Development and Study of an Electron Cyclotron Resonance Waveguide Plasma Cathode for Electric Propulsion Applications.

Abstract

In electrostatic ion thrusters and Hall thrusters, electron sources are used for propellant ionization and neutralization of the thruster beam. Thermionic emitter-based sources are commonly used, but they possess inherent lifetime limitations due to emitter depletion, poisoning, and sputtering of the emitter surface. For long duration electric propulsion (EP) driven missions or semi-permanent plasma contactor installations, these emitters have become primary limiting components on thruster life. There are two goals to this work: first, to develop and demonstrate the feasibility of an emitterless plasma cathode for EP; and second, to study the underlying physics of emitterless cathodes. The waveguide plasma cathode uses traveling 2.45 GHz microwaves in a cylindrical waveguide geometry, with permanent magnets, to generate an electron cyclotron resonance (ECR) discharge. Electron current is extracted from this source plasma through a downstream aperture. This device delivered up to 4.2 amperes of electron current, at low power (90 W/A) and high gas utilization. The device was tested with argon, krypton, and xenon. Probe diagnostics were used to measure axial profiles of electron density, electron temperature, and plasma potential, inside the device and in the external plume. These measurements show that some trace plume ionization is necessary for substantial current extraction. Plasma potential in the plume tracks with a biased anode, and a weak electric field in the plume transports current across the anode-cathode gap. Internal plasma conditions are also discussed. The plasma density in the extraction aperture increased by orders of magnitude, relative to the source discharge density, during electron current extraction. This is attributed to the formation of a dense plasma structure at the aperture. Laser collision-induced fluorescence (LCIF) was used to create two-dimensional images of plasma density and effective electron temperature at the aperture. The structure had a high density core, surrounded by a layer of high energy electrons accelerated by a double layer. Probe diagnostics verified the existence of a potential gradient between the aperture and bulk plasma. The aperture plasma acts as an effective loss area for electrons, and may be a common feature of plasma cathodes that should be included in models of these devices.