New Retarding Field Energy Analyzer for Ion Temperature and Energy Distribution Measurements at the DIII-D Lower Divertor*

A prototype retarding field energy analyzer (RFEA) probe has been developed for the DIII-D lower divertor, using the Divertor Material Evaluation System (DiMES), to measure the ion temperature (T_i) from 10 to 200 eV and the ion energy distribution. The divertor plasma conditions introduce unique engineering and interpretation challenges for the RFEA. The heat load (~ 100 MW/m²) and Debye length (~ 10 µm) necessitate compact RFEA cavity geometry comparable to the ion gyroradius (~ 1 mm), and narrow particle entrances (~ 10 µm). Thus, small signal level and significant distortion of the energy distribution of the collected ions are anticipated. Therefore, multiple novel solutions have been adopted, including a unique multi-slit tungsten entrance, which improves ion collection while enduring the extreme heat flux, achieved by laser micro-machining, and bi-directional voltage sweeping (± 500 V) along with a new data interpretation algorithm utilizing 3-D particle-in-cell (PIC) simulations of the full probe cavity geometry to account for ion losses due to space charge and internal geometry. We have shown that T_i can be measured around 10% error from synthetic signals with Maxwellian and near-Maxwellian ions, and the reconstruction of ion energy distribution was reasonably consistent.