Dissipation of post-disruption runaway electron plateaus by shattered pellet injection in DIII-D

Effective runaway electron (RE) mitigation strategies are essential for protecting ITER from the potential damage of a first wall strike. In DIII-D, shattered pellet injection (SPI) with large Ne pellets demonstrates the dissipation of post-disruption RE plateaus by collisions with high-Z impurities, while equivalently sized $D_2$ pellets lead to a reduction of the impurity content of the background plasma, reducing RE dissipation. Varying the relative quantities of $Ne/D_2$ in mixed species pellets shows that the effect of $D_2$ may be dominant in determining the RE/pellet interaction. Compared with injection of the same quantity of Ne by massive gas injection, SPI achieves a similar initial RE current decay rate, but residual RE current remains after SPI. This may be due to the effects of a small quantity of $D_2$ (used as a “shell” for firing of the Ne pellets) displacing high-Z impurities. These results will help guide the optimization of injection schemes and pellet compositions for the RE mitigation system in ITER.