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Due to its unique properties tungsten is a promising candidate as plasma-facing-material (PFM) in future nuclear fusion reactors. Tungsten features an exceptionally high melting point, high thermal conductivity, low tritium inventory and comparatively low erosion rate under plasma loading [1]. But given the extreme loads on the PFM during operation of a fusion reactor, the lifetime of plasma-facing components (PFC)s is limited. Currently, it is planned to replace damaged PFCs when they reach the end of their service life. However, the lifetime of PFCs could be increased by in situ repair using additive manufacturing technology (AM) in the form of direct-energy-deposition (DED). The wire–based laser metal deposition process (LMD-w) meets several necessary conditions for operation in the vessel and could be used for performing such in situ repairs.It was investigated if the LMD-w process is able to heal thermal induced surface cracks and roughening by remelting the substrate during deposition of tungsten. For this purpose, tungsten samples of 12 × 12 × 5 mm3, which later served as substrate plates for the LMD-w experiments, were treated with combined steady-state and transient thermal loads in the electron beam facility JUDITH 2. These samples were brazed to a copper cooling structure and exposed to 105 thermal shocks of 0.5 ms duration and an intensity of Labs = 0.55 GW m−2 (FHF = 12 MW s0.5 m−2) at a base temperature of Tbase = 700 °C. This way, edge localized mode (ELM) like thermal load damage was induced on the tungsten samples. On these samples, different LMD-w and laser remelting process strategies were performed. Subsequently, these samples were analyzed, and it was examined that the healing of the pre-damaged substrate material was successful. In parallel, the laser remelting process was modeled in a thermal transient finite element method (FEM) simulation in order to gain an insight into the temperatures prevailing in the material during the process.

Divertor components for ITER and even beyond will be subjected to cyclic steady state heat loads with a duration of several minutes to hours, repeatedly occurring slow transients during reattachment or ramp-up and down, as well as heat loads during ELMs applying a combination of low cycle fatigue and creep as well as high cycle fatigue via thermal shock loads. While for the qualification of components the duration of the fatigue cycles up to now has been kept small, i.e., close to the required time to reach thermal saturation which is 10 s for typical divertor components, creep in these components has not yet been assessed.In this study divertor tungsten monoblock mock-up manufactured via hot radial pressing in the ITER-like geometry consisting of 4 monoblocks and quality checked via ultrasonic testing are exposed to high heat flux loads in the electron beam facility JUDITH 2 using a high temperature cooling circuit with controlled water chemistry. Thereby, cyclic loads up to 1000 cycles with a duration of 10 to 600 s and a power density of 20 MW/m2 were applied, representing strike point loading conditions in DEMO during strike point sweeping scenarios. Each of the tungsten monoblocks is loaded individually providing the possibility to study different scenarios on one single mock-up. The aim is to assess the performance and degradation of performance due to the applied loads, which is supported by characterization via metallography, profilometry, SEM and hardness testing after the high heat flux tests.

The basic principle of electron beam melting (EBM) technology is the additive generation of structures by the selective melting of metal powder layer by layer with an electron beam under vacuum conditions. The cooling rate of the EBM process can be reduced drastically by increasing the temperature of the powder bed to avoid the formation of solidification cracks by brittle materials such as tungsten (W). This refractory metal is a promising candidate as plasma facing material for future fusion reactors. The selection of tungsten is owing to its physical properties such as the melting point of 3420 °C, the high strength and high thermal conductivity, the low thermal expansion and low erosion rate. Disadvantages are the low ductility, and fracture toughness at room temperature. Furthermore, the manufacturing by mechanical machining, such as milling and turning, is extremely cost and time consuming. An interesting alternative process route to conventional manufacturing technologies is EBM. It allows the near-net shape fabrication of prototype structures with geometrical freedom and has proven its capability for mass production by the manufacturing of hip prostheses made of titanium.This manuscript describes the fabrication of tungsten parts via electron beam melting, with application to the manufacturing of divertor armour. The investigation comprises the microstructure examination, crystallographic texture, as well as mechanical characterization via tensile and Charpy impact testing. This is followed by the presentation of process routes to fabricate mock-ups with different designs and copper cooling structures.Furthermore, the different mock-ups were exposed to high heat flux (HHF) applying transient thermal loads to assess thermal shock and thermal fatigue performance of EBM tungsten.Post mortem analyses were performed quantifying the occurring damage with respect to reference tungsten grades by microscopical means.The achieved results demonstrate the high potential to process tungsten via electron beam melting.

Plasma-facing components (PFC) in nuclear fusion reactors are exposed to demanding conditions during operation. The combination of thermal loads, plasma exposure as well as neutron induced damage and activation limits the number of materials suitable for this application. Due to its properties, tungsten (W) is foreseen as plasma-facing material (PFM) for the future DEMOnstration power plant. It is considered suitable due to its exceptionally high melting point, excellent thermal conductivity, low tritium retention and low erosion resistance during plasma exposure. But even tungsten armored PFCs have a limited lifetime due to, among other factors, surface erosion and the resulting thickness reduction of the armor material.In-situ local deposition of tungsten by means of additive manufacturing (AM) could counteract surface erosion and thus increase the service life span of PFCs. After evaluation of the potential AM processes qualified for this task, the wire-based laser metal deposition (LMD-w) process was selected as the most suitable process. First trials were conducted to examine if it is possible to reliably deposit tungsten onto tungsten substrate using the LMD-w process. In these first studies, single welding beads were generated, and in later experiments, entire layers were created from several welding beads which are arranged next to each other. To ensure reproducibility of the results, the substrate temperature was kept constant. Further experiments aimed at the elimination or minimization of problems such as oxidation, occurrence of balling defects, porosity, cracking, surface waviness and insufficient connection to the substrate. To increase the welding bead quality, the input parameters like laser power, deposition velocity, wire feed rate, inert gas flow, as well as the wire position were optimized. Furthermore, stacking of several layers, as well as the remelting of an already created layer, were carried out and investigated. This study represents the first steps in testing the feasibility of an in-situ surface regeneration concept for PFCs.

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.

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.

Graded tungsten/steel composite is a potential interlayer to alleviate the thermal expansion mismatch between tungsten (W) armour and steel structure in the first wall (FW) of a future fusion reactor. However, existing thermomechanical finite element (FE) numerical simulations of the FW featuring this graded interlayer have modelled the composites inappropriately by assuming their properties to follow an elementary rule of mixtures; linear interpolation of the properties of W and steel based on the volume content of W. This opens up the question of determining the properties of the composites appropriately. Thus, in this study, a microstructural image based modelling technique is proposed to predict the macroscopic mechanical properties of the composites. As a case study, plasma sprayed W/steel composites of three compositions were investigated. FE simulations of the corresponding microstructures, captured via scanning electron microscopy (SEM), were carried out with the help of an open source code (OOF2) which transforms the SEM images into a 2-dimensional mesh. For the determination of macroscopic mechanical properties, image based finite element (FE) simulations of the mapped mesh were carried out. These simulations were done on microstructures of different physical sizes, including mesoscale and microscale morphological artefacts. Also, for each physical size, several SEM images were captured at different sites in the composites to consider the randomness of the material. FE simulations were conducted at various virtual temperatures between 20 °C and 700 °C. The predicted mechanical properties agreed much better with the few available experimentally determined literature values of the composites than the simple linear interpolation.

This work reports about preparation of W/Fe co-sputtered layers between tungsten and steel in order to ease their coefficient of thermal expansion mismatch.Iron and tungsten have been gradually co-sputtered on W tiles with real time deposition parameters changing to obtain a graded interlayer: SEM, EDS, XPS, HR-TEM have been used to characterize the W/Fe co-sputtered layer.W tiles co-sputtered with the W/Fe layers have been hot pressed to steel tiles: morphology and mechanical strength have been investigated for joints with and without W/Fe co-sputtered layer.Lap-shear tests in compression at room temperature have been made on hot pressed W/steel joints with and without W/Fe co-sputtered layer, showing a different behaviour.High flux test on joints with and without W/Fe co-sputtered layer are reported and discussed. The mechanical test results of manufactured joints are in good accordance with high heat flux test results, showing that the behaviour of hot-pressed joints with W/Fe co-sputtered layer was slightly worse if compared to joints without co-sputtered layer at the interface.

Prompt re-deposition strongly reduces the migration of eroded material in current fusion devices, thus lowering core dilution and extending the lifetime of the plasma-facing components. However, under the high-density ( $n_{\mathrm{e}} \gt 2 \times 10^{20}$ m ^−3 ), low-temperature ( $T_{\mathrm{e}} \lt 5$ eV) divertor plasma conditions anticipated for future fusion devices like ITER, Coulomb collisions become highly efficient. These collisions not only prevent the ions from completing their Larmor orbits but also strongly couple the sputtered impurities to the plasma, leading to impurity entrainment. Entrainment drags impurities with the plasma flow to the surface, leading to enhanced impurity impact energies as discussed in the companion paper (Cornelissen M.J.H. et al 2025 Nucl. Fusion 65 026009), and the re-deposition of sputtered particles. This entrained re-deposition was studied with the linear plasma generator Magnum-PSI, whose linear geometry allowed for a clear distinction between entrained and prompt re-deposition. Five W targets were exposed to high-density ( $n_{\mathrm{e}} \approx 5 \times 10^{20}$ m ^−3 ), low-temperature ( $T_{\mathrm{e}}\approx 1.2$ eV) argon (Ar) plasmas. The deposition pattern of tungsten (W) on molybdenum (Mo) witness plates was determined using ion beam analysis and electron scanning microscopy. As the ion-impact energy of Ar ^+ increased from $E_{\mathrm{i}}\approx$ 43 eV to $E_{\mathrm{i}}\approx$ 94 eV, the local re-deposition rate decreased from $R\approx$ 93% to $R\approx$ 68%. Entrained re-deposition became evident from an up-downstream asymmetry in the re-deposition profile and was dominant over prompt re-deposition for the exposures investigated. Although the ionization length was long, high re-deposition rates were found, revealing that neutral-ion interactions were important. Thus, entrainment will be a crucial ingredient in the erosion and re-deposition studies of future fusion reactors.

Tungsten-steel metal matrix composites are consolidated using electro-discharge sintering. At first steel and tungsten powders are sintered separately and then 25 vol% W, 50 vol% W and 75 vol% W mixed powders are sintered. A thorough process parametric study is carried out involving analysis of the influence of particle size distribution, sintering pressure, and discharge energy on the maximum discharge current and obtained residual porosity. Thermal expansion coefficient and the specific heat capacity of the optimized sintered composites are almost same as their theoretical values, however the thermal conductivities and the mechanical properties are lower than the expected values.

The realization of the first wall (FW), which is composed of a protective tungsten (W) armor covering the structural steel material, is a critical challenge in the development of future fusion reactors. Due to the different coefficients of thermal expansion (CTE) of W and steel, the direct joining of them results in cyclic thermal stress at their bonding seam during the operation of the fusion reactor. To address this issue, this study benchmarks two joining concepts. The first concept uses an atmospheric plasma sprayed graded interlayer composed of W/steel composites with a varying content of W and steel to gradually change the CTE. The second concept uses a spark plasma sintered graded interlayer. Furthermore, in order to benchmark these concepts, a directly bonded W-steel reference joint as well as a W-steel joint featuring a vanadium interlayer were also tested. These joints were tested under steady-state high heat flux cyclic loading, starting from a heat flux of 1 MW/m2 up to 4.5 MW/m2, with stepwise increments of 0.5 MW/m2. At each heat flux level, 200 thermal cycles were performed. The joints featuring a sintered graded interlayer survived only until 1.5 MW/m2 of loading, while the joint featuring plasma sprayed graded interlayer and V interlayer survived until 3 MW/m2.

Highly conductive TiO2 films with different Nb doping levels (up to 5 at%) were prepared by reactive DC magnetron sputtering under precise control of the oxygen partial pressure. They were deposited on unheated substrates, covered with a protective Si3N4 layer, and subsequently annealed at 300 °C. The doping efficiency of Nb is greater than 90%. Conductivity is a maximum for a partly oxidized target in the transition range. The best films exhibit a resistivity of 630 µΩ cm and a mobility of 7.6 cm2/Vs combined with a high transparency above 70%. Comparing the behavior of undoped and Nb-containing films, intrinsic limits of the conductivity in the TiO2−x:Nb system could be observed, and a consistent model explaining these findings is presented. The conductivity is limited—by decreasing electron density due to Nb oxidation—by increasing incorporation formation of Nb2O5 clusters as scattering centers with increasing oxygen partial pressure and Nb concentration, by a transition from the crystalline to the amorphous state of the films below a critical oxygen partial pressure.