First predictive simulations for deuterium shattered pellet injection in ASDEX Upgrade

TL;DR

This study presents the first simulations of deuterium shattered pellet injection in ASDEX Upgrade, analyzing MHD activity, thermal quench dynamics, and plasma behavior to inform runaway electron mitigation strategies.

Contribution

It introduces the first predictive simulations of deuterium SPI in ASDEX Upgrade, exploring parameter effects on MHD activity, TQ dynamics, and plasma recovery, with insights into ablation and flux surface re-formation.

Findings

Thermal quenches occur rapidly, typically within 100 microseconds.

Toroidal harmonics up to n=10 influence plasma stochastization.

Post-quench density profiles become monotonic, aiding runaway electron mitigation.

Abstract

First simulations of deuterium shattered pellet injection (SPI) into an ASDEX Upgrade H-Mode plasma with the JOREK MHD code are presented. Resistivity is increased by one order of magnitude in most simulations to reduce computational costs and allow for extensive parameter scans. The effect of various physical parameters onto MHD activity and thermal quench (TQ) dynamics is studied and the influence of MHD onto ablation is shown. TQs are obtained quickly after injection in most simulations with a typical duration of 100 microseconds, which slows down at lower resistivity. Although the n=1 magnetic perturbation dominates in the simulations, toroidal harmonics up to n=10 contribute to stochastization and stochastic transport in the plasma core. The post-TQ density profile remains hollow for a few hundred microseconds. However, when flux surfaces re-form around the magnetic axis, the density has become monotonic again suggesting a beneficial behaviour for runaway electron avoidance/mitigation. With $10^{21}$ atoms injected, the TQ is typically incomplete and triggered when the shards reach the q=2 rational surface. At a larger number of injected atoms, the TQ can set in even before the shards reach this surface. For low field side injection considered here, repeated formation of outward convection cells is observed in the ablation region reducing material assimilation. Responsible is a sudden rise of pressure in the high density cloud when the stochastic region expands further releasing heat from the hot core. After the TQ, strong sheared poloidal rotation is created by Maxwell stress, which contributes to re-formation of flux surfaces.