Designing an energy analyzer for measuring non-thermal electron energy distribution functions in the PHAse space MApping (PHASMA) experiment

Retarding field energy analyzers (RFEAs) measure the energy distribution function (EDF) of a charged species in a plasma. We present the design of an RFEA whose intended use is to measure the EDFs of non-thermal electrons along the axial magnetic field during electron-only magnetic reconnection in the PHAse Space MApping (PHASMA) experiment. The design considers a combination of transparency and space-charge effects described here. Though PHASMA's parallel Thomson scattering system could be expected to provide the same measurement as the RFEA, the measured photon counts from the tail of the electron velocity distribution function measured along the axial magnetic field are dominated by line emission from impurities in the experiment and statistical noise. The RFEA design presented here enables measurements of the EDFs of electrons in the tail.