Polarisation-OTDR based optical fibre sensor for plasma current measurement in ITER: effect of fibre bending, twisting and temperature dependence of Verdet constant on the measurement accuracy

In ITER, polarimetric optical fibre sensors measure the plasma current (0-17 MA) by exploiting the Faraday effect induced state of polarisation (SOP) rotation of a polarised light propagating in the sensing fibre, placed on the outer surface of the ITER vacuum vessel (VV) section. In the discussed here system, the polarisation-OTDR (POTDR) is employed to analyse the SOP rotation and a spun fibre is used as the sensing fibre. The Verdet constant is the proportionality constant between the SOP rotation and the axial magnetic field induced by the current. The presence of unwanted birefringence in the sensing fibre will degrade the measurement accuracy. In this paper, we analyse the effect of the birefringence induced by the fibre bending and twisting together with the effect of the temperature dependence of the Verdet constant. Due to the diffculty in taking measurements in the ITER representative conditions, a simulation approach is developed|using Jones formalism|to show that the performance of the sensing fibre in terms of plasma current measurement can be characterised by the ratio of precursor fibre linear beat length (LB) over its spun period (SP ) i.e., LB SP. Finally, we estimate the minimum required LB SP ratio of the sensing fibre needed to satisfy the ITER plasma current measurement specifications.