Advanced Material and Component Behavior Under Fusion Loading Conditions

The first wall and divertor components in a fusion reactor are exposed to extreme environmental conditions. A high number of energetic particles, hydrogen isotopes from the fusion fuel, helium and neutrons as fusion reaction products in a reactor, as well as impurity atoms impinge on the plasma-facing armor. The deposited energy leads to extreme surface temperatures – strongly depending on the location within the device – and consequently to transient or steady state stress gradients between surface and cooling structure. Transient loads impose fatigue phenomena on components, as well as to the armor material itself. On the component level, this can limit the lifetime of joints between dissimilar materials in a component. On the material level, surface roughening, crack and void formation, up to loss of grains are observed. Depending on the particle kinetic energy, erosion by sputtering needs to be considered and avoided in areas with very high particle fluxes like the divertor. Material degradation is further observed due to thermal effects like recrystallization and grain growth. In addition, the fusion environment combines all these loads with a flux of neutrons up to 14 MeV from the deuterium-tritium reaction. As a consequence, collisional damage and transmutation reactions (both to light and heavy nuclei) modify the first wall materials significantly.<br/>For a fusion reactor which aims at a high duty cycle, consequently extreme particle and neutron fluences to the plasma-facing materials arise. Material degradation and erosion behavior must be tailored to allow an economical operation. Finally, safety aspects like a self-passivating behavior of first wall armor in case of a loss-of-coolant event need to be considered.<br/>In this presentation, the development of cutting-edge material solutions for first wall and divertor components of a fusion power device will be presented. Tungsten is still the material of choice for a fusion reactor, since its benign behavior in terms of erosion, fuel retention, thermal resistance and activation is unsurpassed. Recent developments of tungsten-based composite materials, like fiber-reinforced tungsten composites produced by chemical infiltration techniques or via powder metallurgy, avoid the brittleness issue of pure tungsten. Graded materials allow the combination of tungsten-based armor with structural and heat sink materials, based on reduced-activation ferritic-martensitic steels or copper alloys. Consequences of accidental loss-of-coolant scenarios with air or water ingress, leading to the mobilization of activated tungsten and its transmutation products, can be addressed by “smart” self-passivating tungsten alloys.<br/>These advanced material concepts for plasma-facing armor require testing and qualification in environments as close to the fusion first wall conditions as possible. Since the operational complexity of fusion confinement devices in most cases interferes with dedicated material tests and therefore material exposure in these devices leads to ill-defined experimental conditions, dedicated test environments are used. Linear plasma devices provide particles in the correct energy and flux regimes as at the first wall of a fusion device. They allow a combination with lasers in pulsed operation mode to apply transient and periodic loads to the materials. Steady state and transient power fluxes, up to very high power ranges (MW to GW per m2) as in transient fusion plasma events can be applied to components by electron and neutral beam devices, still allowing a very controlled experiment and variability of the loading parameters. From both the in operando and the post exposure examinations in such extreme loading tests, the material behavior can be characterized, failure modes identified, and critical material parameters determined. This presentation will give an overview on the material and component characterization of fusion plasma-facing components in extreme conditions.