Effect of optical fibre bending on plasma current measurement in ITER using a polarisation-OTDR setup

Abstract

Accurate plasma current measurement is mandatory for the safe operation of the ITER. We investigate the possibility of performing this task using the polarisation-OTDR (optical time domain reflectometry). With this approach, an optical fibre placed on the outer surface of the vacuum vessel (VV) is used to measure the plasma current by analysing the Faraday effect induced SOP (state of polarisation) rotation of the backscattered light. The measurement accuracy depends on the extra linear birefringence resulting from the fibre bending related to the placement of the fibre on the vessel. In the present work, we assess this effect. Considering the difficulty in imitating such a bending geometry together with relevant currents in a lab environment, a simulation-based approach is developed to quantify the bending effect in the context of the ITER VV. The effect of fibre bending on the plasma current measurement accuracy in the expected ITER operating current range (0–17 MA) is analysed for different spun periods (SP) and beat lengths (LB) of the spun fibre placed around the VV. It is shown that a proper choice of LB and SP can alleviate the bending effect and achieve the ITER specifications regarding plasma current measurement accuracy.