Heterodyne reflectometer for phase and time delay measurements in W7-AS

Abstract

A broadband heterodyne reflectometer operating in the frequency range 75–110 GHz in extraordinary mode polarization, is used at the W7-AS stellarator for density profile and density fluctuation studies. The system allows for broadband operation and permits unambiguous determination of the phase delay independent of the amplitude fluctuations in the reflected beam. The system uses two different gauss beam optics for both signal launch and detection with a slim beam waist of about 2 cm at the relevant plasma location. An amplitude modulation system has been incorporated operating at 133 MHz modulation frequency sharing with the phase sensitive system the main oscillators, antennas, and the receiver front end. AM detection is accomplished in the IF part by synchronous detection after recovery of the carrier by narrowband filtering. Time delay measurements can be done simultaneously with the carrier phase determination. The main purpose of the diagnostic in AM mode is fast edge density profile monitoring, conventional carrier phase detection is used for density fluctuation measurements and planned correlation studies. The agreement between the WKB approximation and the 1 D full wave numerical solutions for the W7-AS parameters and geometry guarantees the good localization of the measurements and permits the estimation of density fluctuation level from the measured phase fluctuations. Density fluctuation studies have been carried out in NBI and ECRH (L- and H-mode) plasmas. Fluctuation levels and spectral characteristics have been analyzed. In addition 2D effects possibly due to rotating turbulent structures have been observed.