Characterizing the effect of various D₂ pellets and gas insertion frequencies on W divertor erosion in DIII-D

Inter- and intra-ELM tungsten erosion during D₂ pellet injection has been assessed from the DIII-D Metal Rings Campaign, where toroidally symmetric W-coated tiles were installed in the lower open divertor. Injecting D₂ pellets triggers smaller, more frequent ELMs, while the addition of neutrals increases the ion flux to the divertor but decreases the average ion impact energy. Results show a 32% increase in the total gross intra-ELM W erosion rate with 20-40 Hz pellet injection frequencies relative to both the no-pellet case and to a nominal injection frequency of 60 Hz. An inverse correlation between inter-ELM density and W erosion during D₂ pellet injection is also observed. Simulations predicted by the ‘free-streaming plus recycling model’ (FSRM) under and over-predict the average W erosion per ELM by approximately 25% when incorporating and excluding a C/W mixed material model, respectively. Experiments are planned to inject D₂ pellets and puff D₂ gas at the same rate into the W-coated Small Angle Slot (SAS) tightly baffled closed upper divertor to distinguish the effect that adding cold neutrals and triggering smaller, more frequent ELMs has on W erosion. This study presents the use of collector probes and ultraviolet (UV) spectroscopy to investigate net W erosion as well.