Role of the equilibrium and perturbative central current density in sawtoothing and non-sawtoothing discharges in KSTAR

The recent experiment [1] on excitation of the m/n=3/3 mode with a current blip induced by ECH and successive evolution to the m/n=2/2 and m/n=1/1 mode in the core of the sawtoothing discharge in KSTAR suggests that the central safety factor (q₀) may have to change from below ∼1 (before crash) to slightly above ∼1(after crash). This interpretation is consistent with the q profile condition for MHD simulation necessary for the growth of higher order modes which require q₀ slightly above ∼1 until the 1/1 mode becomes dominant [2]. Experimental observation of a long lived higher order mode in non-sawtoothing discharge (presumably q₀>1) is consistent with the fact that the q₀ has to be below ∼1 to support the growth of the m/n=1/1 mode and subsequent crash.