10-channel far-infrared polarimeter for the tokamak configuration variable

A new far-infrared polarimeter diagnostic for the tokamak a configuration variable (TCV) is under construction at CRPP. It uses two FIR lasers at 432.5 mu m, optically pumped by a 120 W continuous wave CO2 laser. The two FIR cavities will be detuned such that the combination of the beams, using a method proposed by Dodel and Kunz [Infrared Phys. 18, 773 (1978)] , produces a single beam with a linear polarization rotating at the difference frequency (set to 750 kHz). For measurements across the minor radius of TCV, this beam will be split into ten beams, each equipped with a Schottky barrier diode as detector. Faraday rotation angles will be measured by coherent detection. In order to optimize the sensitivity of the polarimeter for the parameter range of interest [n(e)(0) < 3 x10(19) m(-3)], we have chosen to keep it separate from the existing 14-channel interferometer operating at 214 mu m. This also leads to substantial simplification of the design and signal processing. The design of the system as well as its expected sensitivity are presented and discussed. The required accuracy to measure profiles of current density and safety factor for typical operating scenarios with internal transport barriers on TCV (including cases with reversed magnetic shear) is assessed by numerical simulations. (c) 2006 American Institute of Physics.