The effect of plasma toroidal rotation on n = 1 resonant magnetic perturbation field penetration under low neutral beam injection torque in EAST

An experiment was conducted to study the mode penetration of $n = 1$ resonant magnetic perturbation (RMP) in EAST under low neutral beam injection torque in the co-current direction (Co-NBI). The experimental results indicate that the threshold current ${I_{{\text{RMP,th}}}}$ for field penetration decreases with higher input torque ${T_{{\text{NBI}}}}$ . Furthermore, it is observed that the plasma mode frequency $| {{f_{{\text{MHD}}}}} |$ at counter-current direction is greatly reduced when the plasma toroidal rotation frequency ${f_\phi }$ increases. The theoretical scaling of mode frequency $( {{I_{{\text{RMP,th}}}} \propto {{| {{f_{{\text{MHD}}}}} |}^{0.70}}} )$ predicted by the field penetration theory is in good agreement with the experimental observation ( ${I_{{\text{RMP,th}}}} \propto {| {{f_{{\text{MHD}}}}} |^{0.53}}$ ). The role of $| {{f_{{\text{MHD}}}}} |$ and ${f_\phi }$ on the mode onset threshold was separately investigated using the full toroidal geometry initial value code MARS-Q (Liu et al 2013 Phys. Plasmas 20 042503). The numerical scaling based on the experimental mode frequency is consistent with the experimental and theoretical ones. Numerical results suggest that evaluating the total mode frequency $| {{f_{{\text{MHD}}}}} |$ is crucial in field penetration analysis, in contrast to toroidal rotation frequency ${f_\phi }$ . With the increase of ${T_{{\text{NBI}}}}$ , the decreasing $| {{f_{{\text{MHD}}}}} |$ leads to a reduction in the field penetration threshold. This suggests that more attention should be paid to error field tolerance in low Co-NBI torque scenarios, where the electron diamagnetic frequency may be canceled out by NBI-driven toroidal plasma rotation.