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

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