Electron Energy Distribution Measurements in the Plume Region of a Low Current Hollow Cathode Public
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A hollow cathode is an electron source used in a number of different electric thrusters for space propulsion. One important component of the device that helps initiate and sustain the discharge is called the keeper electrode. Cathode keeper erosion is one of the main limiting factors in the lifetime of electric thrusters. Sputtering due to high-energy ion bombardment is believed to be responsible for keeper erosion. Existing models of the cathode plume, including the OrCa2D code developed at Jet Propulsion Laboratory, do not predict these high-energy ions and experimental measurement of the electron energy distribution function (EEDF) could provide useful information for the development of a high fidelity model of the plume region. Understanding of the mechanism by which these high-energy ions are produced could lead to improvements in the design of hollow cathodes. The primary focus of this work is to determine the EEDF in the cathode plume. A single Langmuir probe is used to measure the current-voltage (I-V) characteristic of the plasma plume from a low current hollow cathode in the region downstream of the keeper orifice. The EEDF is obtained using the Druyvesteyn procedure (based on interpretation of the second derivative of the I-V curve), and parameters such as electron temperature, plasma density and plasma potential are also obtained. The dependence of the EEDF and other parameters on the radial position in the plume is examined. Results show that the EEDF deviates from the Maxwellian distribution, and is more accurately described by the Druyvesteyn distribution directly downstream of the cathode. Off-axis measurements of the EEDF indicate the presence of fast electrons, most likely due to the anode geometry. The cathode used in these tests is representative of the cathode used in a 200W class Hall thruster. Data is presented for a hollow cathode operating on argon gas for two cases with different discharge currents.
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