Validation of the measured q0 > 1.0 in MHD quiescent period after the sawtooth crash in KSTAR

Abstract

Validation of the central safety factor (q0) during the sawtooth oscillation in the core of the Korean Superconducting Tokamak Advanced Research (KSTAR) is achieved by combination of the measured q-profile by Motional Stark Effect (MSE) and comparative study of the tearing modes experimentally excited with the current blip with the M3Dc1 (reduced MHD equations with finite resistivity) code. The measured central safety factor (q0) was ~1.0 and uncertainty in off-set may hinder whether it is below or above ~1.0 in sawtoothing L-mode discharge. The comparative study further validates that the central safety factor (q0) has to be above ~1.0, in the MHD quiescent regime after the sawtooth crash. The fact that transformation of the excited 3/3 mode into the 1/1 kink mode through 2/2 mode before the crash in one sawtooth period (~12ms) suggests that q0 goes below ~1.0 before the crash. Experimental observation of long lived (~400 ms) tearing modes (m=3 and m=5) in non-sawtoothing H-mode discharge (presumably  ≥ 1.0) further supports the fact that q0 has to be above ~1.0 in the MHD quiescent period of the sawtoothing discharge.