Electron Retarding Potential Analyzer (ERPA)

ERPA is a retarding potential analyzer that measures current by collecting ambient electrons using a sweep voltage in order to repel electrons below a certain energy and accept electrons with energy greater.  The measured current can be used to determine electron temperature.  ERPA has been a standard instrument for many missions including all of those described here.

photos by Hyomin Kim

For more information about the ERPA see Mark Widholm's description at: