Neutron-yield reduction in sawtooth crashes from n = 2 resonant magnetic perturbations in EAST

Abstract

A significant reduction in neutron yield in the plasma core has been observed in EAST neutral beam injection into plasmas with a low-edge safe factor of q95=3.5 and a n = 2 resonant magnetic perturbation (RMP) during a sawtooth crash. Harmonics of the m/n = 1/1 mode (m and n being the poloidal and toroidal mode numbers) develop simultaneously. A sawtooth crash is triggered when the current of the RMP exceeds a critical value; saturated fishbone phase occurs with small RMP currents. Soft x-ray (SXR) tomographic patterns obtained during sawtooth crashes have annular hot-ring shapes outside the q = 1 surface that are similar to those of double tearing modes triggered by off-axis sawtooth crashes in TFTR reversed magnetic shear plasma. M3D nonlinear simulations confirmed the initial penetration of the n = 2 RMP in the plasma core and further demonstrated a complete sawtooth crash with the appearance of a right annular hot ring near the q = 1 surface. Reconstruction of the trajectory of SXR signals before the crash shows chaotic behavior. Moreover, a high-frequency mode was seen to appear just before the sawtooth crash, accompanied by an edge geodesic acoustic mode. This edge mode is free in the saturated fishbone phase. The role of high-frequency mode in the sawtooth crash remains unclear.