At this operating pressure, an increase in input power beyond 850 W did not result in an increase in the extraction current, indicating that a cutoff plasma density had been reached.