Speaker
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 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.
* 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 |
