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Description
The first divertor of DTT [1] will accommodate different magnetic configurations while maximizing heat exhaust capability. Therefore, most of its plasma-facing surface is based on ITER-like plasma-facing units (PFUs), consisting of CuCrZr cooling tubes protected with tungsten monoblocks [2]. To intercept the strike points of the reference magnetic configurations (Signe Null, X-Divertor, and Negative Triangularity), four targets are integrated into the divertor: Inner and Outer Targets for the SN configuration, and Central and Horizontal Targets for compatibility with X-D and NT.
The design and qualification of the Central Target (CT) are particularly challenging. Due to variations in radial position and PFU number, CT monoblocks are toroidally wider than those of the Inner and Vertical Targets, increasing their susceptibility to macro-crack formation under high thermal loads. In addition, the central location of the CT prevents shielding of PFU ends by First Wall modules, requiring a sharp 90° bend that cannot be protected by monoblocks. As a result, the current CT design includes a 25 mm long section clad with tungsten flat tiles, designed to withstand a heat load of 5 MW/m², mainly radiative.
To qualify both the design and manufacturing process, dedicated mock-ups were produced. These consist of copper blocks joined to tungsten flat tiles using different joining technologies and subsequently joined to the CuCrZr tube by diffusion bonding (HRP). Although not fully representative of the final component, the mock-ups were designed to assess the thermal fatigue performance of the W/Cu joint with tile dimensions foreseen in the design (26.5 mm toroidal and 8 mm poloidal). Two of the manufactured mock-ups—one with the W/Cu joint produced by diffusion bonding and one by molten copper casting—were tested at the GLADIS facility. The test protocol consisted of 5000 cycles at 5 MW/m² and 300 cycles at 10 MW/m². The high heat flux (HHF) tests were conducted in successive steps, and non-destructive ultrasonic inspections were performed to monitor joint quality as the number of cycles increased. In this work, the results obtained during the tests are described and the final metallographic investigation is presented.
[1] F. Romanelli et al., Nucl. Fusion 64, 111, (2024)
[2] S.Roccella et al., IEEE TRANSACTIONS ON PLASMA SCIENCE, 52, 9, 2024
[3] P. Innocente et al., Nuclear Materials and Energy, 33, 2022, 101276