Development and Study of an Electron Cyclotron Resonance Waveguide Plasma Cathode for Electric Propulsion Applications.
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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.PhDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89714/1/brweathe_1.pd