Experimental Study Of MHD Instabilities Based On Electron Cyclotron Emission Imaging Diagnostic

In the tokamak plasma physics, high confinement mode (H-mode) is believed to be the most promising mode to achieve fusion reaction. However, the physical mechanism and threshhold of L-H transition are still under misunderstanding. A variety of magneto-hydrodynamic (MHD) instabilities, such as sawtooth, tearing mode, interchange mode and ballooning mode, highlight complexity of the H-mode physics. Thus, it is vital task to study H-mode. To study the high temperature plasma physics, it is very important to measure the plasma parameters like electron temperature, ion temperature, density,current, rotation velocity and so on, and characteristics of the MHD instabilities.Electron cyclotron emission imaging (ECEI), which is an advanced two dimen-sional diagnostic, has a high special and temporal resolution to cover a huge area in the cross-section of tokamak. ECEI can provide a real and local image of electron tempera-ture fluctuation. Comparing with the traditional one dimensional diagnostic as electron cyclotron emission radiometry (ECER) and traditional two dimensional diagnostic as tomographic soft X-ray imaging, ECEI has more advantages. Basic principle and hard-ware of ECEI are introduced in the second chapter. More details and matters of ECEI needing attention are discussed. Design and installation of three dimensional ECEI on J-TEXT tokamak are also shown in this chapter. The 3D-ECEI system provides exper-imental evidence of helical symmetry of sawtooth precursor mode.In the third chapter, it is fully discussed about the ECEI observation of compound sawtooth, 2/1 mode and plasma disruption. Comparing with the normal sawtooth, com-pound sawtooth contains four phases: ramping phase, partial collapse, ramping phase,full collapse. The time evolution of the compound sawtooth phases is investigated by ECEI, especially the collapse phases. Taking the advantage of the huge coverage of ECEI in the cross-section of EAST, all of the electron temperature fluctuation inside q=1 surface are summed up at each moment. Energy flux transporting out of q=1 sur-face during partial collapse and full collapse are compared. With this method, a new definition of sawtooth crash time is raised up and statistical work is done about the sawtooth crash time, sawtooth period and energy flux during the crash. Some anormal sawtooth crash processes are revealed. All of these anormal processes provide more experimental material for the study of sawtooth behavior.During the study of plasma major disruption, ECEI provides a complete process of the major disruption, especially the time evolution of characteristics of the 2/1 mode before the disruption. The observation of changes of the mode structure and island width supply more direct experimental data, which is helpful to understand the major disruption. It is also found that emergence of the 2/1 mode is associated with flatting of electron temperature profile and current density profile. Besides, intensity of line emissions from intrinsic impurities decreases at the same time, and so does the neutron flux. All of these phenomena maybe taken as an omen of the 2/1 mode, and may shed light on the prediction of the major disruption. Vertical displacement event (VDE) is also studied by ECEI in this paper. The movement of plasma center is observed and the trace is shown.Due to the MHD instabilities including edge localized mode (ELM), the pedestal physics are very complicated and difficult to understand. In the fourth chapter, clas-sification and theories of ELM are introduced, and so does the application of ECEI at edge plasma region. Since the condition of optical thick cannot always be guaranteed,the ECEI data can be interpreted as radiation temperature containing both electron tem-perature and electron density, instead of the local electron temperature. In the 2014 EAST campaign, a quasi coherent mode is measured by ECEI during co-current neutral beam injection (NBI) heating plasma. Characteristics of the coherent mode and process of ELM crash are demonstrated and discussed in this paper. It is found that the mode localizes just inside the last closed flux surface (LCFS) during H-mode. Ballooning-like mode structure of the mode can be identified, and the radial island width is only about 4cm. From the images given by ECEI, it is clearly shown that the mode rotates in the electron diamagnetic direction in the laboratory frame. During ELM-free H-mode,the mode frequency downshifts from around 45kHz to 15kHz, which is accompanied with decrease of amplitude, poloidal wave number, phase velocity of the mode. The characteristics including structure, frequency, rotation direction and localization of the coherent mode during inter-ELM H-mode are same with the mode during ELM-free H-mode. Thus, it is believable that the modes during ELM-free H-mode and inter-ELM H-mode are belong to same physical mode. According to the amplitude of divertor Dαsignal, small ELM crash and big ELM crash are newly defined in this paper. By calcu-lating cross correlation of two channels of ECEI inside and outside LCFS, energy flux transporting cross LCFS during the two kinds of ELM crash are compared. With images provided by ECEI, the structural evolution of the two kinds of ELM crash is discussed.Finally, the conclusion of this paper is given in the fifth chapter. In the part of development prospect, disadvantages of ECEI system on EAST are discussed, and so are the potential improvement projects. Talking about the ECEI data, techniques of big data and machine learning are introduced. With these methods, quality of ECEI signals can be judged, and the sawtooth behaviors may be identified in the signal. What is more important, statistical law of some physical phenomena may be found, which is very helpful to understand these phenomena. By mining the data, it is believed that ECEI can contribute more to the confined fusion plasma physics.