Far-infrared polarimetric characterization of metallic mirrors exposed to a tokamak plasma

We describe the polarimetric characterization in the far infrared $(λ=118.8μm)$ of a set of metallic mirrors and of a corner cube retroreflector whose surface has been exposed to a tokamak plasma. The objective of the measurements was to investigate possible changes of the polarization state of the incident radiation due to plasma depositions and identify possible sources of errors for the measurement of the ITER $q$ profile by far-infrared (FIR) polarimetry. Tests have been made on Mo and Cu mirrors exposed to the plasma in TEXTOR and Tore Supra. A corner cube retroreflector exposed in Tore Supra was also tested. The thickness and composition of the plasma depositions on these optical elements were known by previous surface analysis and visible ellipsometry studies. Our tests have been carried out in the FIR laboratory of the RFX experiment in Padova, Italy, and in the Laser Research Laboratory of the Department of Electrical and Electronic Engineering at University College Cork, Ireland, using a Stokes polarimeter assembled from components of the RFX FIR polarimetric diagnostic. The tests indicate that the plasma depositions on these elements do not affect the polarimetric response of the surface in the FIR, the only noticeable effect being a significant decrease of the reflectivity in a highly damaged region of limited area on the surface of one mirror. No other effect that may prevent the use of mirrors or corner-cube retroreflectors with this kind of surface damage in the poloidal ITER polarimeter has been observed.