Theoretical and experimental studies of the ITER ECRH polarizer and rotator gratings

Abstract

Electron Cyclotron Resonance Heating (ECRH) will be one of the major heating and current drive mechanisms for the ITER fusion experiment. A pair of reflection grating polarizers will be used in the ECRH high power microwave transmission lines to generate the required elliptically polarized microwave beam for ideal plasma coupling. A 'polarization rotator' and a 'circular polarizer' are used together to convert a linearly polarized beam, generated by a gyrotron, to an arbitrary elliptically polarized beam. This thesis presents numerical and experimental results characterizing the elliptical properties of microwaves reflected from a pair of polarizer gratings designed for operation at 170 GHz. First, a theoretical basis is presented for understanding the polarizing behavior of a reflection grating with an arbitrary groove shape. Vector transformations between incident and reflected fields calculated in High Frequency Structural Simulator (HFSS) are used to find the phase shift between the field components that reflect from the top and bottom grating surfaces. Using these results, we characterize the reflecting field by its ellipticity (#) and the angle of rotation of the main polarization axis (a). Next, detailed experimental measurements of the fields reflected from the aforementioned polarizer pair were taken with a Vector Network Analyzer. Very good agreement was seen between the numerical and experimental results and, to our knowledge, these are the first measurements of a polarizer/rotator pair in corrugated waveguide to be successfully compared with theory. Based on these results, we also calculated full polarization maps for grating pairs with alternative groove profiles. We also experimentally studied the mode conversion introduced by the polarization rotator as the grating is rotated about its axis. The presence of higher order modes will increase the ohmic losses along the transmission line.