Polarization Issues with High Power Injection and Low Power Emission in Fusion Experiments
All tokamak experiments using ECCD require setting of the beam elliptical polarization for proper coupling to the plasma. This is done either in the matching optics unit (MOU) at the output of the gyrotron, or in a couple of miter bends of the transmission line. Similarly, oblique ECE receivers require selection of the correct elliptical polarization to provide localized measurements. For the TCV tokamak at the CRPP, gyrotron and oblique-ECE polarizers are characterized during either high- or low- power testing of equipment: for the gyrotrons the behaviour is determined at a single frequency, but for the oblique-ECE the broadband response is needed. These characteristics are included in the calibration database and used during subsequent analysis of the power coupling to, or from, the sources (gyrotron, plasma, or low power transmitting antenna). A more detailed characterization has been carried out (at low power) with the MOU for the EU, 170GHz, 2MW, gyrotron prototype for ITER. This paper discusses the methodology and results of these measurements, as well as a review of nearly a decade's worth of experimental data from the 6 gyrotron, 3MW, 82.6GHz TCV system. In particular, the consistency between the calibrations and the subsequent data from tokamak experiments is analysed.