Experimental Investigation Of Edge Coherent Modes In The EAST By Using Doppler Backscatter System

The thesis is based on the Experimental Advanced Superconducting Tokamak (EAST), and a Doppler backscatter system is designed and set up in it. Distribution and fluctuation of radial electric field as well as density fluctuation are measured in the EAST by using advanced microwave technology. The main work of this thesis include:set up of the Doppler backscatter system (DBS), measurement of DBS in the laboratory, set up of the inject wave’s angle adjustment control system of Doppler backscatter system, use of the frequency synthesis source of the Doppler backscatter system, set up of the back-end acquisition system and network, research on the disper-sion of over-mode waveguides, calculation of Doppler shift, extract of radial electric field and its fluctuation, measurement of density fluctuation. The first observation of geodesic acoustic mode (GAMs) and edge-coherent-mode in the EAST by using Doppler backscatter system is presented, what is more, poloidal mode numbers are confirmed through correlation analysis. Duo to the enhancement of the parameters of EAST, the Q-band system is replaced by another V-band system. These two poloidal separated V-band systems compose the correlation reflectometry.At first, this thesis reviewed the development and current situation of controlled nuclear fusion, and introduced the EAST, the type of plasma heating:ohmic heating and plasma auxiliary heating, including the electron cyclotron resonance frequency (E-CRF), the ion cyclotron resonance frequency (ICRF), the lower hybrid resonance fre-quency (LHRF) and neutral beam injection (NBI). Three plasma confinement regime, high confinement regime (H-mode), low confinement regime (L-mode) and I-phase are introduced, as well as the measurement of distribution and fluctuation of radial electric field in the tokamak plasma.Doppler backscatter system in this thesis which has been set up in the EAST contains two systems:Q-band (33-50GHz) and v-band (50-75GHz). Heterodyne detection technique is used to identify the positive or negative of Doppler shift, and the zero frequency interference can be avoid at the same time. It also benefits to the noise-signal ratio of the system. In order to change the incident angle of incident wave, a mirror system was designed which include a fixed flat mirror and a rotatable ellipsoidal mirror. The angle range is ±20° (Plus or minus is the propagation direction relative to the horizontal plane, positive upward, downward is negative). The measurement wave number coverage is 4-22cm-1. Frequency synthesis source is the heart of a microwave system, and a good source with low phase noise can avoid the introduction of a lot of unnecessary noise. We choose company Giga-tronics’s GT2520B as our source; it can output a fixed frequency or a step by step frequency signal. So the fluctuation of the radial electric field in a fixed position or the distribution of radial electric field can be detected. In experiment, in order to achieve the long-distance transmission with low loss, over-mode waveguide is chosen as the transmission. In order to explain the intermediate frequency of density profile reflectometer changes with incident frequency, we do some research on the dispersion of the over-mode waveguide, and the results display that this is induced by over-mode waveguide.The Doppler backscatter system was successfully installed in the window O of the EAST during 2012 spring experimental campaign, and a lot of effective data were obtained. In this experiment, the distribution of radial electric field was measured for the first time under L-mode and H-mode. Opposite direction of Doppler shift was ob-tained through changing the direction of incident angle. Geodesic acoustic mode was observed during the density decline of plasma discharge, whose poloidal mode num-ber is 0 through correlation analysis of the two poloidal separated Doppler backscatter system. This is consistent with the expected in=0, n=0 structure. Conditionally averaged bispectral analysis shows a strong nonlinear interaction between the GAM and the broadband turbulence, and edge coherent modes are observed. Until now, it has not formed conclusion on the understanding and processing of Doppler backscat-ter system. This thesis develops three useful methods to extract coherent modes. They are centre of gravity of the complex amplitude spectrum (COG), derivative of the DBS phase, and phase spectrum. These three methods are all feasible to extract GAM. Re-sults of applying them to other modes show low frequency information, for example, low frequency zonal flow, is more obvious by phase compared with derivative of the phase. Derivative of the phase is more suitable to extract high frequency information. The selection of points is the key point for the COG method.At last, the thesis describes the construction and test of a two V-band (50-75GHz) Doppler backscatter system. The preliminary results in the EAST are also presented.Duo to the enhancement of the parameters of EAST, the previous system is upgraded to two V-band (50-75GIIz) Doppler backscatter system. It can cover the low field side plasma in the EAST. Transmitting antenna and receiving antenna are the same antenna. An intermediate frequency amplifier is added after the first mixer. This is helpful to reduce the interference between signal and zero frequency noise. The influence of the signal strength and distribution is tested systematically after the mirror with lithium coating. No big differences are found, the signal strength only decays about 1 dB. In order to reduce the damage to microwave devices from elec-tron cyclotron resonance heat, notch filter is used which can reflect microwave in the frequency 140GHz.