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Influence of hydrogen/helium fluxes on the thermal shock response of tungsten

Beside the thermal loading conditions, plasma facing materials are also exposed to high particle fluxes such as hydrogen, helium and neutrons. Both loading conditions induce a wide range of surface modifications on the loaded surface such as roughening, cracking, blister and fuzz formation, as it can be seen in figure 1.

SEM imagesFigure 1. SEM images showing surface modifications of tungsten induced by helium plasma loading in combination with thermal shocks (0.76 GW/m², 1 ms, 1000 pulses, base temperature = RT) applied by laser before plasma loading (top left), simultaneously (top right, base temperature ~ 850 °C), and after plasma loading (bottom right). The picture on the bottom left shows tungsten fuzz after helium plasma loading without additional thermal shocks.

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However, most of the experimental facilities currently in operation cover only certain areas of the above mentioned exposure conditions. To achieve more realistic exposure conditions and to investigate the influence of particle fluxes namely hydrogen on the thermal shock behaviour, tungsten targets were exposed to both loading conditions successively. Therefore, tungsten targets are exposed to high flux hydrogen-plasma in linear plasma or neutral beam devices such as Pilot-PSI, Magnum-PSI, GLADIS and many more. Thermal shock tests are performed with the electron beam devices JUDITH 1 and JUDITH 2.

The obtained results and surface characterisations show that the combined loading with hydrogen-plasma and electrons has a significant influence on the thermal shock behaviour of tungsten, especially the order of these loads. Some representative examples of loaded surfaces and the induced surface modifications/damages are shown in figure 2. Targets that were preloaded with hydrogen-plasma show a significantly different damage formation and pattern in contrast to the only or first electron beam loaded samples. They show lower crack depth and less plastic deformation and a proceeding of cracks into areas which were not exposed to thermal shock loads. Beside this, the surface temperature during the hydrogen-plasma exposure influences how pronounced these observed differences are. The combination of these results suggests that the main reason for this effect is the hydrogen embrittlement of tungsten and stresses induced by super-saturation of hydrogen in the surface near crystal lattice.

In addition to this it can be stated that the combination of high particle fluxes, like hydrogen, helium and neutrons, with thermal shock events, as it will be the case in a fusion device, will lead to a degradation of the thermal shock response of tungsten. This has to be taken into account for the design and construction of plasma facing components.

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Overview of the induced surface modifications and damagesFigure 2: Overview of the induced surface modifications and damages in dependence on the loading conditions and sequence. The plasma and electron beam loaded areas are indicated by the red circle (d = 8 mm) and the blue square (4 x 4 mm²). a) sample exposed to electron beam only, b) sample first exposed to electron beam afterwards to hydrogen plasma, c) sample first exposed to hydrogen plasma and afterwards to electron beam, d) sample only exposed to hydrogen plasma.