Study of Plasma Fluctuation Visualization Using Imaging Reflectometry

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During the period of this research project, following results were obtained.1) Millimeter-wave two-dimensional imaging array has been improved, and applied to the GAMMA 10 tandem mirror for the simultaneous measurement of phase imaging and ECE (electron cyclotron emission) imaging in one plasma shot.2) A novel detector using MMIC technology has been fabricated in collaboration with Teratec Corporation. The detector consists of the integration of a bow-tie antenna, a down-converting mixer using a Schottky barrier diode, and HBT (hetero-junction bipolar transistor) amplifiers on a GaAs substrate. The heterodyne characteristics of the detector were evaluated at 70-140 GHz. The response of intermediate frequency (IF) up to 10 GHz was confirmed, which is one order broader than that of a hybrid detector.3) The optics for ECE imaging on Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS) were designed by using a ray-tracing method to focus radiation signals onto a detector array. It consists of an ellipsoidal mirror and a plane mirror. The Airy pattern of a point source and the magnification factor at 140 GHz were in good agreement with the designed values.4) The millimeter-wave imaging system was installed in LHD.The ECE signal was confirmed at IF=1-4 GHz channels of two detectors, simultaneously.5) An FM and an ultrashort-pulse reflectomer have been applied to an inductively coupled plasma with a radio frequency source. In the latter case, the reflected waves are directly recorded to a digitizing scope with 50 GHz bandwidth, and analyzed by a wavelet transform based phase extraction method and a tomographic method which rely on a row signal waveform rather than on the group delay as a function of frequency. The results demonstrate the potential of the current system for time resolved density profile measurements of LHD plasmas as well as low temperature processing plasmas.