A new far-infrared polarimeter on the tokamak à configuration variable

Summary form only given. A new far-infrared polarimeter diagnostic is under installation on the TCV tokamak at CRPP. Its aim is to obtain the poloidal current distribution via measurements of Faraday rotation angles of the beam polarization along 10 different vertical lines. The Faraday rotation angle is proportional to the product of the plasma density and the vertical magnetic field, providing constraints for improving the reconstruction of the internal magnetic field distribution. The setup consists of two FIR lasers operating at a wavelength 432.5e-6 m, optically pumped by a 120 W continuous wave CO₂ laser. The diagnostic is based on a method proposed by Dodel and Kunz. The FIR laser beams are detuned by 750 kHz by adjusting the cavity lengths. They are relayed to the tokamak by separate dielectric waveguides and combined into a single beam with a linear polarization rotating at the difference frequency. This probing beam is divided into five vertical beams across a poloidal section of the device. At the top of the tokamak each beam is equipped by 2 polarisation sensitive waveguide Schottky diode detectors. Measurements of Faraday rotation angles are obtained using a heterodyne detection scheme, with a reference signal from a line not passing through the plasma. The main aspects of the system and the use of the Faraday rotation measurements for the equilibrium reconstruction by the LIUQE equilibrium code will be presented.