Analysis of structural vibration effect on plasma current measurement using FOCS

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.