Simulation of the loss of passing fast ions induced by magnetic islands in EAST tokamak plasmas

The loss of beam ions due to magnetic islands is investigated in a tokamak. The perturbed guiding-center drifts of passing particles including the effect of the finite orbit width are demonstrated. The widths of the drift islands under resonant conditions are studied theoretically and numerically. The ORBIT code is used to simulate the action of the neoclassical tearing mode with a toroidal mode number n = 1 and poloidal mode number m = 2 on passing fast ions generated by neutral beam injection in the Experimental Advanced Superconducting Tokamak. Two loss channels for passing fast ions are identified as the resonant interaction and the stochastic interaction. The lost fast ions in the loss detector zone (LDZ) to simulate the fast-ion loss detector assemble around two regions in phase space, namely, (i) a pitch angle of θ = 28° both with and without the mode and (ii) θ = 59° when the mode amplitude is large enough, where θ=arccosv∥/v. The number of these lost ions in the LDZ evolves in the period of the mode. The fraction of the total lost ions evolves in the period of the n = 1 oscillation in the toroidal direction. The fraction of lost beam ions has a linear relationship with the mode amplitude in first 10 µs and a quadratic one thereafter. The corresponding characteristics of the lost beam ions in phase space are also discussed.