Performance Degradation of a Spacecraft Electron Cyclotron Resonance Neutralizer and Its Mitigation

摘要

The electron cyclotron resonance microwave discharge ion thruster μ 10, installed in the Japanese asteroid explorer Hayabusa, experienced trouble during a round-trip voyage between Earth and an asteroid due to degradation of the neutralizer. The neutralizer is a critical component limiting the thruster lifetime. The mechanism of the performance degradation is investigated here to find out methods to extend its lifetime for next-generation μ10 thrasters. Fault tree analysis during on-the-spot inspections of the neutralizer, previously used for 20,000 h endurance testing on the ground and in experimental simulations, leads to the following proposed degradation mechanism. The internal-surface materials are sputtered by doubly ionized xenon and accumulate on other surfaces, forming thin films containing iron. Next, thermal cycles result in the peeloff of the thin films due to the difference between the thermal expansions of the films and the base plates. Finally, thin films containing iron (that is, ferromagnetic flakes) are magnetically attracted to the tips of the magnetic circuit These flakes inhibit plasma production, resulting in a performance degradation of the neutralizer. A magnetic circuit covered with molybdenum as a sputtering-resistant material is shown to be effective in prolonging the life of the neutralizer.