The University of Michigan
Department of Aerospace Engineering
| Plasmadynamics & Electric Propulsion Laboratory |
Plasma Diagnostics

Retarding Potential Analyzer (RPA)

An electrostatic probe that uses a series of grids to measure the ion energy distribution.

Figure 1. Cross-sectional illustration of a RPA showing the multi-grid design and particle filtration process (from ref. 5).

Figure 2. Photograph of a RPA in VF12. A movable shutter was used to protect the RPA grids when the probe was not being used. Before testing, a laser was used to align the RPA with respect to the NASA-173Mv2. (From ref. 4).

Figure 3. PEPL's micro RPA version 2 (MRPAv2, with 19 mm outer casing diameter) photograph (upper) and schematic (lower). (From ref. 6).

The Retarding Potential Analyzer diagnostic uses a series of grids to selectively filter ions and determine the ion energy distribution. The outer grid (Grid 1, see Figure 1) exposed to the plume is floating to minimize the perturbation between the probe and ambient plasma. Grid 1 also provides additional attenuation of the plasma, which leads to reduced number density and increases the internal probe Debye length. The second grid acts to repel essentially all of the plasma electrons. The third grid is used to retard the ions so that only the ions with energy-to-charge ratios greater than the grid voltage can pass through the retarding grid and reach the collector. The voltage of the retarding grid can be varied to determine the characteristic. The derivative of the current-voltage characteristic is proportional to the ion energy distribution. Sometimes a fourth grid, between the ion retarding grid and the collector is used to suppress secondary emission electrons that emanate from the first two grids and the collector. These secondary emission electrons can alter the true ion current measurement.


Selected Relevant Publications

  1. 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.
  2. King, L. B., "Transport-property and Mass Spectral Measurements in the Plasma Exhaust Plume of a Hall-effect Space Propulsion System," Ph.D. Dissertation, University of Michigan, 1998.
  3. Beal, B. E., and Gallimore, A. D., "Energy Analysis of a Hall Thruster Cluster," IEPC-2003-035, 29th International Electric Propulsion Conference, Toulouse, France, March 17-20, 2003.
  4. Hofer, R. R., "Development and Characterization of High-Efficiency, High-Specific Impulse Xenon Hall Thrusters," Ph.D. Dissertation, University of Michigan, 2004.
  5. Brown, D. L., "Investigation of Low Discharge Voltage Hall Thruster Characteristics and Evaluation of Loss Mechanisms," Ph.D. Dissertation, University of Michigan, 2009.
  6. Lemmer, K. M., "Use of a Helicon Source for Development of a Re-Entry Blackout Amelioration System," Ph.D. Dissertation, University of Michigan, 2009.
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