Plasma current measurement in ITER with a polarization-OTDR: impact of fiber bending and twisting on the measurement accuracy

Using a polarization-sensitive optical time domain reflectometer (OTDR), plasma current in the International Thermonuclear Experimental Reactor (ITER) can be measured by investigating the Faraday effect-induced polarization rotation in a spun fiber placed around the Vacuum Vessel. However, intrinsic birefringence and external effects like fiber bending and twisting generate unwanted polarization changes and decrease the measurement accuracy. In this paper, a simulation-based approach is developed, considering bending and twisting effects to assess the performance of the reflectometer in measuring plasma current at ITER. The results demonstrate that, for a proper choice of spun sensing fiber parameters (intrinsic beat length and spun period), the performance of the sensor satisfies ITER accuracy.