Validation of q₀ ≥ 1.0 in the MHD Quiescent Time after Crash of the Sawtooth Instability in KSTAR

Full text: Long standing issue of full versus partial reconnection model of the sawtooth instability is revisited in Korean Superconducting Tokamak Advanced Research (KSTAR). The measured central safety factor, q₀ by Motional Stark Effect (MSE) is ∼ 1.0 and this measurement alone cannot validate the sawtooth instability model definitively due to nontrivial off-set error. Study of controlled experiment of the tearing modes (m > 1) sensitive to magnetic shear with the resistive magnetohydrodynamic (MHD) theory supports the full reconnection model. Here, the radial position of the excited tearing modes (m/n > 1/1) and their time evolution into the 1/1 kink mode before the crash in sawtoothing plasma suggests that q₀ ≥ 1.0 in the MHD quiescent period after the crash and q₀ ≥ 1.0 before the crash. Experimental observation of long lived tearing modes with constant mode number in non sawtoothing discharge (presumably q₀ ≥ 1.0) further supports the fact that q₀ ≥ 1.0 in the MHD quiescent period after crash in sawtoothing discharge and hence the complete reconnection model. (author)