Polarization-sensitive reflectometry-based plasma current measurement in ITER: influence of operating temperature

Abstract

ITER is a tokamak-based fusion device where the knowledge of plasma current is crucial for its safe and successful operation. A polarimetric optical fibre sensor, installed around a section of the ITER Vacuum Vessel (VV), can provide a measure of the plasma current, by exploiting the Faraday effect-induced state of polarization (SOP) rotation of the light launched into the sensing fibre. In the system discussed here, spun fibre is used as the sensing fibre and a polarization-sensitive reflectometer (PSR) is used as the interrogator. In this paper, we analyse the impact of the ITER VV wall ambient temperature on the sensor’s plasma current measurement accuracy, when the other inevitable perturbation effects, namely fibre bending, and twisting are also considered. As ITER is not yet operational and there is no practical way of imitating the ITER operating conditions, we resort to the Jones formalism-based simulation approach to estimate the minimum required LB/ SP ratio of the spun sensing fibre that satisfies the ITER plasma current measurement specifications, under the considered perturbation effects.