Measurement of Electron Density Fluctuations Using O-Mode Microwave Imaging Reflectometry in a TST-2 Spherical Tokamak

Abstract

An O-mode microwave imaging reflectometry system has been installed in a spherical tokamak named TST-2. The illumination wave frequency is 23 -32 GHz, which corresponds to the cutoff electron density of 0.65 -1.3 × 10 19 m −3 . The microwave image of the scattered wave is formed on an imaging detector named horn-antenna millimeter-wave imaging detector by imaging optics that consist of an ellipsoidal aluminum mirror, a Teflon lens, and a dielectric plate. The detected channel numbers are 6 (poloidal) × 6 (toroidal) × 2 (radial). The data of power and phase of the scattered wave are sampled every 0.5 µs and are stored by the LABCOM system at the National Institute for Fusion Science via a private network named SNET. The scattered wave quickly fluctuates (∼5 µs) and has a large amplitude (>100 times). From the time evolution of the phase at the internal reconnection event, it is inferred that the reduction in amplitude may be owing to the phase mixing of scattered wave in each detector channel.