Akiyama T, Kawahata K, Ashikawa N, Tokitani M, Okajima S, Nakayama K, Yoshida N, Ebihara A, Tokunaga K, Ohtawa Y, Tsuji-Iio S
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, Gifu 509-5292, Japan.
Rev Sci Instrum. 2007 Oct;78(10):103501. doi: 10.1063/1.2786938.
This article describes changes of optical properties of retroreflectors installed on the inner wall of the vacuum vessel of the Large Helical Device. They were made of stainless steel and were used for a CO(2) laser polarimeter. The reflectivity for a CO(2) laser beam dropped up to 40% after the start of glow discharge cleanings and main plasma experiments. Then it decreased gradually. The reflectivity in a wavelength range shorter than 10 mum decreased significantly through one experimental campaign (about 4 months). On the other hand the reflectivity in a wavelength range longer than 50 mum was larger than 70%. Decrease in the reflectivity in the central region of mirrors was more significant (the reflectivity for visible beams became almost zero) than that in the edge. The distribution of the reflectivity along the radial direction is expected to be related to the shape of the retroreflector. The parallelism of the reflected beam to an incident one also deteriorated. The changes in the polarization angle and in the ellipticity of reflected light polarization were not observed from the visible to the far infrared range.
本文描述了安装在大型螺旋装置真空容器内壁上的后向反射镜光学特性的变化。这些后向反射镜由不锈钢制成，用于二氧化碳激光偏振计。在辉光放电清洗和主等离子体实验开始后，二氧化碳激光束的反射率下降了高达40%。然后它逐渐降低。在一个实验周期（约4个月）内，波长小于10μm范围内的反射率显著下降。另一方面，波长大于50μm范围内的反射率大于70%。镜子中心区域反射率的下降比边缘区域更显著（可见光束的反射率几乎变为零）。沿径向方向的反射率分布预计与后向反射镜的形状有关。反射光束与入射光束的平行度也变差了。从可见光到远红外范围，未观察到反射光偏振的偏振角和椭圆率的变化。
