Microwave Plasma Cathode


Background

Electron sources are employed in many electric propulsion (EP) systems, such as electrostatic ion thrusters and Hall thrusters. In both of these devices, accelerated electrons are used to break down the propellant gas, providing ions that can be accelerated to produce thrust. As positive ions are accelerated out of the thruster, the floating spacecraft will have a tendency to build up a negative charge. An additional electron source known as a neutralizer is used to neutralize the positive ion beam with an equal amount of negative electron current.

At the U-M Plasma Science and Technology Laboratory, a waveguide microwave plasma cathode is being studied as a proof of concept for a long-lived alternative to emitter-based electron sources. The device utilizes electron cyclotron resonance (ECR) heating, wherein electrons orbiting in a static magnetic field are accelerated, through the resonant absorption of microwaves at a frequency equal to the electron cyclotron frequency. These electrons are then used to break down the feed gas within a waveguide structure, and the resulting discharge acts as a source of electrons. Electrons are extracted from this microwave discharge through a downstream aperture by an anode or a combination of anode and keeper electrode.

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The approach of using propagating microwaves in a waveguide as the heating mechanism for a plasma cathode circumvents several constraints that are inherent to many electron sources.Since there is no thermionic emitter situated in the microwave plasma cathode itself, the lifetime of the device is not limited by emitter erosion from sputtering. The microwave plasma cathode should be able to run on a variety of gases – including carbon dioxide and water vapor – because there is no concern about poisoning of an emissive insert through surface chemistry. Additionally, since the microwave launching antenna is not in direct contact with the plasma within the waveguide, there is no possibility of sputtering erosion of the antenna surface.

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Performance to Date / Current and Future Work

  • The U-M waveguide microwave plasma cathode has demonstrated the ability to deliver up to 4.2 Amperes of electron current, which is well within the range of required neutralizer current for low to medium power thruster systems.
  • The source has demonstrated electron current on xenon, krypton, and argon feed gases, with more gases to be tested in the future.
  • Total (microwave + beam extraction) power consumption as low as 89 W/A has been demonstrated on xenon.
  • Gas utilization factors of up to 32 electrons per feed gas atom have been demonstrated on xenon as well. A high degree of ion recycling within the source reduces the amount of neutralizer feed gas that is required.
  • Langmuir probe data are being compiled to investigate the dependence of plasma parameters on operating conditions like extraction bias, microwave power, and gas flow rate, both inside the microwave source and within the visible plume between the plasma cathode and anode.
  • Future experiments include plasma potential mapping using probes, 2-D mapping of plasma parameters using laser diagnostics, and the optimization of the electron beam extraction circuit in order to increase deliverable current and efficiency.
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Further Reading:

Weatherford, B. and Foster, J., “Initial Performance of an ECR Waveguide Plasma Cathode with Permanent Magnets,” Proceedings of the 31st AIAA International Electric Propulsion Conference, Ann Arbor, MI, Sept. 20 –
24,2009, Paper No. IEPC-2009-211

Weatherford, B., Foster, J., and Kamhawi, H., “Improved Performance of the ECR Waveguide Plasma Cathode with Permanent Magnets,” Proceedings of the 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and
Exhibit, Nashville, TN, July 25-28, 2010, Paper No.AIAA-2010-6519