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The exhaust of power and particles is currently considered as one of the ultimate challenges in view of the design of a power producing magnetic confinement thermonuclear fusion device, like DEMO. One predominantly challenging aspect in this regard is the design and manufacture of divertor target plasma-facing components (PFCs) that have to sustain substantial particle, heat and neutron fluxes during fusion operation. With respect to the design of highly loaded actively cooled PFCs, copper (Cu) alloys are currently regarded as state-of-the-art structural heat sink materials (HSM). However, it has been underlined that the use of Cu alloys in PFCs implies issues mainly due to the behaviour of these materials under neutron irradiation characterised by a pronounced loss of ductility at lower and a loss of strength at elevated temperatures. These operating temperature limitations impose a strong constraint on the design of divertor PFCs and have regarding DEMO in the literature been termed a high impact design engineering risk. Against this background, the development of tungsten-copper (W-Cu) composites as potentially advanced HSMs for highly loaded PFCs was pursued by the authors during recent years. The progress of these developments is discussed in the present paper in terms of results of high-heat-flux tests conducted on PFC mock-ups that comprised W-Cu composite material heat sinks. Overall, the results of these tests indicate that W-Cu composites can indeed be regarded a viable class of advanced materials for the heat sink of highly loaded PFCs.

In the framework of the DEMO divertor project an extensive R&D programme has been carried out to develop advanced design concepts for hot water cooled divertor targets. These plasma-facing components are made of W monoblocks as plasma facing material bonded on CuCrZr cooling tubes. They are designed to allow a reliable DEMO operation of 2 h long pulses up to a 20 MW m-2 maximum heat flux. Considering the current experiences of the qualification of ITER divertor components, the operation at significantly higher neutron fluence, the longer required lifetime, and the coolant temperature of 150 °C are challenges for design and manufacturing. The aim of the performed HHF tests is to support the identification of the most promising design concepts for further development. Eight different types of monoblock mock-ups were designed and manufactured by the involved research groups. These mock-ups were assessed by HHF examination in the test facility GLADIS at IPP Garching. The HHF tests were focused on 20 MW m-2 loading with 500-1000 cycles using hot water cooling to simulate slow thermal transients in DEMO. In screening tests heat fluxes up to 32 MW m-2 were applied to study the performance of the components against single thermal events exceeding the expected heat flux limits. This paper gives an overview of the main results obtained from the HHF tests of about 45 components. The microscopic examination after loading analyses recrystallization effects and investigates the W/Cu interfaces in more detail.

We analyzed data from ultrasonic testing and infrared thermography non-destructive examinations performed on a subset of 25 small scale water-cooled target mockups of four target designs developed in the 2nd R&D phase of WPDIV. Examinations were performed before high heat flux tests for the 25 mockups and after high heat flux tests for 18 of them. The detected manufacturing defects are various in size and location. Widths of defects range from 0.2 to 12 mm, which is the entire block width. Angular sizes of defects range from few degrees to 360°. Defects having an angular size less than 50° or a width less than 4 mm are likely to be missed by infrared thermography examinations. After high heat flux tests at 20 MW m−2 up to 500 cycles, we noticed no significant evolution of pre-existing defects.