Characterization of Plasma Hitting the DIII-D Main Chamber Wall during SPI Shutdown*

The shattered pellet injection (SPI) is the primary disruption mitigation technique chosen for ITER due to a higher assimilation rate of injected material by the plasma compared to other techniques, such as massive gas injection (MGI). However, previous experiments on DIII-D using a lower-single-null H-mode configuration have estimated the assimilation rate of injected deuterium to be 20 to 30% during the middle of a current quench (CQ), which is considerably below 100% and the expected assimilation rates from energy balance. To understand this discrepancy, particle balance was estimated by utilizing absolutely calibrated H-alpha brightness for the ionization source term, interferometers for the in-plasma electron number, and a midplane probe for particle flux to the wall. Although the particle flux to the wall exhibits significant error bars, it cannot be neglected as the main chamber particle sink seems to be larger than 10% of the ionization source term. In addition, the plasma hits the main chamber wall throughout the disruption, with the largest impact occurring near the start of the CQ. The midplane particle flux is roughly consistent with the calculated ExB particle flux during the CQ but appears significantly larger than the ExB particle flux during the thermal quench (TQ).