Assessment of the environmental effects on the ITER FOCS operating in reflective scheme with Faraday mirror

摘要

Plasma current measurements will play an important role in ITER to provide real-time plasma control and machine protection. Performance of standard electromagnetic sensors may be not sufficient to fulfill this task due to a combined effect of steady-state operation and the presence of strong nuclear radiation. The Fiber Optics Current Sensor (FOCS) is a back-up system included in the ITER design. Faraday mirror (FM) can be a part of the FOCS, which allows to improve the system performance. It is required that the polarization rotation angle is equal to 90 degrees with acceptable deviations below 0.3 degrees. We have experimentally investigated the effect of thermal perturbations and gamma-radiation on the rotation angle of commercial FMs. The FM rotation thermal sensitivity was found to be in a range 0.105 - 0.124 degrees/degrees C. Gamma-irradiation resulted in a 1.2 degrees change, saturated after a 12.5 kGy dose. Based on these results we conclude that the cubical area is the preferable location for the FM despite that implies placing additional link fibers to reach the ITER vacuum vessel.