Validation of the 'full reconnection model' of the sawtooth instability in KSTAR

Abstract

The central safety factor (q(0)) during sawtooth oscillation has been measured with a great accuracy with the motional Stark effect (MSE) system on KSTAR and the measured value was similar to 1.0 +/- 0.03. However, this measurement alone cannot validate the disputed full and partial reconnection models definitively due to non-trivial off-set error (similar to 0.05). Supplemental experiment of the excited m = 2, m = 3 modes that are extremely sensitive to the background go and core magnetic shear definitively validates the &apos;full reconnection model&apos;. 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 q(0) >= 1.0 in the MHD quiescent period after the crash and q(0) < 1.0 before the crash. Additional measurement of the long lived m = 3, in = 5 modes in a non-sawtoothing discharge (presumably q(0) >= 1.0) further validates the &apos;full reconnection model&apos;