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

Imazawa, Ryota; Kawano, Yasunori; Ono, Takehiro; Kusama, Yoshinori

This paper describes results from the first integrated analysis on the feasibility of a retro-reflector (RR) installed in the ITER vacuum vessel. We make thermal and structural analyses of passively-cooled RR and terrace retro-reflector array (TERRA) by using heat load conditions on ITER. In the case of RR, the temperature reaches over 600$$^{circ}$$C, and the flatness of deformed mirror surface forming RR is approximately 80 $$mu$$m. Although the maximal displacement of mirror surface is large, 69% of the incident-laser power returns to a diagnostic room, disturbance of phase is $$pi/4$$ and rotation angle of polarization plane is less than 0.06 $$^{circ}$$. On the other hand, the temperature of TERRA is approximately 350$$^{circ}$$C, and the flatness of the deformed mirror surface is less than 10 $$mu$$m. The flatness of the mirror surface of the deformed TERRA is much smaller than laser wavelength (118 $$mu$$m), but the global shape is curved to ruin 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, disturbance of phase is $$pi/5$$, and rotation angle of polarization plane is less than 0.06$$^{circ}$$. Since the optimized incident angles of flat and curved TERRA are different, a real-time feedback control of the incident angle is necessary for applying TERRA to ITER poloidal polarimeter.

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