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10-15 May 2021
Nice, France
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Edge Fluctuation Dynamics in RMP-Driven ELM Suppression and ELM-free H-mode Plasma in KSTAR

13 May 2021, 15:20
20m
Nice, France

Nice, France

Rapporteured Magnetic Fusion Experiments EX/4 MHD and ELM

Speaker

Dr Jaehyun Lee (National Fusion Research Institute (NFRI))

Description

Next step fusion devices such as ITER will need a reliable method for controlling the quasi-periodic expulsion of a large amount of heat and particles onto the plasma-facing components caused by edge-localized modes (ELMs). Several options are currently being considered to achieve the required level of ELM-crash control in ITER; this includes operation in plasma regimes which naturally have no or very small ELMs and suppression of ELM-crashes by active control of edge pedestal with resonant magnetic perturbations (RMPs). Many experiments at different tokamak devices have been dedicated to providing a solution that can be applied to ITER by the application of different approaches. However, due to a lack of understanding of the ELM control mechanism, a reliable ELM control is far from perfect yet.
The characteristics of two different non-ELM-crash plasmas have been studied using 2D fluctuation imaging systems on the KSTAR; (1) RMP-driven ELM crash suppressed H-mode, (2) ELM-free (or ELM-less) H-mode plasmas.
The time history of integrated spectral powers of filamentary mode (blue; 5-30kHz) and turbulence (red; 30-70kHz) along with the RMP coil current (left). Auto-bispectrum of a single ECEI channel in the ELM-crash suppression phase.
The ELM-crash suppressed H-mode plasma by the RMP is characterized by the coexistence of the filamentary mode and smaller-scale turbulent eddies at the plasma edge $[1]$. We have identified filamentary mode similar to ELM filament that still maintained, even when the ELM crash has been completely suppressed on $H_{\alpha}$ signal by RMP. Correlation analysis among the ECEI channels showed that the RMP would keep enhancing turbulent fluctuations at the plasma edge toward the ELM-crash suppression. A cross-phase analysis showed that such edge turbulence has a rather broad dispersion with a wide range of wavenumber ($k_\mathrm{\theta}<1.1$ cm$^{-1}$) and frequency ($f<100$ kHz). A detailed analysis suggests that energy exchange between filamentary mode and RMP-driven turbulent fluctuations would be responsible for the ELM-crash suppression (Fig. 1). Also, the plasma perpendicular rotation and its local shear estimated by the movement of the turbulent eddies decreases rapidly at the transition of the ELM-crash suppression $[2]$. Such reduction of the perpendicular rotation and its local shear could affect the turbulent fluctuation level by RMP.
In contrast, the ELM-free H-mode plasma was accompanied by benign edge harmonic oscillations (EHOs) near the separatrix. The EHOs had long-wavelength (toroidal mode number, $n\le4$) and remained stable during the ELM-free phase, and the ELM crashes rarely occurred at this stage. The observed EHOs appeared discontinuously and synchronized with quasi-periodic RF ($f\sim500$ MHz) spikes, providing an enhanced particle transport from the core plasma. This could avoid an increase in plasma edge pressure and prevent ELM crashes. The bicoherence analysis revealed that there is a strong nonlinear interaction between EHOs, and the nonlinear interaction of EHOs has a significant effect on the ELM structure and dynamics (Fig. 2). In addition to the EHOs, rotational shear found to play a significant role in the ELM-free phase. If the rotational shear is sufficiently large enough compared to the typical ELMing H-mode plasma, the ELM crashes disappeared, and the fluctuation level from the EHO increased, resulting in an intense transport event.
Coherence spectrums in the ELM-less and ELMing phase and contour plot of perpendicular rotation shear at the plasma edge (left). Bicoherence analysis during the ELM-less phase (right).
* This work supported by the Korea Ministry of Science and ICT under NFRI R&D programs (NFRI-EN2001-11) and National Research Foundation of Korea (NRF) (No. 2019R1F1A1057545).

References:
$[1]$ J. Lee et al, Phys. Rev. Lett. 117, 075001 (2016).
$[2]$ J. Lee et al, Nucl. Fusion 59, 066033 (2019).

Affiliation National Fusion Research Institute (NFRI)
Country or International Organization Korea, Republic of

Primary authors

Dr Jaehyun Lee (National Fusion Research Institute (NFRI)) YoungMu Jeon (National Fusion Research Institute) Dr Yongkyoon In (Ulsan National Institute of Science and Technology (UNIST)) Dr Gunyoung Park (National Fusion Research Institute (NFRI)) Minjun J. Choi (National Fusion Research Institute) Gunsu YUN (Pohang University of Science and Technology) Minwoo Kim (UNIST) Jayhyun Kim (National Fusion Research Institute) Prof. Young-chul Ghim (Korea Advanced Institute of Science and Technology) Jaewook Kim (KAIST) Jongha Lee (National Fusion Research Institude) Won Ha Ko (Korea, Republic of) Hyeon K. Park (UNIST)

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