Deuterium shattered pellet assimilation in DIII-D

In DIII-D, radial transport, MHD growth, and intrinsic impurity species radiation are found to be important in determining the disruption dynamics following D₂ shattered pellet injection (SPI). SPI with hydrogen species is an essential capability for disruption mitigation in ITER, to reduce runaway electron generation by maximizing particle assimilation and reducing hot-tail seed generation. Particle flux and profile measurements indicate significant radial particle transport occurs during the pre-thermal-quench (TQ) and current quench (CQ) of D₂ SPI shutdowns. Radial penetration is deeper for bigger pellets, significantly shortening the pre-TQ. When assimilation rates are insufficient to directly initiate a CQ, the pre-TQ timescales are found to be determined by the growth of n=1 MHD instabilities. TQ onset, and the variability in this timescale, is consistent with existing thresholds for non-SPI disruptions due to locked modes [1]. Subsequent CQ rates are dependent on carbon impurity levels, suggesting that sputtering sources may become important if additional radiating impurities are not injected. 

[1] P. de Vries, et al. Nucl. Fusion 56 (2016) 026007