Fuel Retention and Removal from the Carbon First-Wall in DIII-D

Experiments to determine the short-term retention of deuterium fuel (D) in a graphite first wall are done using a global particle balance. The global particle balance is calculated continuously through a discharge and shows a majority of the D wall retention occurs during the initial ohmic and L-mode phases. Typical wall uptake rates in these phases are $\sim$30$\pm$3.5~Torr-L/s which is $\sim$2\% of the measured divertor ion flux. The continuous global particle balance is compared with a shot-integrated balance, and they agree to $\sim$5\%. During the H-mode phase of typical ELM-y discharges, the uptake is near zero ($\sim \pm$5~Torr-L/s), which is $\leq$0.2\% of divertor ion flux. This fluctuating retention rate leads to a wall inventory reduction during the H-mode phase that can be as much as 100\% though typically $\sim$20-30\%. A vacuum bake of first wall at 350$^{\circ}$C after plasma operations recovered $\sim$80\% of the retained D, and left $\sim$9\% of the total fuel injected during the run-day in the first wall.