Sawtooth Suppression by Flux Pumping on HBT-EP*

We report systematic investigation of sawtooth suppression using a unique adjustable wall system in the High Beta Tokamak-Extended Pulse (HBT-EP) device. When the normalized wall radius is within a critical value, the amplitude of the m/n = 3/1 external kink mode is eliminated or significantly reduced and large sawtooth events occur. In contrast, as the plasma-wall distance increases, MHD activity increases and sawtooth events are significantly suppressed. 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. The observed correlation between MHD activity and sawtooth suppression is clear and consistent with the role of magnetic flux pumping as a mechanism for sawtooth suppression [1]. Central to the flux pumping mechanism is the dynamo effect, which leads to the flattening and broadening of the current profile, thereby suppressing sawtooth events. Even minor variations in plasma-wall coupling lead to clear change in the intensity of the dynamo effect, resulting in significantly different discharge styles, categorized either as ‘sawtoothing discharges’ or as ‘sawtooth-suppressed discharges' [2]. These observations demonstrate the potential to achieve flux-pumping and suppress sawtooth events via external mode control. Given that flux-pumping is a promising route to sawtooth suppression, which is required for the favorable confinement and stability properties of the Hybrid scenario [3], these controlled experiments provide valuable insights for sawtooth control strategies in advanced scenarios for future burning plasma experiments like ITER.

[1] I. Krebs et al 2017 Phys. Plasmas 24 102511.

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

[3] C. C. Petty et al 2009 Phys. Rev. Lett. 102, 045005.