Visualization of core and edge MHD instabilities in 2D using ECEI

G.S Yun,Y.M Jeon,S.I Park,M.J Choi,A J H Donné,J.H Lee,W Lee,C.W Domier,S.W Yoon,Y.S Bae,M Joung,J Lee,H.K Park,N.C Luhmann,G.H Choe

doi:10.1051/epjconf/20123203002

Abstract

MHD instabilities in the core and edge of the KSTAR plasmas have been visualized in 2D using an electron cyclotron emission imaging (ECEI) system with sufficient time and space resolutions for the study of the underlying dynamics. In the core region where the ECE optical depth is large, the ECEI has provided localized measurements of fast MHD phenomena such as the crash of internal kinks and coalescence of dual flux tubes. In the edge pedestal region of H-mode plasmas where the optical depth is marginal, the ECEI measurements were found to be still localized and were able to provide detailed 2D images of edge localized modes (ELMs) such as the growth of multiple filamentary structures and the crash of the pedestal characterized by fast localized bursts of the filaments [3]. The effect of electron temperature and density fluctuations on the ECE signals has been analysed to understand the limitations of the edge ECEI measurements.

Highlights

Each flux surface of ideal tokamak plasma in equilibrium (Fig.1a) has uniform electron temperature (Te) and density owing to the fast heat and particle transport along the helical magnetic field lines constituting the flux surface
In the following example images of MHD instabilities, it is important to note that the apparent poloidal rotation (V*pol) of the instability structure in the electron cyclotron emission imaging (ECEI) view at a fixed toroidal location is a combined effect of the plasma toroidal rotation (Vtor), the true poloidal flow (Vpol), the phase velocity (Vph) of the instability mode as illustrated by Fig.3:
EPJ Web of Conferences (1) The KSTAR ECEI system has been routinely operated for simultaneous 2D visualization of core and edge MHD instabilities to study the interaction between them

Summary

Introduction

Each flux surface of ideal tokamak plasma in equilibrium (Fig.1a) has uniform electron temperature (Te) and density (ne) owing to the fast heat and particle transport along the helical magnetic field lines constituting the flux surface. The ECEI systems have been recently installed in systems in several tokamaks and made substantial contributions to the physics understanding of sawtooth instabilities [1], tearing modes [2], Alfvén eigenmodes [3], and edge localized modes (ELMs) [4]. EPJ Web of Conferences harmonic (X2) ECE radiations were measured and interpreted as local Te measurements as usual [5], except two cases: (1) during the sawtooth crash phase where nonthermal electrons could be generated and enhance the ECE intensity or radiation temperature (TECE) above the thermal level (Te) and (2) the edge pedestal region where the optical depth is marginal and decreases rapidly toward the separatrix

KSTAR ECEI

Summary of KSTAR ECEI results

Discussion on the edge ECE images