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Study on the influence of boronization on the first mirror unit in EAST
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DOI：10.1016/j.nme.2026.102085.png)
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Abstract
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The new ITER baseline adopts full-tungsten (W) plasma-facing components (PFCs). Boronization is employed as the primary wall-conditioning technique to suppress oxygen (O) impurities during the initial operation stage. To evaluate its impact on diagnostic mirrors, a generic first mirror unit (FMU) mock-up was exposed to eight boronization cycles during the 2024 spring campaign in EAST. The FMU consists of the first mirror (FM), second mirror (SM), and third mirror (TM). These mirrors were protected by an aluminum (Al) baffle with a plasma-facing aperture adjacent to the FM. To better characterize the samples, each mirror in the FMU was composed of 16 small mirror samples. Reflectivity, surface morphology, and elemental composition were measured before and after exposure. The results revealed non-uniform deposition and varying degrees of reflectivity degradation across the three mirrors. Significant boron-based (B-based) layers were found only on the FM samples close to the aperture, exhibiting a symmetric spatial distribution consistent with the FMU geometry, with a thickness of approximately 200-400 nm. The B concentration reached up to similar to 30 at.%, leading to a maximum reduction of specular reflectivity from similar to 55% to similar to 1% at the wavelength of 380 nm. No obvious B-based deposits were detected on FM samples away from the aperture or on the SM and TM. However, these locations accumulated thin mixed films with thicknesses of several tens of nanometers, causing reductions of up to similar to 40 percentage points in specular reflectivity over 300-800 nm. All samples showed a pronounced increase in diffuse reflectivity, indicating a modification of the surface roughness. Given that each boronization typically produces similar to 100 nm of B-based coating on the first wall (FW) in EAST, these findings highlight the critical role and effectiveness of the baffle in mitigating direct deposition on diagnostic mirrors. Nevertheless, deposition induced by neutral particles during boronization and the re-deposition of sputtered FM material can still form non-uniform layers on the SM and TM, inevitably impairing their optical performance. These results provide important guidance for next-generation fusion devices, particularly regarding mirror protection, cleaning strategies, and reflectivity recovery.
Keywords:
Boronization
First mirror unit
Deposition
EAST
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