Frequency modulated continuous wave reflectometry for density profile measurement in front of ICRH antenna in Wendelstein 7-X

A Frequency Modulated Continues Wave (FMCW) reflectometry with heterodyne detection regime is developed for measuring the electron density profile in front of the ICRH antenna in Wendelstein 7-X (W7-X). The dual-band set up consists of the one at E-band (60GHz-90GHz) and another at W-band (75GHz-110GHz), enabling to measure the local density at two different poloidal positions. The system is polarized in extraordinary mode (X-mode), corresponding to a measurable density of ne ≤ 6.0 × 1019m−3at a central magnetic field of B0= 2.5T. The transmission line consists of an oversized wave guide in Ka-band (WR-28) in the vacuum, and is tapered to the fundamental wave guide, respectively. Two pairs of sectoral horn antenna are mounted between the ICRH antenna straps. The horn mouth is designed to keep the E-plane at the Ka-band dimension whereas the elongated H-plane is customized for a sufficient gain and directivity. In this paper the layout of the front-end and the design of electronic module are presented. Furthermore, the evaluated density profiles from the exper-iments demonstrate a good agreement with profiles measured by other diagnostic, e.g. Akali metal beam. And the density profile inside the magnetic island is presented as well. The diagnostic will be operated routinely for density profile measurement and will contribute to related physical study on W7-X.