Interpretative 3D MHD modelling of deuterium shattered pellet injection into a JET H-mode plasma
The pre-thermal quench (pre-TQ) dynamics of a pure deuterium (D2) shattered pellet injection (SPI) into a 3 MA/7MJ JET H-mode plasma is studied via 3D non-linear MHD modelling with the JOREK code. The interpretative modelling captures the overall evolution of the measured density and radiated power. The simulations also identify the importance of the drifts of ablation plasmoids towards the tokamak low field side (LFS) and the existence of background impurities in fragment penetration, assimilation, radiative cooling and MHD activity in D2 SPI experiments. It is found that plasmoid drifts lead to an about 70% reduction of the central line integrated density in the JET D2 SPI discharge considered. Background impurities are shown to dominate the radiation in the considered discharge, with tungsten being the main pre-SPI contributor and neon the post-SPI contributor given its ability to radiate more strongly at lower temperature. With inputs from JOREK simulations, modelling with a Lagrangian particle-based pellet code PELOTON reproduces the deviation of the SPI fragments in the direction of the major radius as observed by the fast camera. This confirms the role of rocket effects and thus plasmoid drifts in the considered discharge and reinforces the validity of the JOREK modelling. The limited core density rise due to plasmoid drifts and the strong radiative cooling and MHD activity with background impurities (depending on impurity species and concentration) could limit the effectiveness of LFS D2 SPI in runaway electron avoidance and are worth considering in the design of the ITER disruption mitigation system.