Change of the toroidal mode number of the edge localized modes during ELM evolution in KSTAR plasma

Abstract

Changes of the toroidal mode number (n) of the edge localized modes (ELMs) have been observed during the ELM evolution phase in several KSTAR discharges. The mode transition always accompanined a discrete change of the mode frequency and it typically lasted for a few hundreds of micro seconds. While the global parameters (stored energy, plasma current, shape, etc.) did not change very much (~2%), the toroidal velocity gradient and the ion temperature gradient at the pedestal considerably changed through the mode transition.Two classes of mode number transitions were identified: overlapping transitions and non-overlapping transitions. In the former case, another filamentary mode structure appears and co-exists with the original mode structure and it persists after the original one perishes. On the other hand, in the latter case, the coherent filamentary structure disappears temporarily and re-appears with a different mode number during the transition. These phenomena may help to understand the previous observation on the ELM crash dynamics where a rapid mode number change preceded the ELM crash [1]. *Work supported by the NRF of Korea under contract No. 2014M1A7A1A03029881 and the BK21+ program.