On the role of preexisting MHD activity for the plasma response to massive deuterium injection

As part of a reliable disruption mitigation system (SPI) for ITER, pure deuterium shattered pellet injection (SPI) has been proposed as a way of avoiding hot tail runaway electron generation. It offers the possibility of diluting the plasma and, thereby, cooling it down by a large factor without immediately triggering a thermal quench (TQ). However, the reliability of this and similar SPI approaches could be reduced by preexisting MHD modes, which are usually present during the pre-TQ phase, when the disruption mitigation scheme is being triggered. To address this question, this theoretical study investigates massive deuterium injection into an MHD active ASDEX Upgrade plasma using the non-linear MHD code JOREK. Cases with and without preexisting 2/1 islands are studied. Scans are performed in the preexisting island size, the number of atoms injected, and the relative phase of the injection location with respect to the island. Realistic values of resistivity and heat diffusion anisotropy are considered. This provides insights into the physical mechanisms at play and the relevant time scales involved. Results largely indicate that plasma dilution by deuterium also seems to work reliably in the presence of preexisting MHD activity. Nevertheless, when injecting in phase with the X-point of a large preexisting island, the TQ can occur earlier than without. Altogether, simulations increase confidence in the reliability of plasma dilution by deuterium injection and its applicability to ITER.