Investigation of sawtooth oscillations and quasi-interchange mode in the experimental advanced superconducting tokamak

Abstract Saturated sawtooth oscillations of the (1,1) mode and its higher harmonics are observed in ELMy H-mode discharges at the EAST facility. Their spatial and temporal characteristics are ascertained using singular value decomposition (SVD) and short-time Fourier transform (STFT). In non-NBI heating discharges, partial collapse is detected during the mid-precursor stage of sawtooth oscillations when the minimum safety factor ( q min ) is ≤1. However, in NBI heating discharges, higher harmonic modes are detected during the precursor and collapse phases of the sawtooth when q min ≥ 1. Furthermore, these mode patterns are developed at the onset of NBI heating, during which the plasma exhibits a gradual decrease in poloidal beta ( β p ) and electron temperature, along with a loss of fast ions due to ELM activity. However, the ion temperature remains unchanged due to the continuous effect of NBI heating. These saturated modes are susceptible to quasi-interchange instabilities, influenced by the hybrid operational mode of the EAST facility, which is characterized by q min ≥ 1, low core magnetic shear, and high-pressure gradients. These experimental findings are consistent with interchange instability theories and previous numerical studies, particularly in conditions where q min ≥ 1 and higher harmonic modes are critical in sawtooth collapse. However, these modes deviate from the predictions of the Kadomtsev model.