Reducing tungsten plasma-material interactions with boron and boron nitride powders in the DIII-D V-shaped divertor*

Boron (B) and boron nitride (BN) powder injections into DIII-D’s V-shaped tungsten divertor is found to curtail heat fluxes, recycling, and tungsten (W) erosion without significantly impacting H-mode plasma performance. Experiments have been conducted using the closed SAS-VW divertor incorporating a W coated outer ring in H-mode (ion B×▽B directed into the divertor) at a neutral beam power of P_NBI = 6 MW and density of 5-6 x 10¹⁹ m^-3. Injection of boron powder at rates up to 25 mg/s at the OSP in the SAS-VW divertor showed no substantial reduction in heat flux but an ~20% decrease in neutron rates and stored magnetic energy. W deposition dropped up to 50<!-- Question, perhaps, is whether a 50% drop is consistent with the drop in Te at the surface, and the subsequent drop in the W physical erosion yield? -->% while B deposition increased by over 100% for shots with <10 mg/s B powder injection, observed through MiMES and DiMES probes. BN powder, however, increased the divertor neutral pressure tenfold and reduced peak parallel heat flux by 90% near the injection point<!-- …consistent with detachment (?) -->. BN has proved to be an effective heat dissipator in both the original SAS divertor with all-carbon walls and the high-Z SAS-VW divertor, outperforming pure B powder in the closed divertor at equivalent rates [1]<!-- Is this because N is higher Z, and so a better radiator at divertor-relevant temperatures? -->. These findings confirm powder injection as a suitable method for controlling plasma-material interactions in tungsten environments in future fusion machines.