Thermal and optical analyses of retro-reflector for poloidal polarimeter on ITER

This paper describes results from the first integrated analysis of the feasibility of a retro-reflector (RR) installed in the ITER vacuum vessel. We have carried out thermal and structural analyses of a passively-cooled RR and a terrace retro-reflector array (TERRA) using heat load conditions expected in ITER. In the case of the RR, the maximum temperature is approximately 620°C, and flatness of the deformed mirror surface of the RR is approximately 80 µm. Although maximal displacement of the mirror surface is large, 69% of the incident-laser power returns to the diagnostic room (89.9% returns when the RR is not deformed), phase disturbance is π/4 and the rotation angle of the polarization plane is less than 0.06°. On the other hand, the maximum temperature of TERRA is approximately 350°C, and flatness of the deformed mirror surface is less than 10 µm. The flatness of the mirror surface of a deformed TERRA is much smaller compared to the laser wavelength (118 µm), but the global shape curves in a manner which disrupts the coherent phase of laser light. The curvature radius of TERRA is 21 m, and 49% of the incident-laser power returns to the diagnostic room (88.6% returns when TERRA is flat), phase disturbance is π/5 and the rotation angle of the polarization plane is less than 0.06°. Since the optimized incident angles of a flat and curved TERRA are different, real-time feedback control of the incident angle is necessary for applying TERRA to the ITER poloidal polarimeter. (author)