25th IAEA Fusion Energy Conference - IAEA CN-221

Pedestal Characteristics during the Edge Localized Mode Mitigation by Super-Sonic Molecular Beam Injection on KSTAR

17 Oct 2014, 14:00

4h 45m

Green 8-9 (Hotel Park Inn Pribaltiyskaya)

Poster Poster 8

Speaker

Dr Hyunyong Lee (Korea Advanced Institute of Science and Technology)

Description

This paper describes the pedestal characteristics during the Edge Localized Mode (ELM) mitigation by Super-sonic Molecular Beam Injection (SMBI) on KSTAR. After SMBI, whose pressure and duration time are 1 MPa and 8 ms, respectively, the reductions in both amplitude and time interval of D_alpha signals indicate the mitigation of type I-like ELM by SMBI. Despite the cold neutral gas injected by SMBI, the stored energy (W_p) did not change by the injection event. The fractional changes in electron density (delta n_e/n_e) and stored energy (delta W_p/W_p) by a single ELM burst and the change in H factor are small after SMBI. The ELM frequency (f_ELM) was increased by a factor of 2 - 3 due to SMBI, on the other hand, the change in f_ELM owing to the increase in the line-averaged electron density is only 8-15 %. The influence time, which is the time length of ELM mitigation, was about 300 ms. During the mitigated ELM phase by SMBI, both pedestal electron density gradient (nabla n_e^ped) and the pedestal electron temperature (T_e^ped) were reduced. As a result, the pedestal electron pressure (P_e^ped) and its gradient were decreased by about 50 %. According to a diffusive, bi-stable cellular automata model, the key mechanism of ELM mitigation is that the controlled relaxation of the nabla P_e^ped changes large ELMs to small ELMs. This prediction is qualitatively consistent with the experimental result and it suggests that the modification of pedestal profiles by SMBI is closely related to the ELM mitigation.