Contrasting non-axisymmetric field influences on L-H Transition in KSTAR

An amazingly contrasting influence of resonant and non-resonant fields on L-H transition has been identified using high precision non-axisymmetric in-vessel control coils in KSTAR. Specifically, we have demonstrated a rather ‘non-flat’ dependence of low-n resonant magnetic field on the threshold power of L- to H-mode (PTH), while observing literally no variation of non-resonant magnetic field on PTH. We have found no sensitivity of non-resonant field on L-H transition even at the maximum affordable level of non-axisymmetric field current. Thus, we speculate that the L-H power threshold might not be determined solely by ExB shearing rate but that a turbulent transport driven by resonant magnetic field would be much more stochastically involved than that of non-resonant magnetic field. At the same time, this strongly suggests that the ITER might not need any dedicated correction of non-resonant magnetic fields, while requiring a high-quality resonant magnetic field control to secure the access to H-mode, in particular, at the marginal level of auxiliary heating sources.