The Doppler Backscattering System On EAST Tokamak

Doppler Backscattering system(DBS) is a combination of the microwave reflec-tometer and Bragg backscattering, and is proved to be a powerful tool of turbulence measurements for its excellent spatial, temporal and wave number resolution. This thesis concentrates on the building of the DBS on EAST, the data analysis, the EAST experiments with DBS and the full wave simulation.Firstly, we introduce the principle of the DBS and the DBS on the others tokamak and stellarator, and to calculate the location of the cutoff layer and the wave number, we develop the ray tracing code for EAST experiments. The input parameters of the ray tracing code are from the profile reflectometer and the EAST EFIT. We use the three dimensional quais-optial Gaussian code to evaluate the angle between the microwave and the magnetic field, the two dimensional code to calculate the radial and wave number resolution, and the fast single ray tracing for radial location and wave number calculation. The WKB approximation is used in the ray tracing code, so when the incidence angle is small where the WKB approximation don’t satisfied, the ray tracing will be broken. We also compare the three different doppler shift calculation method:the center of gravity or the weighted mean, fit to the power spectrum, and fit to the asymmetric part of the power spectrum, and develop the data analysis method for EAST experiments.Secondly, we finish the design of DBS on EAST which can satisfy the need of EAST experiments. The DBS on EAST includes two channels:the Q-band(f=33-50GH) X mode system and the V-band X mode system(f=50-75GHz), and both of them are heterodyne measurements. A stable synthesizer is chosen as the microwave source, and the source can fix the frequency for output, or scan the frequency step by step. A20MHz single side band frequency modulator is used to modulate the carrier frequency for the heterodyne measurements. We use two mirrors to optimize the wave propagation:a flat mirror and a rotatable ellipsoidal mirror. The ellipsoidal mirror is carefully designed to focus the microwave to the cutoff layer, and the beam radii at the cutoff layer is2.5cm for V-band and3cm for Q-band. We can rotate the ellipsoidal mirror to change the incidence angle, and the angle range is±20°. The wave number coverage is4-22cm-1, and it can cover the whole minor radius for the L-mode and also can reach the top of the pedestal for H mode. The wave number resolution is better than0.3, and the radial resolution is better than0.2. In the lab, a wheel with fluctuation on the surface is used to represent the plasma in tokamak. By launching the microwave oblique to the wheel, we can measure the doppler shift which is caused by the wheel whirl, and then we can test the system in the lab.Thirdly, we show the results of DBS experiments on EAST tokamak. The DBS was installed on the0port, and in the EAST experiments we finished the frequency and wave number scanning measurements. We found that the doppler shift increased with the incidence angle, while the amplitude of the peak decreased with the angle. And when we changed the direction of the incidence angle, the doppler shift would also change the sign. In the EAST experiments, we study the effect of LHCD. It showed that the poloidal velocity became larger when the LHCD was on, and once the LHCD turned off, the velocity became small. In the thesis, we got the radial electric field profile for L mode and H mode, and it shows that there is a well at the edge of plasma for both L mode and H mode, but the depth of the H mode well is much larger than L mode, and is more closed to the edge. We also showed the evolution of the electric field during L-H and H-L transition. It showed that the electric field will increase during the L-H transition at both the edge and the core plasma, but the density fluctuation decreases at edge and increases in the core. In EAST experiments, we study the I-phase oscillation during L-H transition. The results shows that the electric field and density fluctuation will oscillate with the I-phase, and the change of poloidal velocity is earlier than the fluctuation and the Ha signal. And we also found that the electric field and the density fluctuation goes in the different direction, which means the electric field can modulate the turbulence, and control the transport finally. In this thesis, we use the DBS to measure the mean electric field and the fluctuation of the poloidal velocity. The measurements shows that when the poloidal velocity fluctuation is large enough, the I-phase transforms to H mode; and when the fluctuation decreases to a certain level, the I-phase goes back to L mode.In this thesis, we study the effect of the turbulence in the ELMy mitigation and suppression. When the supersonic molecular beam injection mitigates the ELMs, in- termittent samll scale turbulence(f～600kHz) was found in the DBS signal. It shows that the intermittent small scale turbulence can increase the transport, and then control the ELMs, and finally mitigates the ELMs. When the LHCD modulation mitigates and suppresses the ELMs, besides the intermittent small scale turbulence, we found a kind of high frequency turbulence (f～4MHz). When the ELM is mitigated, the high frequency turbulence is intermittent, and it came out in turn with the small scale turbulence. While the ELMs are totally suppressed, we found the high frequen-cy turbulence become continuous. But we still don’t know how this high frequency turbulence comes out, and what kind of mode it is.Finally, we finish the two dimensional full wave simulation of the DBS. We verify the location measurements of the microwave reflectometer and the doppler backscat-tering system by the simulation. We also simulate the relationship between the ampli-tude of the doppler shift and the density fluctuation, and the results shows that the am-litude grows up with the density fluctuation. This thesis study the difference between the back scattering and the forward scattering. It’s found that the frequency of the shift which comes from the forward scattering is between zero and the doppler shift of the back scattering, and the shift frequency from the forward scattering increase with the density fluctuation until it reaches the frequency from the back scattering.