Recent achievements in R&D on fibre optics current sensor for ITER

ITER is expected to achieve new milestones on the way to commercial fusion energy production. This will be still a research machine where numerous diagnostic systems are deployed, both already used on existing tokamaks and developed specifically for burning plasma operation. One of the innovative diagnostics is the Fibre Optics Current Sensor (FOCS). This paper describes recent results of the ITER FOCS development, concentrating on the front-end design. The FOCS sensing fibre, likely acrylate coated, will be located directly on the external surface of the vacuum vessel. During operation, it will be exposed to temperatures of 100–110 °C and multi-MGy level total radiation doses, while during vacuum vessel baking intervals the temperature will be 200–210 °C with low levels of radiation. Such temperatures are outside the nominal range for acrylate coatings even for acrylate coatings specially designed for high temperature applications. We provide experimental results that indicate that acrylate coated fibres FOCS fibres nevertheless can survive ITER environment without critical degradation and attribute this to the system design, for which the fibre operates in an inert gas (neon) atmosphere.