Validation of the ‘full reconnection model’ of the sawtooth instability in KSTAR

Abstract

The central safety factor (q0) during sawtooth oscillation has been measured with a great accuracy with the motional Stark effect (MSE) system on KSTAR and the measured value was ∼1.0±0.03.However, this measurement alone cannot validate the disputed full and partial reconnection models definitively due to non­trivial off­set error (~0.05). Supplemental experiment of the excited m = 2, m = 3 modes that are extremely sensitive to the background q0 and core magnetic shear definitively validates the ‘full reconnection model’. The radial position of the excited modes right after the crash and time evolution into the 1/1 kink mode before the crash in a sawtoothing plasma suggests that q0 >= 1.0 in the MHD quiescent period after the crash and q0 < 1.0 before the crash. Additional measurement of the long lived m = 3, m = 5 modes in a non­sawtoothing discharge (presumably q0>= 1.0) further validates the ‘full reconnection model’.