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 has been developed for electron density profile measurement in front of the ion cyclotron resonance heating (ICRH) antenna on Wendelstein 7-X (W7-X). The dual-band setup consists of one band at E-band (60–90 GHz) and another at W-band (75–110 GHz), enabling measurements of local density at two different poloidal positions. The system is polarized in the extraordinary mode (X-mode), corresponding to a measurable density of n_\mathrme \leqslant6.0 \times 10^19\,\rmm^-3 at a central magnetic field of B_\mathrm0 = 2.5 T. The transmission line includes an oversized K_a-band (WR28) waveguide in vacuum, which is tapered to a fundamental waveguide at E-band or W-band, respectively. Two pairs of sectoral horn antennas are mounted between the ICRH antenna straps. The horn mouth is designed to maintain the E-plane at the K_a-band dimension, while the elongated H-plane is customized to achieve 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 experiments show good agreement with profiles measured by other diagnostics, such as Alkali beam emission spectroscopy (ABES). Additionally, the density profile inside the magnetic island is presented. This diagnostic will be operated routinely for density profile measurements and will contribute to related physical study on W7-X.