Development of a Retarding Field Energy Analyzer for Ion Energy Distribution Measurements in the DIII-D Divertor

A retarding field energy analyzer (RFEA) system is currently under development for the DIII-D divertor. The RFEA is designed to measure main ion temperature (T_i) at the divertor target ranging from 10 eV to 200 eV, under typical density of n_e ~ e¹⁹ m^-3 and high parallel heat flux ~ 100 MW/m², at ~ 1 ms temporal and ~ 1 mm spatial resolutions.The initial testing with a prototype probe will be accomplished at the lower divertor with the Divertor Material Evaluation System (DiMES), where the probe will be facing the “upstream” field-aligned plasma flow. With refined probe geometry and material choice, some of the critical issues of RFEA were evaluated and addressed by testing with compact plasma source and 3D simulations of fields, particles, and heat flux, including space-charge, distortion of the ion distribution function, and the heat load on the structure where it is shown by the simulation that surface temperature on the entrance plate rises < 3000 K during 5s operation with local peak heat flux ~ 100 MW/m². The long-term objective is to develop a permanent RFEA diagnostic array capable of providing reliable T_i readings in the SOL, which can be utilized in the studies of heat flux management, divertor detachment, and impurity transport.