Description
ITER will rely on shattered pellet injection (SPI) to mitigate the damaging consequences of plasma disruptions. Its disruption mitigation system (DMS) is designed to support various injection schemes and geometries to maximise material assimilation across the evolving plasma conditions during a disruption. The SPI system at ASDEX Upgrade has been adapted for the recent campaign to test a variety of injection schemes and to assess post thermal quench (TQ) injections which mimic the limited fragment plume-plasma intersection as expected for upper port injection in ITER. Experiments show that post-TQ injections suffer from weak fragment ablation due to the low plasma temperature, reducing densification and acceleration of the current quench (CQ) for minimising electromagnetic loads. In ohmic density limit disruptions, neon assimilation decreases by ~50% when fragments arrive 2 ms into the CQ compared with pre-TQ injection, leading to only partially mitigated CQs due to the comparatively low plasma resistivity.
Early deuterium injection aimed at avoiding the triggering of MHD activity and subsequent global reconnection event can dilute the plasma and reduce hot-tail runaway seeds. However, initial assimilation remains low (≈20%) and is further reduced by transport losses before TQ onset. Adding neon improves assimilation without shortening the pre-TQ phase to below the injection duration. Although smaller and faster fragments in general lead to a steeper density rise, their preferential edge deposition increases material losses by the time of the TQ. Intrinsic impurities, particularly tungsten in degraded H-mode plasmas, strongly influence mitigation performance. Longer pre-TQ phases and lower temperatures enhance fragment penetration and assimilation, and the presence of intrinsic impurities relaxes CQ mitigation requirements. Multiple Ne/D SPI has revealed that the assimilation deteriorates if the fragment plumes arrive more than 1 ms apart, which can be attributed to the already initiated plasma cooling. This imposes a strict requirement on the allowable fragment arrival jitter when material must be injected from several locations.
This contribution summarises results from recent ASDEX Upgrade SPI experiments and their implications for ITER’s disruption mitigation strategy.