EC waves polarization control in the TJ-II stellarator

In the TJ-II stellarator, the plasmas are created and heated by two 53.2 GHz gyrotrons, each of them delivering 300 kW. The power is transmitted to the plasma by two quasi-optical transmission lines (QTL1 and QTL2). The last mirror of each line is a steerable mirror located inside the vacuum vessel, which allows for perpendicular and oblique injection. In order to have a good coupling of the X mode, for the ECRH as well as for the ECCD experiments, a complete control of the polarization of the launched waves is required. In the QTL1, the polarization ellipse angle and the ellipticity are varied using a pair of grooved mirrors. One mirror varies the ellipticity, whereas the second mirror rotates the main axis of the polarization ellipse. In this paper, the calculation of the general polarization for any propagation angle in respect to the static magnetic field is presented, as well as the code used to get the appropriate orientation angles of the grooves of the polarizers. The results are compared with the measurements that have been carried out using the power monitor integrated in the last mirror of the line, outside the vacuum vessel. This monitor is a directional coupler, which couples part of the incident beam with a tapered holes array and both components of the electric field can be discriminated. The measurements are also used to calibrate the code by determining an effective groove width and period of each polarizer. The second transmission line (QTL2) only has a polarization twister, which allows us to change the direction of the linear polarization. A single grooved mirror polarizer has been designed to replace the present twister and thus get the required elliptical polarization without replacing additional mirrors in the line.