Effective Runaway Electron Mitigation through MHD Instabilities and Passive Coils in J-TEXT Tokamak

Disruptions and runaway electrons (REs) pose a significant challenge to the safety and stability of tokamak-based fusion reactors. The passive coil has been proposed as a strategy to mitigate RE generation during the current quench phase of tokamak disruptions. Recently, the J-TEXT team installed a helical coil in the tokamak to test the concept of runaway electron mitigation coil (REMC) in their disruption experiments. When the coil was activated, a higher likelihood of suppressing the RE current plateau was observed, indicating the coil’s effectiveness. However, simulations using NIMROD and DREAM suggested that the coil’s perturbed magnetic fields alone were insufficient to form large magnetic islands—due to a lack of resonances—or to significantly diffuse REs, which contradicts the experimental results.

In this work, we employed the M3D-C1 code to perform a self-consistent simulation of J-TEXT disruptions in the presence of REMC, incorporating both RE current generation and its interaction with MHD modes. To accurately represent the helical coil in J-TEXT, the resistive wall model in M3D-C1 was upgraded to simulate a low-resistivity helical channel within the wall. The simulations reveal that, although the magnetic perturbations generated by the passive coil alone are insufficient to produce large magnetic islands or induce global field stochasticity, they can serve as seeds for MHD instabilities. Accurately modeling this process requires capturing the complex coupling among the RE current, MHD activity, and the passive coil. In scenarios with a peaked RE current profile, a cascade of MHD instabilities with various mode numbers (m,n) can be triggered by the perturbation fields from the coils. The resulting MHD-driven island overlap leads to stochastic magnetic fields that play a crucial role in transporting REs from the core to the wall.

These results indicate that the effectiveness of REMC during the current quench should be evaluated using a self-consistent approach that couples MHD and RE modeling, as the growth of RE current can significantly influence MHD stability characteristics.