High heat flux test results for a thermal break DEMO divertor target and subsequent design and manufacture development

This paper focuses on the development of the water-cooled divertor target concept known as Thermal Break, which was carried out in two phases. In Phase 1, six small scale mock-ups were fabricated and subjected to high heat flux (HHF) testing of up to 25 MW/m(2) and thermal cycling of up to 500 cycles at 20 MW/m(2). All six mockups survived the campaign and maintained 20 MW/m(2) heat exhaust capability. Detailed examination of mockups was carried out to understand the damage mechanisms. One mock-up, which was tested beyond its design intent at 500 cycles, shows signs of progressive damage. Potential damage modes were identified and influenced subsequent Phase 2 mock-up design. Although there are signs of tungsten surface cracking, the predominant damage mode is not by "deep cracking" but substantial permanent deformation in the interlayer features. Therefore, in Phase 2 the manufacturing procedure was updated, the interlayer grooves were given stressre-lieving radii which have significantly reduced the interlayer plastic strain range. Interlayer design parameters were selected following the use of response surface-based design search and optimization. Mock-ups of the Phase 2 design have been manufactured and HHF testing is planned within 2018.