Retarding Potential Analyzer (RPA): Measures ion energy distribution

The Retarding Potential Analyzer diagnostic uses a series of grids to selectively filter ions and determine the ion energy distribution.   The first grid acts to repel essentially all of the plasma electrons.  The second 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 third 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.

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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-03-035, 29th International Electric Propulsion Conference, Toulouse, France, March 17-20, 2003.