Experiments Toward Understanding Impurity Assimilation During Massive Gas Injection for Disruption Mitigation in DIII-D

Impurity assimilation following massive gas injection (MGI) is desirable for collisional suppression of runaway electrons (RE). Experiments on the DIII-D tokamak have shown that impurity ions created at the plasma edge by MGI initially mix inward quite slowly toward the plasma core. When the associated cold front reaches the $q$=2 rational surface, impurity mixing is accelerated due to destabilization of low-order tearing modes, leading to the thermal quench (TQ). Average core mixing efficiencies of impurities injected into the vacuum vessel up through the TQ are of order 10{\%}. Typically, RE suppression ratios $\gamma _{crit}$~= $E_{crit}$/$E_{\vert \vert }$~$\approx $ 0.01 are obtained using argon. Better suppression ratios $\gamma _{crit}$~$\approx $ 0.06 are obtained with low-$Z$ (H$_{2}$ or He) injection and firing five MGI valves simultaneously.