Abstract
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.
Highlights
The anomalous transport of fast ions (FIs) caused by magnetohydrodynamic (MHD) instabilities has been studied widely in magnetic fusion energy research.1–3 Magnetic islands formed by MHD modes break the magnetic topology and trigger the transport
Measurements of the non-inductive current by using a neutral beam current drive in the DIII-D tokamak have shown that the current is reduced considerably in the core region during the neoclassical tearing mode (NTM) burst, which implies that NTMs contribute to FI transport
The effect of the finite orbit width of an energetic particle is important for the transport of beam ion (BI), giving rise to additional drift islands of ions in phase space
Summary
The anomalous transport of fast ions (FIs) caused by magnetohydrodynamic (MHD) instabilities has been studied widely in magnetic fusion energy research. Magnetic islands formed by MHD modes break the magnetic topology and trigger the transport. The radial drift of trapped FIs in the presence of an NTM was demonstrated to model the losses of trapped FIs in the AUG tokamak and found that the significant variation of the parallel velocity of trapped particle due to the magnetic field was important for the transport.. The dynamics of passing FIs in low-frequency modes including the effect of the finite orbit width were studied to obtain drift islands of these ions instead of magnetic. When the NTM amplitude is large enough, the produced drift islands can lead to orbit stochasticity to cause the transport of these particles. The corresponding FI loss was investigated numerically in the presence of an NTM and toroidal-field ripple in the EAST..
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