The extreme brittleness of tungsten (W) is one of the challenges of using W as first wall material. One attempt to alleviate this problem is to use W alloys with better mechanical properties. However these alloying elements must not degrade the favorable properties of W with respect to its application at the first wall of fusion devices: low sputter yield and hydrogen inventory. In this work we investigate the hydrogen retention in the recently proposed W/Ta alloys under deuterium ion bombardment. By directly comparing pure W and W/Ta alloys with 1% and 5% Ta content we found that the W/Ta alloys retain significantly more hydrogen than pure W under identical implantation conditions. Our finding of increased hydrogen retention together with the fact that the Ta alloying did not improve the brittleness makes W/Ta alloys an unacceptable choice for the first wall of fusion devices.

Abstract.The surface morphology and deuterium retention were investigated of polycrystalline tungsten targets that were exposed to deuterium plasmas at widely varying conditions. By changing only one parameter at a time, the isolated effects of flux, time and pre-damaging on surface modifications and deuterium retention were studied. The sample exposed to low-flux plasma (10 20 m −2 s −1 ) is mostly smooth with only a few areas containing very large blisters (50 -500 µm). The samples exposed to high-flux plasmas (10 24 m −2 s −1 ) show large numbers of smaller blisters (1 -10 µm) and in addition even smaller protrusions (<750 nm). The size of the blisters and their density strongly increase with fluence. Pre-damaging tungsten with MeV ions leads to less blisters but to more protrusions. In addition to these (sub-)micrometer-sized structures, all samples show formation of nanostructures. Comparison of a low-flux and high-flux sample exposed to similar fluence showed that the variation in morphology is dominated by the flux differences. It is shown that the blisters and protrusions originate in inter-and intra-granular cavities, respectively. The depth of the cavities underneath the surface correlates well with the depth distributions of the retained deuterium. Trapping of significant amounts of deuterium therefore seems to take place in and/or close to these cavities and gives rise to an additional peak in the thermal desorption spectrum at 700 K.PACS numbers: 28.52.Fa, 28.52. Nh, 52.40.Hf, 52.77.Dq, 61.80.Jh, Submitted to: Nuclear Fusion Surface morphology and deuterium retention at high-flux deuterium plasmas 2

Fundamental understanding of hydrogen-metal interaction is challenging due to lack of knowledge on defect production and/or evolution upon hydrogen ingression, especially for metals undergoing hydrogen irradiation with ion energy below the reported displacement thresholds from literature. Here, applying a novel low-energy argon-sputter depth-profiling method with significantly improved depth resolution for tungsten (W) surfaces exposed to deuterium (D) plasma at 300 K, we show the existence of a 10-nm-thick D-supersaturated surface layer (DSSL) with an unexpectedly high D concentration of ~ 10 at. % after irradiation with ion energy of 215 eV. Electron back-scatter diffraction reveals that the W lattice within this DSSL is highly distorted thus strongly blurring the Kikuchi pattern. We explain the strong damage by the synergistic interaction of the energetic D ions and solute D atoms with the W lattice. Solute D atoms prevent the recombination of vacancies with interstitial W atoms, which are produced by the collisions of energetic D ions with W lattice atoms (Frenkel pairs). This proposed damaging mechanism could also be active on other hydrogen-irradiated metal surfaces. The present work provides a deep insight into hydrogen-induced lattice distortion at plasma-metal interfaces and sheds light on its modelling work.

The microstructure of tungsten samples is systematically modified by recrystallization to investigate the structure dependence of deuterium (D) retention. After a thorough characterization by scanning and transmission electron microscopy, nonrecrystallized samples and samples recrystallized by different degrees are loaded with deuterium in a low-temperature plasma device. The deuterium inventory is measured by nuclear reaction analysis and thermal desorption spectroscopy. The post implantation surface morphology is investigated by scanning electron, optical and atomic force microscopy. The modification by recrystallization allows a wide variation in the crystallite size and has a strong impact on the measured D retention. Both the total amount and the binding state of the retained D are changed.