Impact of ELM control techniques on tungsten sputtering in the DIII-D divertor and extrapolations to ITER

TL;DR

This study tests a model predicting tungsten erosion during ELMs in tokamaks, compares it with experiments, and extrapolates findings to ITER, highlighting the effects of different control techniques on tungsten sputtering.

Contribution

The paper introduces a validated model for tungsten erosion during ELMs and assesses the impact of various control methods, providing predictions for ITER conditions.

Findings

Pellet pacing reduces W sputtering by flushing C impurities.

RMPs do not reduce W sputtering as predicted by the model.

Impurities like N or Ne can significantly increase W erosion in ITER.

Abstract

The free-streaming plus recycling model (FSRM) has recently been developed to understand and predict tungsten gross erosion rates from the divertor during edge localized modes (ELMs). In this work, the FSRM was tested against experimental measurements of W sputtering during ELMs, conducted via fast WI spectroscopy. Good agreement is observed using a variety of controlling techniques, including gas puffing, neutral beam heating, and plasma shaping to modify the pedestal stability boundary and thus the ELM behavior. ELM mitigation by pellet pacing was observed to strongly reduce W sputtering by flushing C impurities from the pedestal and reducing the divertor target electron temperature. No reduction of W sputtering was observed during the application of resonant magnetic perturbations (RMPs), in contrast to the prediction of the FSRM. Potential sources of this discrepancy are discussed. Finally, the framework of the FSRM is utilized to predict intra-ELM W sputtering rates in ITER. It is concluded that W erosion during ELMs in ITER will be caused mainly by free-streaming fuel ions, but free-streaming seeded impurities (N or Ne) may increase the erosion rate significantly if present in the pedestal at even the 1% level. Impurity recycling is not expected to cause significant W erosion in ITER due to the very low target electron temperature.