The avalanche of runaway electrons (REs) during ITER disruptions could potentially generate several megaamperes of RE current during the current quench (CQ) process which might damage the plasma-facing components. Previous studies have suggested that avoiding the formation of such a large RE current would be difficult [1]. However, before the number of REs increases to a large value, some REs might be lost due to magnetic stochasticity from the growth of MHD instabilities such as tearing modes [2]. In our work, MHD-induced RE transport is modeled with the JOREK code, using a 3D reduced MHD model and a fluid description of RE avalanche and transport. The stability of the tearing modes is primarily determined by the current density profile which is uncertain in the plasma after the thermal quench (TQ). The scan over various current density profiles has revealed that the flattening of the current profile, which is expected during TQ according to experimental data on present tokamaks, leads to a higher growth rate of edge tearing modes due to the increased current density gradient on the edge. Therefore, the CQ process under two post-TQ scenarios with peaked or flat current profiles is modeled in this work to study their MHD activity and RE transport. Under the flat profile, global magnetic stochasticity is observed when the RE current is still negligible. The consequential massive RE deconfinement essentially reduced the potential risk of an MA-scale RE current.

references

[1] O. Vallhagen et al, NF 62(11):112004

[2] L. Carbajal et al, PP 27(3):032502