Gamma ray irradiation effects on optical elements of ITER diverter impurity monitor

ITER divertor impurity monitor (DIM) is a diagnostic system that directly observes light emissions from a nuclear fusion plasma in the range of 200-1000 nm. Since DIM observes a wide range from infrared to ultraviolet with a single optical system, high optical performance such as spatial resolution and aberration is required for optical elements. In addition, it must be highly robust against external environments such as temperature, humidity, vibration and magnetic field. Because of its high radiation environment, it is necessary to use components with high radiation resistance. In this study, DIM optical elements installed in the interspaces and the port cells are investigated, because gamma ray irradiation can impact the optical performance of those elements installed there. The irradiation experiments were made at Gamma-ray irradiation facilities at QST, Takasaki. The attenuation of transmittance due to gamma irradiation were evaluated for silica, fluorite and LiCaAlF6 as the glass material in the spectrometers installed in the port cell. The influence of irradiation on a silica polka dot beam splitter was found to be negligible. For optical fibres, it was found that a high OH fibre for measurement of UV lights has a significant decrease in transmittance by irradiation and it is necessary to take some measures. For these experimental results, we obtain information necessary for proceeding the ITER DIM design