Design of a Retarding Field Energy Analyzer for the Large Plasma Device

This article details the construction of a retarding field energy analyzer (RFEA) designed to measure the ion energy distribution function (IEDF) in a moderately dense laboratory plasma, such as that produced in the Large Plasma Device (LAPD). The RFEA was specifically developed to study ion acceleration in a magnetic reconnection experiment involving two kink-unstable flux ropes. It features four independently biasable grids, an electrically and thermally insulating exterior, a mesh grid stack as the entrance grid to collect more current, a solder-less design for easy assembly, and is constructed with readily available off-the-shelf materials for quick turnaround time. In this experiment, the RFEA demonstrated continuous operation for several days under LAPD conditions. It measured low-energy thermal ions with energies below 20 eV and observed field-aligned ion beams with energies between 9 and 15 eV near the reconnection region between the two ropes. For brevity, a detailed analysis of the ion beam and supporting 3-D gyrokinetic simulations are presented in a related manuscript [Tang et al., Phys. Plasmas 30, 082104 (2023)]. Ion temperature measurements of thermal ions within the flux ropes from the RFEA were consistent with those inferred from the spectroscopy of Doppler-broadened helium II spectral lines (320.3 nm).