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Design and integration of femtosecond fiber Bragg gratings temperature probes inside actively cooled ITER-like plasma-facing components

Abstract : Measuring the temperature in plasma-facing components (PFCs) provides information both on plasma parameters in the divertor region and on the thermal stress experienced by PFCs. Fiber Bragg gratings (FBGs) are interesting candidates for this application because they are immune to electromagnetic interferences and their ability to be multiplexed allows an extended spatial coverage. Four fibers, each of them including eleven regenerated Bragg gratings, have been embedded in tungsten-coated graphite components and operated up to their signal-collapsing limit at 800°C. Extending the measurement range towards higher temperatures increases the sensitivity to plasma parameters and allows withstanding higher energy experiments. To overcome thermal limitations, the system is upgraded using femtosecond laser inscribed fibers. In addition to their outstanding thermal stability, femtosecond FBGs benefit from higher signal-to-noise ratios than regenerated FBGs. The paper addresses femtosecond FBGs design and issues relative to their integration inside the actively cooled ITER-like PFCs of the WEST tokamak. Gratings period and length is designed to increase the number of measurement spots to fourteen gratings per fiber, regularly distributed over 17cm, while ensuring robust detection even with strong thermal gradients (no overlapping or deformation of Bragg peaks). The system operates up to 1200°C with gradients reaching 200°C/mm perpendicularly and 40°C/mm in parallel to the fiber. FBGs are inserted in actively cooled ITER-like PFCs through a 2.5mm deep lateral groove localized at 5mm beneath the top of bulk tungsten mono-blocks. A PFC mock-up machined with a groove has been tested under HHF facility to assess the effect of the groove on mono-blocks thermal behavior. The test demonstrates that machined mono-blocks behave as expected from simulation and can withstand 20 MW/m 2 heat flux (i.e. 1200°C in the fiber) with 20% overheating as compared to intact mono-blocks.
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Submitted on : Tuesday, February 22, 2022 - 8:31:20 AM
Last modification on : Friday, April 1, 2022 - 3:54:27 AM
Long-term archiving on: : Monday, May 23, 2022 - 6:11:15 PM

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N Chanet, y Corre, Romain Cotillard, J Gaspar, Guillaume Laffont, et al.. Design and integration of femtosecond fiber Bragg gratings temperature probes inside actively cooled ITER-like plasma-facing components. Fusion Engineering and Design, Elsevier, 2021, 166, pp.112376. ⟨10.1016/j.fusengdes.2021.112376⟩. ⟨hal-03583704⟩

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