Alteration of Low Density Operation Range by Resonant Magnetic Perturbation in KSTAR

Abstract

Knowing the operation range of tokamak is important since the operation parameters determine the fusion output power and plasma operation scenarios. In this thesis the operation range of KSTAR was revealed via Hugill diagram and Greenwald density fraction diagram. In total 1740 experimental shots were selected carefully for this analysis which were performed from 2011 to 2014 in KSTAR. By comparing the operation range of KSTAR with that of DIII-D via Hugill diagram, it was found that KSTAR can reach far lower density, insisting that KSTAR has extremely low intrinsic error field. The range of the achievable Greenwald fraction of KSTAR was shown to be 6% to 60% in the present database. The two factors, runaway electron and error field, which can alterate the achievable low density operation range were investigated. The condition for the generation of the runaway electron in KSTAR was found via Dreicer condition with dedicated runaway electron generation experiments. Although these experiments satisfy the condition for generating runaway electrons, all of these shots could be sustained without significant interruptions. Based on this observation the runaway electrons are not thought to be the main factor to set the low density operation limit in KSTAR. On the other hand, plasma shots with the increased error field by applying the resonant magnetic perturbation(RMP) often showed the plasma locking phenomenon so that access to the low density operation range was significantly constrained. The plasma locking threshold is found to be differed by the position of the applied RMP coil and the value. The RMP coil current magnitude needed for plasma locking from the middle coil is about half of that from the top or the bottom coil. And the lower value showed the lower density operation range, with in cases and in cases. The influence of RMP to the low density range is investigated in terms of the L-mode to H-mode transition power which shows that access to the optimal density range of the L-H transition can be limited by applying RMP. Therefore, it is suggested to take this effect into account in designing operation scenarios for tokamaks employing RMP to suppress edge localized modes in H–mode plasmas in particular.