Retarding potential analyzer: Principles, designs, and space applications

The main emphasis of this paper is on the use of Retarding Potential Analyzers (RPAs) for measuring the electron and ion distribution functions, density, and temperature of space plasmas and the charging level of spacecraft surfaces interacting with the plasmas. Multiple grids of progressively negative potential are used to suppress the secondary electrons produced at the entrance of an RPA. We point out that it is impossible to achieve complete suppression. The secondary electrons collected by the receiver may produce a spurious lump in the distribution measured. Improved designs and space applications for spacecraft charging and ionic liquid ion beam diagnostics are discussed. Spacecraft charging will be very important for planetary explorations, whereas ionic liquid ion beams will be important for spacecraft propulsion in the future.