Influence of 3D helical magnetic perturbations on runaway electron generation in J-TEXT tokamak

A large number of runaway electrons (REs) generated during disruption can cause significant damage to next-generation large-scale tokamaks. The influence of three-dimensional (3D) helical magnetic perturbations on the suppression of RE generation was explored using a set of 3D helical coils in J-TEXT tokamak, which can excite m/n = −2/2 helical magnetic perturbations. Experimental evidence shows that the −2/2 magnetic perturbations caused by the opposite coil current direct plasma toward the high-field side, simultaneously enhancing the magnetic fluctuations, which would enhance the radial loss of REs and even prevent RE generation. On the other hand, −2/2 magnetic perturbations can also reduce the cooling time during the disruption phase and generate a population of high-energy REs, which can interact with high-frequency magnetic fluctuations and in turn suppress RE generation. The critical helical coil current was found to correlate with electron density, requiring higher coil currents at higher densities. According to the statistical analysis of RE generation at different electron densities, the applied −2/2 magnetic perturbations can increase the magnetic fluctuations to the same level at lower electron densities, which can decrease the threshold electron density for RE suppression. This will be beneficial for RE mitigation in future large tokamak devices.