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During the 2019 experimental campaign, the WEST tokamak was partially equipped with ITER-like plasma facing units (PFUs) made of discrete monoblocks (MBs) in one of the twelve lower divertor sectors. The magnetic field lines can enter the gaps between two MBs and strike their leading edges (LEs) with near normal incidence, leading to high localized heat flux, temperature and thermomechanical stress during both stead-state operation and transients. Exposed leading edges are a particular matter of concern because of potential crack formation, recrystallization or even melting of tungsten. During the 2019 experimental campaign in WEST, five of them were misaligned (vertical misalignment h = 0.30 ± 0.1 mm) with their poloidal leading edges exposed to the plasma heat flux. A medium wavelength IR filter (MWIR: 3.9 ± 0.1 μm) was installed in the very high resolution infrared system, featuring a submillimeter spatial resolution (∼0.1 mm/pixel). This system has a temperature detection threshold of T threshold, BB ≈ 250 °C. In this paper, thermal analysis will be presented with a specific focus on overheating of poloidal and toroidal edges using post-mortem measured emissivity maps. The study reveals an unexpected hot spot in the top LE corner of misaligned PFUs at the outer strike point (OSP). Photonic calculations were performed with different emissivity on top and lateral surfaces, in order to consider the complex reflectance characteristics in the toroidal gaps. During this experiment, the Larmor radius (0.35 mm) is comparable to the gap size. Thus, the helical trajectory of the particles may significantly affect the heat load distribution in the edges vicinity. Heat load simulations were performed with ion orbit modeling that consider the Larmor radius, to study its impact on the thermal distribution of the MB. The photonic simulations showed that it was possible to observe a false hot spot at the top LE corner, with both optical approximation (OA) and ion orbit (IO) modeling, due to reflections and different emissivity values. [ABSTRACT FROM AUTHOR]

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 up-graded 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. The period and length of the gratings are designed to increase the number of measurement spots to fourteen gratings per fiber, regularly distributed over 17 cm, 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 2.5 mm deep lateral groove localized at 5 mm beneath the top of bulk tungsten monoblocks. A PFC mock-up machined with a groove has been tested under HHF facility to assess the effect of the groove on monoblocks thermal behavior. The test demonstrates that machined monoblocks behave as expected from simulation and can withstand 20 MW/m2 heat flux (i.e. 1200 °C in the fiber) with 20 % overheating as compared to intact monoblocks. [ABSTRACT FROM AUTHOR]