Fluctuation Measurements in Tokamaks with Microwave Imaging Reflectometry

To study the mechanism of anomalous transport in tokamaks requires the use of sophisticated diagnostic tools for the measurement of short-scale turbulent fluctuations. In this article, we describe an attempt at developing a technique capable of providing a comprehensive description of plasma fluctuations with k(subscript parallel rho i) < 1, such as such as those driven by the Ion Temperature Gradient mode in tokamaks. The proposed method is based on microwave reflectometry, and stems from a series of numerical calculations showing that the spatial structure of fluctuations near the cutoff could be obtained from the phase of reflected waves when these are collected with a wide aperture optical system forming an image of the cutoff onto an array of phase sensitive detectors. Preliminary measurements with a prototype apparatus on the Torus Experiment for Technology Oriented Research 94 (TEXTOR-94) [U. Samm, Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (IEEE, Piscata way, NJ, 1995), p. 470] confirm the validity of these conclusions. Technical issues in the application of the proposed technique to tokamaks are discussed in this article, and the conceptual design of an imaging reflectometer for the visualization of turbulent fluctuations in the National Spherical Torus Experiment (NSTX) [M. Ono, et al., Nucl. Fusion 40, 557 (2000)] is described