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.
The linear plasma device PSI-2 serves as a pilot experiment for the development of components, operational regimes and control systems for the linear plasma device JULE-PSI, which will be located in ...the nuclear environment allowing studies of radioactive and toxic samples. PSI-2 is also used for fusion reactor relevant plasma-material interaction studies. This contribution describes the PSI-2 layout and parameters and summarizes the recent scientific and technical progress in the project, including the installation of a target station for the sample manipulation and analyses.
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.
In this paper the manipulation of power deposition on divertor targets at DIII-D by the application of resonant magnetic perturbations (RMPs) for suppression of large type-I edge localized modes ...(ELMs) is analysed. We discuss the modification of the ELM characteristics by the RMP applied. It is shown that the width of the deposition pattern in ELMy H-mode depends linearly on the ELM deposited energy, whereas in the RMP phase of the discharge those patterns are controlled by the externally induced magnetic perturbation. It was also found that the manipulation of heat transport due to the application of small, edge RMP depends on the plasma pedestal electron collisionality
. We compare in this analysis RMP and no RMP phases with and without complete ELM suppression. At high
, the heat flux during the ELM suppressed phase is of the same order as the inter-ELM and the no-RMP phase. However, below this collisionality value, a slight increase in the total power flux to the divertor is observed during the RMP phase. This is most likely caused by a more negative potential at the divertor surface due to hot electrons reaching the divertor surface from the pedestal area along perturbed, open field lines.
The hydrogen retention in fusion reactors can be significantly influenced by the presence of plasma impurities. Earlier studies showed that helium can reduce the retention in tungsten wall materials. ...This paper gives the results of experiments on this topic in the linear plasma generator PSI-2. Exposures of polycrystalline tungsten samples to a deuterium plasma were performed at low temperatures (380K) under the variation of the impurity species (He, Ar) and concentration (0–5%). For the experiments with He, the total deuterium fluence was varied between 2⋅1024m−2 and 2⋅1026m−2. Subsequently, the surface morphology and deuterium retention were investigated. The results show a reduction of the deuterium retention by a factor of 3 for helium, and an increase by up to 30% for argon. A diffusion model for the helium case was developed, in which a shallow layer of porous helium nanobubble structures reduces the total deuterium content.
Applying liquid metals as plasma facing components for fusion power-exhaust can potentially ameliorate lifetime issues as well as limitations to the maximum allowed surface heat loads by allowing for ...a more direct contact with the coolant. The material choice has so far been focused on lithium (Li), as it showed beneficial impact on plasma operation. Here materials such as tin (Sn), gallium (Ga) and aluminum (Al) are discussed as alternatives potentially allowing higher operating temperatures without strong evaporation. Power loads of up to 25 MW m−2 for a Sn W component can be envisioned based on calculations and modeling. Reaching a higher operating temperature due to material re-deposition will be discussed. Liquids typically face stability issues due to j × B forces, potential pressure and magnetohydrodynamic driven instabilities. The capillary porous system is used for stabilization by a mesh (W and Mo) substrate and replenishment by means of capillary action.
ITER's divertor will have to sustain high particle and heat loads. Tungsten has been chosen as plasma facing material for the entirety of ITER's divertor. Therefore, it is important to study the ...behavior of ITER-grade tungsten under fusion relevant loads. In this work, tungsten samples with two different microstructures were studied, one with needle-like grains transversal to the surface, and the other with larger, isotropic grains obtained after recrystallization. The samples were exposed to a steady state plasma and a pulsed laser in the linear plasma device PSI-2 to simulate the conditions of the divertor in a fusion reactor. The results show that the transversal samples have a higher damage threshold, but at higher power density and pulse numbers, the damage observed, as well as the surface roughness, is similar. The recrystallized samples displayed a shallower crater depth, which might be an important factor to be considered when selecting a material for ITER and should be studied further.
The effects of particle flux and exposure temperature on surface modifications and deuterium (D) retention were systematically investigated on four different tungsten (W) microstructures. ...As-received, recrystallized, and single crystal W samples were exposed to D plasmas at surface temperatures of 530–1170 K. Two different ranges of D ion fluxes (1022 and 1024 D+m−2s−1) were used with the ion energy of 40 eV and particle fluence of 1026 D+m−2. Increasing the particle flux by two orders of magnitude caused blister formation and D retention even at temperatures above 700 K. The main effect of increasing the particle flux on total D retention was the shifting of temperature at which the retention was maximal towards higher temperatures. Diffusion-trapping simulations were used to fit the thermal desorption spectroscopy (TDS) release peaks of D, yielding one or two types of trapping sites with de-trapping energies around 2 eV.
Cracking thresholds and crack patterns in tungsten targets after repetitive ITER-like edge localized mode (ELM) pulses have been studied in recent simulation experiments by laser irradiation. The ...tungsten specimens were tested under selected conditions to quantify the thermal shock response. A Nd:YAG laser capable of delivering up to 32 J of energy per pulse with a duration of 1 ms at the fundamental wavelength λ = 1064 nm has been used to irradiate ITER-grade tungsten samples with repetitive heat loads. The laser exposures were performed for targets at room temperature (RT) as well as for targets preheated to 400 °C to measure the effects of the ELM-like loading conditions on the formation and development of cracks. The magnitude of the heat loads was 0.19, 0.38, 0.76 and 0.90 MJ m−2 (below the melting threshold) with a pulse duration of 1 ms. The tungsten surface was analysed after 100 and 1000 laser pulses to investigate the influence of material modification by plasma exposures on the cracking threshold. The observed damage threshold for ITER-grade W lies between 0.38 and 0.76 GW m−2. Continued cycling up to 1000 pulses at RT results in enhanced erosion of crack edges and crack edge melting. At the base temperature of 400 °C, the formation of cracks is suppressed.
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