Manufacturing and high heat flux testing of advanced target mock-ups for the EU-DEMO divertor target

• High Heat Flux tests on advanced target small mock-ups in DEMO relevant conditions. • Assessment of joining techniques applied to advanced materials for plasma facing components. • Manufacturing process of advanced target small mock-ups using tungsten-fiber reinforced copper (Wf-Cu) as cooling pipe. Among the R&D tasks undertaken for the technological development of plasma facing components, a research activity has been undertaken under the framework of the EUROfusion consortium to pursue the consolidation and verification of the current target concepts envisioned for DEMO, i.e., the ITER-like baseline concept and the back-up concept with tungsten-fiber-reinforced copper (Wf-Cu) pipes. Focus has been addressed on the back-up solution (Wf-Cu pipes) finding alternative technological solutions for monoblock-pipe joining in order to reduce the use of materials having high activation and/or degradation under neutron irradiation. Among the brazing alloys tested for the monoblock/Wf-Cu pipe joint, the Gemco commercial alloy has been selected as the most suitable, thanks to its good joining capability and low content of nickel, which suffers high neutron activation. However, the use of Wf-Cu pipes instead of standard CuCrZr ones complicates the joining. One reason could be that the lower thermal expansion coefficient of W compared to Cu affects the overall expansion behaviour of the tube, making it difficult to fill the gap between the surfaces to be joined (necessary for the assembly) during the brazing process. The present paper describes the manufacturing activities which led to the fabrication of small mock-ups. In particular, fabrication, testing at High Heat Flux (HHF) conditions and analyses of a small mock-up are presented. Non-destructive examinations by ultrasonic testing have been performed pre- and post-HHF tests to assess the structural integrity of the mock-up and the reliability of the joining. Furthermore, microscopic examinations at different magnifications have been carried out to highlight microstructural modifications, recrystallization, and potential defect propagation due to thermal cycling.