Colloid thrusters are under consideration for NASA missions such as the Laser Interferometer Space Antenna (LISA), which requires the continuous cancellation of external disturbances (approximately 25 microNewtons over a 3-10 year mission). Emissive probes are one diagnostic for the measurement of plasma potential, which can provide valuable information on the level of space-charge neutralization in a thruster plume. Understanding how to achieve effective space-charge neutralization of the positive-droplet thruster plume is important for efficient operation and to minimize the risk of contamination. In this Thesis we describe a laboratory electrospray (colloid) source and accompanying power processing electronics developed for testing of diagnostics in colloid thruster plumes. We present results of an initial series of emissive probe measurements using floating probe and swept bias probe techniques. These measurements were carried out using a single needle emitter operating on a mixture of EMI-IM (an ionic liquid) and tributyl phosphate. For a spray operating at a discharge voltage and current of 2.0kV and 200nA respectively, a potential of 5.0V was measured using the floating probe technique with the probe located at a distance of 2.7cm from the electrospray source. The interpretation of this floating potential as the plasma potential is discussed. In a separate set of tests, we used the swept bias emissive probe technique at the same distance and measured a plasma potential of 2.0V at a discharge voltage of 2.0kV. The discharge current in this latter test was somewhat unstable and varied from approximately 250 nA to over 1000nA. Numerical integration of the Poisson equation was performed to better understand space charge limitations of a probe emitting into a low density plasma. These results are presented and some implications for the measurements discussed. While the electrospray droplet number density was not measured, calculations to estimate this number density are also presented. Based on these estimates and our numerical calculations, the“knee" in the current voltage characteristic measured using the swept probe technique is estimated to be within 1.3 V of the actual plasma potential.
Worcester Polytechnic Institute
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Roy, Thomas Robert, "Plasma Potential Measurements in a Colloid Thruster Plume" (2005). Masters Theses (All Theses, All Years). 359.
emissive probes, electrospray, colloid thrusters, plasma potential, Space vehicles, Propulsion systems, Spraying equipment, Colloids, Electric properties, Droplets