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Interpretation of temperature distribution observed on W-ITER-like PFUs in WEST monitored with a very-high-resolution IR system

Abstract : 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 (~0.1mm/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.
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Submitted on : Thursday, February 17, 2022 - 11:51:54 AM
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A Grosjean, M H Aumeunier, Y Corre, M Firdaouss, J Gaspar, et al.. Interpretation of temperature distribution observed on W-ITER-like PFUs in WEST monitored with a very-high-resolution IR system. Fusion Engineering and Design, Elsevier, 2021, 168, pp.112387. ⟨10.1016/j.fusengdes.2021.112387⟩. ⟨hal-03578445⟩



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