The effect of plasmoid drifts on the pellet rocket effect in magnetic confinement fusion plasmas

We detail here a semi-analytical model for the pellet rocket effect, which describes the acceleration of pellets in a fusion plasma due to asymmetries in the heat flux reaching the pellet surface and the corresponding ablation rate. This effect was shown in experiments to significantly modify the pellet trajectory, and previously projected deceleration values of ${\sim} 10^6\,\textrm {m}\,\textrm{s}^{-2}$ for reactor-scale devices indicated that it may severely limit the effectiveness of pellet injection methods. We account for asymmetries stemming both from plasma parameter gradients and an asymmetric plasmoid shielding caused by the drift of the ionised pellet cloud. For high temperature, reactor relevant scenarios, we find a wide range of initial pellet sizes and speeds – particularly those relevant for large fragments of shattered pellet injection for disruption mitigation – where the rocket effect has a major impact on the penetration depth. In these cases, the plasma parameter profile variations dominate the rocket effect. We find that for small and fast pellets, where the rocket effect is less pronounced, plasmoid shielding-induced asymmetries dominate.