Measuring Faraday Effect in Z-Cut Crystal Quartz at Wavelength of 118.8 µm for Polarimeter

抄録

Faraday rotation angle in Z-cut crystal quartz at a wavelength of 118.8 µm was measured for the first time. Z-cut crystal quartz is used in vacuum windows of polarimeters for plasma diagnostics that use far-infrared light. ITER poloidal polarimeter uses double vacuum windows of 10-mm Z-cut crystal quartz discs in compliance with nuclear safety. According to the Becquerel model, thick Z-cut crystal quartz may lead to non-negligible Faraday rotation that must be compensated for to measure the pure polarization change attributable to the plasma. For the first time, this paper measures Faraday rotation at the wavelength of 118.8 µm by means of the rotating linear polarizer method. The Becquerel model is not applicable at the wavelength of 118.8 µm and Faraday rotation resulting from the Z-cut quartz vacuum window is negligible even with a magnetic field of 0.3 T and a 10-mm thick vacuum window.