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Plasma Diagnostics

Faraday Probe

An electrostatic probe comprised of an exposed electrode (often guarded) that is used to measure thruster beam current density.


Figure 1. Schematic (upper) and photo of the AFRL nested Faraday probe (from ref. 3).

Figure 2. Photograph of the JPL-style nude Faraday probe.

The Faraday probe is a diagnostic tool used to measure the ion current density distribution of plasma. While simple in principle and in implementation, in actuality, Faraday probe ion current measurements are extremely difficult to conduct accurately. There are several types of Faraday probes including nude, cupped, collimated, gridded, and a recent PEPL development, magnetically filtered probes. A standard Faraday probe (or nude probe) is biased below plasma potential to ensure that plasma electrons are repelled (Figures 1-3). Thus, a good estimate/measurement of electron temperature and plasma potential is essential to any good Faraday probe survey. Ions flowing to the probe create a current that can be measured to find the ion current density. In situations where the pressure in the vacuum chamber is too high, low-energy ions with random velocities born from charge-exchange (CEX) collisions can be attracted to the probe due to the negative probe bias. Excessive probe collection of CEX ions will inflate the predicted beam ion current density, especially at large off-axis angles where few beam ions reach. In such cases, a collimated or gridded Faraday probe is often used. A collimated Faraday probe is very similar to the nude probe except that the probe is now encased in a long drift tube with a hole in the collection end. The encasement only allows the ions of a specific velocity vector to reach the nude Faraday probe. The collimated probe theoretically acts as a filter for the low-energy, random ions. Gridded probes, a favorite among Russian plume specialists, use a low-voltage ion-repelling grid to filter out low-energy CEX ions. PEPL's magnetically-filtered probe works much the same way except now a drift tube with a strong transverse magnetic field is used to deflect low-energy ions out of the intercept ellipse of the Faraday probe.


Figure 3. Schematics of the JPL style (left) and GRC style nude Faraday probes (from ref. 3).

 

Selected Relevant Publications

  1. Hofer, R. R., Walker, M. L. R., Gallimore, A. D., "A Comparison of Nude and Collimated Faraday Probes for Use with Hall Thrusters," IEPC-01-020, 27th International Electric Propulsion Conference, Pasadena, CA, October 15-19, 2001.
  2. Walker, M. L. R., Hofer, R. R., Gallimore, A. D., "The Effects of Nude Faraday Probe Design and Vacuum Facility Backpressure on the Measured Ion Current Density Profile of Hall Thruster Plumes," AIAA-2002-4253, 38th Joint Propulsion Conference, Indianapolis, IN, July 7-10, 2002.
  3. Brown, D. L., "Investigation of Low Discharge Voltage Hall Thruster Characteristics and Evaluation of Loss Mechanisms," Ph.D. Dissertation, University of Michigan, 2009.
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