The fiber-optic current sensor (FOCS) will be installed in ITER to measure the plasma current for plasma control and machine protection. FOCS uses the Faraday effect in the fiber installed on the outer surface of the vacuum vessel. During plasma operation in ITER, vibrations may change polarization properties of optical fiber installed in the cryostat bridge, and it may affect the sensor accuracy. In this paper, we analyze the vibration effect on the FOCS measurement by applying the Jones matrix formalism. The effect of vibrations on the Jones matrix was addressed using an experimental set-up. A fiber-inserted helical shape metal tube was prepared according to the ITER cryostat bridge design. Vibrations were applied using a shaker, assuming a worst-case scenario of ITER operation. Using experimental data, we were able to estimate the influence of vibrations on the accuracy of plasma current measurement using FOCS. We have also estimated requirements for the spun fiber which is planned to be used for FOCS. It is concluded that it is not possible to satisfy the ITER requirements when using commercially Hi-Bi spun fibers, while a Lo-Bi fiber with a ratio of the linear beat length to the spun period of ~200 allows to achieve the goal.
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 difficulty 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 . Finally, we estimate the minimum required LB/SP ratio of the sensing fibre needed to satisfy the ITER plasma current measurement specifications.
For constructing a quadrature phase interferometer for optical current sensors, we integrated various optical components on a single polymeric waveguide chip. To obtain stable output in quadrature interferometer, it is important to remove the scattered light inside the planar waveguide of integrated optics. In this work, we analyzed the mechanism of optical interference due to the radiated light, and improved the structure of optical waveguide device . After the improvement of waveguide structure, the sensor output signal was maintained within ±0.5% error range regardless of its operating point drift.
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