The Fibre Optic Current Sensor (FOCS) is a system that will perform plasma current measurements at ITER during long plasma discharges under intense nuclear radiation. Plasma current measurements are important for safe machine operation. The FOCS must perform measurements is harsh tokamak environment, which include strong magnetic fields, high temperatures, and also significant levels of neurons and high energy photons. JET is a unique machine where the impact of such conditions on the sensor performance can be investigated experimentally. In particular, future DT operation will be fully ITER-representative in terms of 14 MeV neutron fluxes.
Considering FOCS radiation hardness assessment, we have installed polarisation detection based FOCS systems at JET and performed measurements in various machine operating scenarios at currents up to 3.5 MA. One system uses the Fibrecore Ge-doped core spun fibre SLB 1250 with a 10 mm spun period, and the another system the Crystal Techno pure silica core LB1300 from with a 5 mm spun.
Spun fibres may be more sensitive to radiation due to internal stresses induced as a result of the preform spinning. No information about the RIA in spun fibres could be found in literature. To address the possible problem of the RIA in spun fibres, radiation testing using Co60 gamma-radiation was performed. Four pure silica core fibres drawn from the same preform without and with different spinning rates, corresponding to the spun period of 5, 20, and 80 mm were exposed up to doses of ~200 kGy, relevant for the ITER environment. A general conclusion from the irradiation is that in case of pure silica fibres the spinning influence on the radiation sensitivity is not significant when the FOCS performance is of concern. The RIA at 1550 nm, FOCS operation wavelength, reached saturation at a dose of ~100 kGy at a level acceptable for ITER.
To address the radiation effect on the FOCS operating in a tokamak environment operating with hydrogen, which is relevant for the ITER first phase operation, analysis of the performance of FOCSs installed at JET provides necessary information. Data accumulated during more than four years of operation were analysed. The radiation dose was estimated based on scaling of the integrated plasma current. Possible transient effect have been observed, but on a long term the performance variation are within measurement uncertainties.