Effectiveness of ELM mitigation techniques in reducing intra-ELM and inter-ELM tungsten erosion rates in DIII-D

We analyze ELM erosion that occurred to tungsten (W)-coated tiles installed in DIII-D during the Metal Rings Campaign (MRC) using WI filterscopes and Langmuir probes [1]. This study assesses the effectiveness of three ELM mitigation techniques used in attached plasma conditions: QH-mode plasmas, D₂ pellet injection, and resonant magnetic perturbations (RMPs). Most notably, QH-mode plasmas during the MRC avoided ELMs and reduced total W divertor erosion rate by 18% when compared to an ELMy H-mode plasma, and increased core stored energy by 20%. Fast pellet pacing at 60 Hz and no pellet injection resulted in a similar total W erosion rate, but injection at 60 Hz reduced the average erosion per ELM and fractional carbon impurities at the top of the pedestal by 47% and 36%, respectively. While RMPs had no conclusive effect on erosion, results show that higher injected powers correlated with more erosion. On average, simulations of intra-ELM erosion predicted by the ‘free-streaming plus recycling model’ (FSRM) [1] overestimate experimental measurements of intra-ELM W erosion during RMPs by a factor of 2 and underestimate W erosion during pellet injection by a factor of 0.5. The reasons for these discrepancies may have to do with a mixed-material C/W layer on the W tiles.