Spatial structure and temporal dynamics of edge localized modes in the KSTAR plasmas

Abstract

The greatest increase in demand for energy is envisaged to support the growth of industrial capability and population. With environmental requirements for zero CO2 emission and sustainable energy mix, new energy sources must be developed for future mankind. As nuclear fusion is one of the most promising option for future energy source, the fusion energy sustains all life on Earth without worrying about the energy problem. In order to make the fusion reaction on Earth, extremely high pressure and temperature environment, in which all atoms are fully ionized and form a plasma, is required. To satisfy these requirement there must first be powerful heating, and thermal losses must be minimized. This is achieved by creating a magnetic ‘cage’ made by strong magnetic fields such as Tokamak. A huge effort has been made to increase the temperature and pressure in plasmas over the past five decades. Meanwhile, it is found that a bifurcation occurs that bring the plasma from low- to high confinement state so called the H-mode. In the H-mode, the temperature and density of plasma are twice that of previous one and the confinement time is also improved a lot. However, the H-mode plasmas suffer several instabilities known as the edge-localized modes (ELMs) that can degrade the plasma stability and performance as well. The ELMs collapse the plasma edge and expel the confined heat and particles by repetitive bursting events. The ELM study is very difficult and complex because of nonlinear and non-axisymmetric feature, consequently their survey is one of the most important field of current fusion research. So far, extensive research has been done on the ELMs theoretically as well as experimentally. As a result, it has been found that ELMs are governed by peeling-ballooning mode which is induced by edge pressure gradient and current density. Although the peeling-ballooning model explains the onset of ELMs, present understanding of ELMs is still far from perfect. There are various types and conditions for the occurrence of ELMs. In order to predict ELM behavior, understanding of ELMs experimentally and developing a consistent model is crucial. Furthermore, controlling ELMs is essential to achieve steady-state operation for fusion energy. The work described in this thesis has been carried out at the Korea Superconducting Tokamak Advanced Research (KSTAR). Since the KSTAR attains the typical ELMy H-mode plasmas routinely and installed with high performance diagnostics, it is suitable to investigate the ELMs experimentally. As a main tool, the electron cyclotron emission imaging (ECEI) system was used for this work. The ECEI provides high spatial and temporal resolution of electron temperature fluctuations in 2-D which is required for measuring ELMs. The ECEI consists of a vertically aligned array of antennas, large aperture flexible optics and LO system, and electronics and digitizing system. The KSTAR ECEI system has been almost developed by the POSTECH independently in collaboration with UC-Davis, successfully installed and operated since 2010 KSTAR experimental campaign. First, the two independent ECEI systems, which are toroidally separated, are used to determine the toroidal mode number that is critical parameter for analyzing the stability of ELMs. This method extends the measurable range of toroidal mode number beyond the Nyquist limit of conventional Mirnov coil array and works in the absence of usable Mirnov coil signals. As a subsidiary information, the pitch angle of magnetic field lines can be measured and it is well matched to the equilibrium fitting calculation using magnetics only. Next, the dynamics of ELM flux tubes have been studied with a high resolution 2-D ECEI system. Multiple ELM flux tubes are found to be simultaneously generated with an exponential growth rate in the edge region. The flux tubes have a net poloidal flow as well as toroidal rotation and often reach quasi-steady state after the initial growth. In crash phase, all flux tubes start to poloidally elongate and then one of the flux tubes develops a finger-like structure and bursts, leading to heat and particles propagation. The spatial structure and temporal dynamics of ELMs have been visualized in inboard side (HFS) and outboard side (LFS) simultaneously to check the ballooning feature. The poloidal mode spacing of the HFS ELM is much larger than the ballooning mode spacing prediction from the LFS ELM spacing while the perturbation amplitude in both side is comparable. The HFS ELM also rotates counter-clockwise (or clockwise) poloidally, which is opposite to clockwise (or counter-clockwise) rotation of the LFS ELM. In crash phase, the LFS perturbation is larger compared to that of the HFS suggesting the ELM crash may be initiated by HFS perturbation. Modified ELMs under n=1 resonance magnetic perturbation (RMPs) have been also investigated using ECEI system. The n=1 magnetic perturbations altered both the spatial structure and temporal dynamics of ELMs. 2-D images of ELMs revealed that the RMP changed the poloidal mode spacing, flux tube size, and poloidal rotation velocity of ELM flux tubes. Especially, two distinctly different phases were observed when the ELM is suppressed by RMP: (1) occasional (non-periodic) tiny transport events, (2) filamentary mode structure without bursting. The stored energy has strong correlation with two different phase and thus more elaborate study is needed in the future. In order to understanding underlying physics for RMP effect on ELMs, the correlation technique has been employed. The correlation ECEI provides clear response to poloidal wavenumbers and phase velocity in ELM-crash-suppression phase. The turbulence properties of ELM-crash-suppression phase are similar to low confinement mode plasma which may suggest that frequent transport at the edge is dominant even though the filamentary mode exists. Finally, the velocimetry technique has been applied to ECE images to yield time dependent two dimensional velocity fields and it supports that filamentary structure and transport events coexist at the edge.