1 EX / P 4-12 ELM and Pedestal Structure Studies in KSTAR H-mode Plasmas

The Edge Localized Modes (ELMs) and pedestal stru ctu e are studied in Korea Superconducting Tokamak Advanced Research (KSTAR) ELMy H-mode plasm as. KSTAR H-mode plasmas have three distinctive types of ELMs ; large type-I ELMs with low ELM frequency ( fELM =10 50 Hz) and good confinement (H98(y,2) = 0.9 1), smaller, possibly type-III, ELMs with h ig ELM frequency ( fELM = 50 250 Hz) at a reduced confinement (H98(y,2) = 0.7 0.8), and a mixed, large and small, ELM reg ime with good confinement (H 98(y,2) ~ 1). The NBI power scan shows that ELM frequency is increased with increasing input power for large ELM s, which is a typical behavior of type-I ELMs. In type -I ELMs, precursor-like signals can be measured by the magnetic sensors, while they are not observed in th e o er two ELM types. Low field side (LFS) profile of electron temperature (T e) from the ECE measurement and pedestal profile of toroidal velocity (Vt) from the charge exchange spectroscopy (CES) measurement show continuous to build up on the LFS during the inter ELM period. However, the recovery of T i pedestal from the CES after the ELM crash does not occur until it finally rises back up at the last stage of the inte r-ELM period, i.e. > 80% of the ELM cycle. The estimated electron pedestal collisionality for the type-I ELM y regime is ve*~ 0.5-0.6. A second L-H transition during the Lmode phase after the end of 1 st H-mode stage occurs for some discharges when the d ivertor configuration is restored by the plasma control system. Characterist ics of this “late H-mode” are compared to those for the main H-mode. An ideal MHD stability analysis for ELMy Hmode plasmas is also carried out by the ELITE code in order to investigate unstable domains for the ELM o ccurrence, as well as its sensitivity to a range of estimated/measured profiles. In this work, the expe rimental investigations of the characteristics of t he ELM and the pedestal structure during the ELM in the KSTAR H-mode plasmas is reported, and the preliminary res ults from the ideal MHD analysis for ELMy H-mode plasmas is presented.