From injection to deposition - capturing the drift of ablated pellet material in a tokamak

Paper in proceeding, 2024

Pellet injection is an important means to fuel and control discharges and mitigate disruptions in reactor-scale fusion devices. The disruption mitigation system in ITER is based on Shattered Pellet Injections (SPI), where pellets consisting of a mixture of hydrogen (H) and neon (Ne) will be shattered against a bend at the end of the guide tube before entering the plasma [1]. Shattering the pellet enables optimising the assimilation efficiency in order to quickly reach the densities required. This is important as the density increase must be sufficient to safely dissipate the thermal and magnetic energy content of the plasma uniformly through radiation, and increase the frictional drag enough to reduce runaway electron (RE) formation.
To assess the efficacy of these applications, prediction of the drift deposition of ablated pellet material is a key ingredient. While complex modeling tools exist to this end, there is a need for reduced, but still sufficiently accurate models that can be implemented in numerical frame-works. Here we present a derivation of an equation governing the drift motion of ablation clouds, from first principles, in combination with an approximate model for the cloud expansion parallel to the magnetic field. This model has been implemented in the numerical disruption modelling tool DREAM [2], which we use to compare the simulated density evolution with experiments at the ASDEX Upgrade tokamak. Finally, we investigate the prospects for disruption mitigation
by SPI in ITER.