Application of microwave imaging system for density fluctuation measurements on large helical device
Résumé
After decades of fusion research, plasma transport remains an outstanding issue. The problem of anomalous transport in magnetically confined plasmas and relation of this phenomenon to microturbulence of small amplitude continues to be of critical importance. Understanding the mechanism of this transport requires the use of sophisticated diagnostic tools for the measurement of turbulent fluctuations. Diagnostic systems for fluctuation measurements in plasmas have evolved from basic 1-D systems to multi-dimensional systems due to the complexity of plasma turbulence physics illustrated by advanced numerical simulations. Using the significant advancements in millimeter wave imaging technology, Microwave Imaging Reflectometry (MIR) and Electron Cyclotron Emission Imaging (ECEI), capable of simultaneously measuring density and temperature fluctuations are under development in National Institute for Fusion Science (JAPAN). Since both systems require same collection optics for the reflected waves from the “cut-off layer” in the MIR system and vertically (poloidally) extended emissions in ECEI system and since both systems operate in a close microwave range (the MIR frequency range is 60~70 GHz and ECEI ranges from 70 to 140 GHz for LHD), it is feasible to combine the two systems. The one of the clear advantages will be to combine the detection part, which utilize state-of-the-art millimeter-wave planar imaging arrays positioned at the focal point of the detection system to form an image. The presented work emphasizes the developing of the microwave imaging reflectometry part of this diagnostic for spatially resolved plasma density fluctuations. A prototype millimeter wave imaging reflectometry system was being installed on LHD in October 2004, which was operated at 66 and 69 GHz and reflects from the X-mode cutoff. System details and laboratory characterization results of the prototype reflectometry imaging system will be presented, along with a preliminary data for the plasma density fluctuation measurements.