Enhanced tungsten prompt re-deposition during ELMs in the DIII-D divertor*

Recent measurements conducted in the DIII-D divertor indicate a substantially higher tungsten prompt re-deposition fraction, $f_{redep} $, occurs in H-mode discharges with edge localized modes (ELMs) relative to L-mode discharges. A value of $f_{redep} $ of \textasciitilde 0.72 was inferred on a 15 mm diameter W-coated DiMES sample exposed to 3 repeat L-mode discharges, whereas an $f_{redep} $ value of \textasciitilde 0.88 was calculated on an identical sample exposed to one H-mode discharge and 2 repeat L-mode shots. This result is interpreted using the recently developed free-streaming plus recycling model (FSRM), extended to include the effect of W prompt re-deposition using analytic geometric approximations. The FSRM predicts that $f_{redep} $ should increase from \textasciitilde 0.4 in the inter-ELM phase to 0.9 during the peak of the intra-ELM phase for strongly attached DIII-D discharges, and from \textasciitilde 0.7 to \textasciitilde 0.95 for a typical detached divertor case. This increase in $f_{redep} $ is due to the strong increase in divertor electron density, and correspondingly shorter W ionization mean free path during the ELM. This work represents the first experimental evidence of enhanced prompt re-deposition during ELMs and is a promising result for ITER where nearly all the W sputtered near the strike-point will be during ELMs. Additional ELM-resolved W re-deposition measurements using spectroscopic interpretation of WI/WII line ratios will also be presented.