Thermal Quench Dynamics of Single Shattered Pellet Injections on JET*

Shattered pellet injection (SPI) is the chosen disruption mitigation technique for ITER, with development and validation driven by the ITER disruption mitigation taskforce and the EUROfusion WPTE program. This work presents results from SPI experiments on JET, focusing on thermal quench (TQ) dynamics in plasmas with up to 8MJ of stored thermal energy, the highest in disruption mitigation studies. Noteworthy elements include seeded plasmas and weakly Ne-doped (<1%) pellets to enhance assimilation.

Thomson scattering measurements during ablation, alongside pellet trajectories inferred from a fast filtered camera, show higher velocities and Ne doping (0.1%) can suppress pellet drift. This demonstrates that radiation cooling could mitigate plasmoid drift, addressing low mass assimilation on ITER.

A significant finding is the reduction in pre-TQ duration with pure deuterium (D) pellets dropped from over 100ms in unseeded plasmas to under 10ms with seeding. This indicates that staggered (H + Ne/H) pellet injections on ITER must be quicker than anticipated.

Additionally, TQ mitigation using only pure D SPI by leveraging seeded impurities was explored. This approach offers benefits such as reduced runaway electron generation via increased density and lower Ne content, along with longer TQ durations to decrease heat flux. Pure D pellets achieved up to 85% of the radiated energies seen with 10% Ne pellets and significantly extended TQ durations.

These findings underscore the importance of seeded impurities and weakly Ne-doped pellets in optimizing the disruption mitigation sequence.