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Functionally graded tungsten/steel composites are attractive to be used as an interlayer to join tungsten (W) and steel for the first wall of future fusion reactor to reduce the thermally induced stresses arising from the different coefficient of thermal expansion (CTE) of W and steel. W/steel composites, with three W contents: 25, 50 and 75 vol% W, will serve as individual sublayers of this functionally graded material. Therefore, the present work exploits an emerging sintering technique, field-assisted sintering technology, to produce these composites. Firstly, a systematic parameter study was conducted aiming to reduce the residual porosity to a minimum while keeping the formation of intermetallic phases at the W/steel interface at a low level. The optimized composites 25, 50 and 75 vol% W achieved a relative density of 99%, 99% and 96%, respectively. Secondly, mechanical tests at elevated temperatures reveal that these composites are ductile above 300 °C, which is the minimum operating temperature of the first wall. Lastly, the measured CTE, specific heat capacity and thermal conductivity were consistent with the theoretically expected values.

Plasma-facing materials (PFMs) in the ITER divertor will be exposed to severe conditions, including exposure to transient heat loads from edge-localized modes (ELMs) and to plasma particles and neutrons. Tungsten is the material chosen as PFM for the ITER divertor. In previous tests, bubble formation in ITER-grade tungsten was detected when exposed to fusion relevant conditions. For this study, ITER-grade tungsten was exposed to simultaneous ELM-like transient heat loads and D/He (6%) plasma in the linear plasma device PSI-2. Bubble formation was then investigated via SEM micrographs and FIB cuts. It was found that for exposure to 100.000 laser pulses of 0.6 GWm−2 absorbed power density (Pabs), only small bubbles in the nanometer range were formed close to the surface. After increasing Pabs to 0.8 and 1.0 GWm−2, the size of the bubbles went up to about 1 µm in size and were deeper below the surface. Increasing the plasma fluence had an even larger effect, more than doubling bubble density and increasing bubble size to up to 2 µm in diameter. When using deuterium-only plasma, the samples showed no bubble formation and reduced cracking, showing such bubble formation is caused by exposure to helium plasma.

During the service life of plasma-facing components, they are exposed to cyclic stationary and transient thermal loads. The former causes thermal fatigue and potentially detachment between the plasma-facing material tungsten and the structural Cu-based materials (divertor) and steel (first wall). The latter causes surface roughening, cracking, or even melting, which could drastically increase the erosion rate. Employing thin flexible W wires (Ww) with a diameter of a few hundred µm can reduce mechanical stresses, and we demonstrated their crack resilience against transient loads within first proof of principle studies. Here, status and future paths towards the large-scale production of such Ww assemblies, including techniques for realizing feasible joints with Cu, steel, or W, are presented. Using wire-based laser metal deposition, we were able to create a homogeneous and shallow infiltration of about 200 µm of the Ww assembly with steel. A high-heat-flux test on such a µ-brush (10 × 10 × 5 mm3 Ww on a ~0.5 mm thick steel layer) using 5 MW/m2 for 2000 cycles was performed without loss of any wire. Microstructural examination after and infrared analysis during the test showed no significant signs of degradation of the joint.