Plasma-facing materials and components in a fusion reactor are the interface between the 
plasma and the material part. The operational conditions in this environment are probably 
the most challenging parameters for any material: high power loads and large particle and 
neutron fluxes are simultaneously impinging at their surfaces. To realize fusion in a tokamak 
or stellarator reactor, given the proven geometries and technological solutions, requires an 
improvement of the thermo-mechanical capabilities of currently available materials. In its 
first part this article describes the requirements and needs for new, advanced materials for the 
plasma-facing components. Starting points are capabilities and limitations of tungsten-based 
alloys and structurally stabilized materials. Furthermore, material requirements from the 
fusion-specific loading scenarios of a divertor in a water-cooled configuration are described, 
defining directions for the material development. Finally, safety requirements for a fusion 
reactor with its specific accident scenarios and their potential environmental impact lead to 
the definition of inherently passive materials, avoiding release of radioactive material through 
intrinsic material properties. The second part of this article demonstrates current material 
development lines answering the fusion-specific requirements for high heat flux materials. 
New composite materials, in particular fiber-reinforced and laminated structures, as well 
as mechanically alloyed tungsten materials, allow the extension of the thermo-mechanical 
operation space towards regions of extreme steady-state and transient loads. Self-passivating

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Development of advanced high heat flux and plasma-facing materials

Challenges for plasma-facing components in nuclear fusion

Component development for operation in a large-scale fusion device requires thorough testing and qualification for the intended operational conditions. In particular environments are necessary which are comparable to the real operation conditions, allowing at the same time for in situ/in vacuo diagnostics and flexible operation, even beyond design limits during the testing. Various electron and neutral particle devices provide the capabilities for high heat load tests, suited for material samples and components from lab-scale dimensions up to full-size parts, containing toxic materials like beryllium, and being activated by neutron irradiation. To simulate the conditions specific to a fusion plasma both at the first wall and in the divertor of fusion devices, linear plasma devices allow for a test of erosion and hydrogen isotope recycling behavior under well-defined and controlled conditions. Finally, the complex conditions in a fusion device (including the effects caused by magnetic fields) are exploited for component and material tests by exposing test mock-ups or material samples to a fusion plasma by manipulator systems. They allow for easy exchange of test pieces in a tokamak or stellarator device, without opening the vessel. Such a chain of test devices and qualification procedures is required for the development of plasma-facing components which then can be successfully operated in future fusion power devices. The various available as well as newly planned devices and test stands, together with their specific capabilities, are presented in this manuscript. Results from experimental programs on test facilities illustrate their significance for the qualification of plasma-facing materials and components. An extended set of references provides access to the current status of material and component testing capabilities in the international fusion programs.

https://iopscience.iop.org/article/10.1088/1741-4326/aa4feb/pdf

Material testing facilities and programs for plasma-facing component testing

For the next step fusion reactor the use of tungsten is inevitable to suppress erosion and allow operation at elevated temperature and high heat loads. Tungsten fibre-reinforced composites overcome the intrinsic brittleness of tungsten and its susceptibility to operation embrittlement and thus allow its use as a structural as well as an armour material. That this concept works in principle has been shown in recent years. In this contribution we present a development approach towards its use in a future fusion reactor. A multilayer approach is needed addressing all composite constituents and manufacturing steps. A huge potential lies in the optimization of the tungsten wire used as fibre. We discuss this aspect and present studies on potassium doped tungsten wire in detail. This wire, utilized in the illumination industry, could be a replacement for the so far used pure tungsten wire due to its superior high temperature properties. In tensile tests the wire showed high strength and ductility up to an annealing temperature of 2200 K. The results show that the use of doped tungsten wire could increase the allowed fabrication temperature and the overall working temperature of the composite itself.

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Development of tungsten fibre-reinforced tungsten composites towards their use in DEMO-potassium doped tungsten wire

Advanced tungsten materials for plasma-facing components of DEMO and fusion power plants

DEMO is the name for the first stage prototype fusion reactor considered to be the next step after ITER towards realizing fusion. For the realization of fusion energy especially, materials questions pose a significant challenge already today. Heat, particle and neutron loads are a significant problem to material lifetime when extrapolating to DEMO. For many of the issues faced, advanced materials solutions are under discussion or already under development. In particular, components such as the first wall and the divertor of the reactor can benefit from introducing new approaches such as composites or new alloys into the discussion. Cracking, oxidation as well as fuel management are driving issues when deciding for new materials. Here composites as well as strengthened CuCrZr components together with oxidation resilient tungsten alloys allow the step towards a fusion reactor. In addition, neutron induced effects such as transmutation, embrittlement and after-heat and activation are essential. Therefore, when designing a component an approach taking into account all aspects is required.

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Materials for DEMO and reactor applications—boundary conditions and new concepts

The development of advanced materials is essential for sophisticated energy systems like a future fusion reactor. Tungsten fibre-reinforced tungsten composites (W f /W) utilize extrinsic toughening mechanisms and therefore overcome the intrinsic brittleness of tungsten at low temperature and its sensitivity to operational embrittlement. This material has been successfully produced and tested during the last years and the focus is now put on the technological realisation for the use in plasma facing components of fusion devices. In this contribution, we present a way to utilize W f /W composites for divertor applications by a fabrication route based on the chemical vapour deposition (CVD) of tungsten. Mock-ups based on the ITER typical design can be realized by the implementation of W f /W tiles. A concept based on a layered deposition approach allows the production of such tiles in the required geometry. One fibre layer after the other is positioned and ingrown into the W-matrix until the final sample size is reached. Charpy impact tests on these samples showed an increased fracture energy mainly due to the ductile deformation of the tungsten fibres. The use of W f /W could broaden the operation temperature window of tungsten significantly and mitigate problems of deep cracking occurring typically in cyclic high heat flux loading. Textile techniques are utilized to optimise the tungsten wire positioning and process speed of preform production. A new device dedicated to the chemical deposition of W enhances significantly, the available machine time for processing and optimisation. Modelling shows that good deposition results are achievable by the use of a convectional flow and a directed temperature profile in an infiltration process.

M. Aumann

Papers

Materials for DEMO and reactor applications—boundary conditions and new concepts

Chemically deposited tungsten fibre-reinforced tungsten – The way to a mock-up for divertor applications

Tungsten fibre-reinforced tungsten composite development of a new high performance material

Chemical deposited tungsten fibre-reinforced tungsten – The way to a mock-up for divertor application