First temperature database achieved with Fiber Bragg Grating sensors in uncooled plasma facing components of the WEST lower divertor

• Published in: Fusion engineering and design Vol. 170; p. 112528
• Main Authors: Corre, Y., Chanet, N., Cotillard, R., Gaspar, J., Laffont, G., Pocheau, C., Caulier, G., Destouches, C., Gardarein, J-L., Firdaouss, M., Houry, M., Missirlian, M., Roussel, N., Santraine, B.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.09.2021; Elsevier Science Ltd; Elsevier
• Subjects: Bragg curve; Bragg gratings; Engineering Sciences; High temperature; High temperature measurement; Instrumentation and Detectors; Nuclear Experiment; Optical fiber sensors; Optical fibers; Optics; Photonic; Physics; Plasma; Plasma heat load; Regenerated fiber Bragg grating; Temperature measurement; Thermocouples; Tokamak devices; Ultraviolet radiation; Optical fiber sensors; Regenerated fiber Bragg grating; Plasma heat load; High temperature measurement; Fiber Bragg Grating; regeneration; Regenerated Fiber Bragg grating; WEST; tokamak; Instrumentation; Nuclear instrumentation; optical fiber sensor; plasma heat load; high temperature measurement; Distributed and Quasi-distributed sensing; divertor; optical fiber temperature sensing probe
• ISSN: 0920-3796; 1873-7196
• DOI: 10.1016/j.fusengdes.2021.112528

Plasma Facing Components (PFCs) temperature measurement is required to ensure safe high power for long pulse tokamak operation and for physics studies. A set of twenty thermocouples (TCs) and four optical fiber temperature sensing probes, each of them including eleven wavelength-multiplexed fiber Bragg gratings (FBGs) written with UV radiation, have been integrated and deployed in the WEST lower divertor. To avoid any thermal erasure occurring above 400 °C, the gratings have been regenerated using a high temperature annealing process. The FBG probes are embedded in tungsten-coated graphite components with no active cooling. The diagnostic was working correctly from the first plasma breakdown achieved in WEST (in 2017) until the first dismantling of the lower divertor three years later (2020). Collected data are sorted by temperature range to investigate the ageing of both PFCs and diagnostics. Measurements obtained with FBGs and TCs are consistent, no long-term drift is reported during the overall period of use. The maximum temperature reported by FBGs in the divertor is 830 °C obtained with 90 MJ of cumulated plasma energy in one pulse. A short but reversible collapse of the Bragg peak (from 5 down to 2 dB) has been observed under high temperature due to the strong temperature gradients measured along the fiber (about 20 °C/mm, for an FBG size of 3 mm). This feedback demonstrates that regenerated FBGs are suitable for long-term temperature monitoring in tokamak environment with high heat load, high temperature and gradient, strong magnetic field and vacuum constraint.