Amplitude modulated heterodyne reflectometer for density profile and density fluctuation profile measurements at W7‐AS

A broadband heterodyne reflectometer operating in the frequency range 75–110 GHz in extraordinary mode polarization is used at the W7‐AS stellarator for both fast density profile determination and density fluctuation studies. The probing signal is amplitude modulated at a frequency 133 MHz using the envelope phase for profile evaluation and the carrier phase to determine the fluctuation information simultaneously. Separate Gaussian beam optics for final signal launch and detection permits a beam waist of about 2 cm at the reflecting layer in the plasma. Amplitude modulated detection is accomplished in the intermediate frequency part by synchronous detection after recovery of the carrier by narrow‐band filtering. Voltage controlled solid state oscillators followed by active frequency multiplication allow to scan the full frequency band within less than 1 ms. For typical W7‐AS operation the accessible density range is 1×10¹⁹ to 6×10¹⁹ m⁻³ for on axis magnetic field of 2.5 T and 4.5×10¹⁹ to 10×10¹⁹ m⁻³ for 1.25 T, respectively. The probed radial positions range between 0.2<r/a<1.1 depending on plasma conditions (a≊17 cm).