No abstract

The effect of combined heating and helium particle flux on annealed tungsten samples has been studied in the neutral beam facility GLADIS. He beams with power densities of 2.4 MW/m 2 and 9.5 MW/m 2 were used to adiabatically load the samples to peak surface temperatures from ~ 950 °C (1223 K) to ~ 2700 °C (2973 K). Changes in the surface morphology resulting from combined heat and the flux exposure were studied for He fluences up to 3×10 22 /m 2 . Typical structures for the sample loaded at ~ 950 °C (1223 K) were blisters with a clear grain orientation dependence and the largest blisters formed on grains with <001> surface normal. However at higher temperatures, blistering was more easily suppressed for grains near this orientation because the growth of larger blister takes place more slowly. An evolution from a "porous structure" to a "coral-like structure" with increasing fluence was observed on the samples loaded at the highest temperature. Based on these results mechanisms for surface modification at different temperatures are discussed and a texture with <001> parallel to the normal direction of the grains is suggested to optimize the plasma facing material due to their stronger resistance to early stage blistering.

Melting and boiling behaviour of pure tungsten and 1 wt% lanthanum-oxide-doped tungsten (WL10) are investigated, focusing on the material selection with respect to material loss induced by cavitation. Melting experiments under high heat loads are carried out in the high heat flux facility GLADIS. Pulsed hydrogen neutral beams with heat flux of 10 and 23 MW m−2 are applied onto the adiabatically loaded samples for intense surface melting. Melt layer of the two tungsten grades exhibit different microstructure characteristics. Substantive voids owning to cavitation in the liquid phase are observed in pure W and lead to porous resolidified material. However, little cavitation bubbles can be found in the dense resolidified layer of WL10. In order to find out the gaseous sources, vapour collection is performed and the components are subsequently detected. Based on the observations and analyses, the microstructure evolutions corresponding to melting and vapourization behaviour of the two tungsten grades are tentatively described, and furthermore, the underlying mechanisms of cavitation in pure W and its suppression in WL10 are discussed.

Divertor wall materials in future fusion devices will be subject to both high flux steady state plasma and transient ELM striking which could limit the lifetime of these plasma facing materials. A pulsed plasma source was therefore developed to reproduce these conditions. Laser irradiation of similar pulse length has been used to disambiguate between the effects of particle and heat loads compared with transient heating alone. A lowered threshold for damage of tungsten was observed in the case of simultaneous transient and steady state loads compared to transients alone, while surface damage was also enhanced with repeated laser irradiation following steady plasma exposure, compared to the two processes individually, indicating a synergistic enhancement of surface modification due to plasma exposure.