Particle Balance of Deuterium During Deuterium Shattered Pellet Injection Shutdown in DIII-D

Shattered pellet injection (SPI) is the primary disruption mitigation technique chosen by ITER due to its higher rate of particle assimilation compared to other techniques such as Massive Gas Injection. Deuterium SPI is particularly important for ITER as it lowers the electron temperature to avoid runaway electron hot-tail formation and induces a complete shutdown of the plasma. Previous DIII-D studies have estimated the assimilation rate of injected D₂ to be less than 20% during the middle of a current quench (CQ), which is concerningly low. Therefore, a particle balance analysis, as detailed in this presentation, was performed to determine the D₂ sources and sinks within the vessel. The ionization source term and particle fluxes to the vessel wall were measured using absolutely calibrated D-alpha brightness and Langmuir probes. Using S/XB ~ 20 as determined from PrismSPECT simulations and ADAS, most of the injected deuterium appears to be completely ionized after entering the vessel. During the thermal quench (TQ), D₂ particles move from the divertors to the center post and wall recycling occurs rapidly. The particle flux to outer midplane walls is negligible but is accompanied by hot plasma filaments with T_e exceeding 100 eV. During the CQ, D-alpha emissions are dominated by volume recombination rather than wall recycling. Evidence of toroidal asymmetries during the shutdown also appears in electron density and poloidal magnetic field measurements, which can partially account for the missing D₂ inventory.