Investigation on fast ion losses induced by neoclassical tearing mode in HL-3 high- β plasmas

The fast ion losses (FIL) induced by m = 2 , n = 1 neoclassical tearing mode (NTM) are investigated dedicatedly in HL-3 high- β N plasmas, where m ( n ) is the poloidal (toroidal) mode number. The spectrogram of FIL fluctuation measured by FIL detector (FILD) shows a dominant characteristic frequency of the NTM rotation frequency, indicating that the NTM rotation modulates the fast ion (FI) loss. The lost FI population peaks at pitch-angle of 112 ∘ − 118 ∘ and energy ranges of 40 − 60 keV , consistent with the full energy of neutral beam. The intensity of the FILD signal is proportional to the square of island width, implying dominantly convective FIL. In addition, the NTM island structure passes through the lost orbit of the trapped FI, which is calculated backward in time from the FILD measurement, suggesting there exists an interaction between FI orbit and the NTM island. The calculation using ORBIT code reveals the wave-particle resonant condition is fulfilled between the NTM and the trapped FIs, further confirming the convective loss mechanism. The interaction primarily affects the toroidal canonical momentum rather than the energy of FIs due to the relatively low NTM frequency. These results enhance our understanding of NTM induced FI transport in high- β plasmas, providing valuable insight for optimizing the high-performance operation scenario for future fusion reactors.