Development of Microwave Imaging Reflectometry System for KSTAR

Abstract

Microwave Imaging Reflectometry (MIR) has been developed for unambiguous measurement of electron density fluctuations in tokamak plasmas. The loss of phase information limiting the use of conventional reflectometry can be minimized by a large-aperture imaging optics and an array of detectors in the MIR embodiment. The evaluation of the optical system is critical for precise reconstruction of the fluctuations. The optical systems of the prototype TEXTOR MIR has been evaluated by experimental characterization of the launching optics and calculations using the analytical method and the Finite Difference Time Domain (FDTD) method. The preliminary KSTAR MIR systems improved from the TEXTOR MIR system have been tested with a corrugated target simulating density fluctuations at the cut-off surface. The effects of optical aberrations and interference between lenses on the phase reconstruction have been investigated by the 2-D amplitude measurement of the reflected waves and the diffraction-based optical simulations. Two preliminary designs of the KSTAR MIR optics for minimizing the aberration and interference effects have been suggested. The details of two designs, including the capability of controlling the zoom factors and reducing the interference effects, have been deeply discussed.