Microwave Imaging Diagnostics in LHD

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A new diagnostic tool for the plasma density fluctuations measurements was developed at National Institute for Fusion Science. For the first time the Microwave, Imaging Reflectometer was deployed for plasma measurements at heliotron (stellarator) class of the thermonuclear fusion devices. Based on main LHD plasma parameters (magnetic field strength and electron plasma density) MIR system operates at 66,69GHz with extra-ordinary orientation of the launching microwave radiation. To deliver very broad Gaussian beam and to pick-up reflected radiation from precisely localized spatial regions inside plasma a very sophisticated Quasi-optical system with combination of high-sensitive detectors was installed. To extend the ability of the MIR system to operate with Electron Cyclotron Emission Imaging diagnostic special quasi-optical splitter (dichroic plate) was used. Both the MIR and ECEI techniques take advantage of large aperture optics to form an image of the reflecting/emitting layer onto a … More n array of detectors located at the image plane, enabling localized sampling of small plasma areas. To obtain the information on plasma fluctuations in poloidal, toroidal and radial directions a multiple antenna system was used.Low frequency plasma density oscillation (2-3 kHz) was observed during NBI plasma heating for regime with low plasma electron density. Some partial correlation (with correlation time 2.5 ms) was shown for poloidally separated detection channels. However, there was no clear evidence of the toroidal correlation in the reflected signals. From those data the estimated value Vθ=44 m/s of poloidal rotation velocity was obtained. During the experiments it was found that due to the very complicated 3-D shape of the plasma in the LHD device the issue of reflected beams focusing becomes of great importance. To improve the quasi-optical antenna system new elliptical antenna with variable steering angle is proposed. This will be giving a better flexibility to optimize the beam-line of the reflected radiation in order to achieve the maximum power of the reflected microwaves. Less