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: