Edge MHD Mode Regulation by Plasma-Wall Coupling and Sawtooth Suppression by Flux-Pumping*

The tangential multi-energy extreme ultraviolet and soft x-ray (ME-EUV/SXR) diagnostic system has been installed on the High Beta Tokamak-Extended Pulse (HBT-EP) device for measurements of electron temperature and study of mode dynamics [1]. Using this system, this study examines the mechanisms underlying sawtooth suppression on HBT-EP [2]. It is observed that strong-intensity sawtooth activities correlate with reduced-amplitude m/n=3/1 external kink modes, while sawtooth suppression correlates with larger and saturated MHD activities. By manipulating the plasma-wall coupling via adjusting the positions of the conducting walls in HBT-EP, it was found that strong sawtooth events occur when the normalized wall radius b/a is within a critical value. Even slight variations in the wall location result in significantly different discharge styles, categorized as “sawtoothing discharges” and “sawtooth-suppressed discharges” respectively. Through a series of mode structure analyses, we confirm the coexistence and coupling of the m/n=1/1 helical core, m/n=2/1 tearing mode, and m/n=3/1 external kink mode during sawtooth-suppression, and that this coupling induces anomalous current broadening. Based on these findings, we conclude that sawtooth suppression in the HBT-EP tokamak is consistent with the process of magnetic flux pumping.

[1] Boting Li et al Rev. Sci. Instrum. 1 October 2023; 94 (10): 103503.

[2] Boting Li et al 2024 Nucl. Fusion 64 046020.