An electrostatic probe comprised of an exposed electrode that is electrically heated into electron emission thereby floating to the local plasma potential voltage which can then be measured directly.
Figure 1. Schematic of emissive probe used with the HARP
for internal Hall thruster measurements (from ref. 3).
Floating emissive probes are used to measure plasma potential in the plumes or inside the discharge chambers of Hall and Ion thrusters.
The probe is constructed from a loop of tungsten filament attached to an alumina insulation tube.
A current is applied to the filament, heating it to the point where electrons are thermionically emitted.
The emitted electrons essentially neutralize the sheath surrounding the filament causing the probe to float at the local plasma potential.
A major advantage of emissive probes is that they give a direct measure of plasma potential without the requirement of a voltage sweep or data reduction, both of which are necessary with other types of electrostatic probes.
One drawback is that the presence of a magnetic field and large density gradients may result in space charge effects creating a variation between the measured and actual potential.
This can be avoided by selecting probe wire of diameter much smaller than an electron gyroradius.
Selected Relevant Publications
Haas, J.M., and Gallimore, A.D., "Internal Plasma Potential Profiles in a Laboratory-Model Hall Thruster," Physics of Plasmas, Vol. 8, No. 2, pp. 652-660, February 2001.
King, L.B., Gallimore, A.D., and Marrese, C.M., "Transport Property Measurements in the Plume of an SPT-100 Hall Thruster," Journal of Propulsion and Power, Vol. 14, No. 3, May-June 1998, 327- 335.
Reid, B. M., "The Influence of Neutral Flow Rate in the Operation of Hall Thrusters,"
Ph.D. Dissertation, University of Michigan, 2008.
R. Shastry, A. D. Gallimore, and R. R. Hofer, "Near-Wall Plasma Characterization of a 6-kW Hall Thruster,"
IEPC-2009-133, 31st International Electric Propulsion Conference, University of Michigan, Ann Arbor, Michigan, USA September 20–24, 2009.