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Abstract: Helium plasma irradiation and electron heating experiments were conducted using tungsten in the divertor simulator NAGDIS-II. Helium plasma irradiation to tungsten led to the formation of nanostructures on the surface, while the nanostructures were annihilated after the potential of the specimen was changed to positive for several 10min so that electrons irradiated the sample without ion irradiation. The specimens were analyzed in detail by transmission electron microscope with the help of focused ion beam technique. It is revealed that the helium nano-bubbles still remained even after the nanostructures were disappeared from the surface. Porosity of the nanostructured tungsten was measured from the TEM images. [Copyright &y& Elsevier]

It has been found recently that low-energy helium (He) plasma irradiation to tungsten (W) leads to the growth of W nanostructures on the surface. The process to grow the nanostructure is identified as a self-growth process of He bubbles and has a potential to open up a new plasma processing method. Here, we show that the metallic nanostructure formation process by the exposure to He plasma can occur in various metals such as, titanium, nickel, iron, and so on. When the irradiation conditions alter, the metallic cone arrays including nanobubbles inside are formed on the surface. Different from W cases, other processes than growth of fiberform structure, i.e., physical sputtering and the growth of large He bubbles, can be dominant on other metals during irradiation; various surface morphology changes can occur. The nanostructured W, part of which was oxidized, has revealed a significant photocatalytic activity under visible light (wavelength >700 nm) in decolorization of methylene blue without any co-catalyst. [ABSTRACT FROM AUTHOR]

Tungsten (W) is one of the most promising candidates for plasma facing materials in the fusion reactor. Helium (He) in W can influence the retention of hydrogen isotopes. In the present study, W targets were simultaneously irradiated by He+-H 2 + or He+-D 2 + ion beams with the energies of 1 keV or 3 keV, at fixed temperatures in a range of room temperature (R.T.) to 1073 K. Mechanisms of He and hydrogen interaction in W were discussed, especially from the point of He retention, which was characterized by the high-temperature thermal desorption spectroscopy (TDS) and glow-discharge optical emission spectroscopy (GD-OES) measurements. It is found that He desorption shifts to a lower temperature range for the W simultaneously irradiated by 3 keV He+-1 keV H 2 + at 573 K, under He+ fluence up to 1 × 1022 He+m−2. Transmission electron microscope (TEM) observation and annealing treatment at the temperature of 873–1073 K show that the increased He uptake is caused by the formation of dislocation. Enhanced retention amounts for the hydrogen isotopes were also confirmed. Amounts of the dislocation loops introduced by the H 2 +-only irradiation can be reduced by annealing treatment at 873 K, while that introduced by He+ irradiation are quite stable, which grows larger at elevated temperatures. With an increase of H 2 + energy, Helium uptakes at both weak trapping sites and bubbles are increased, while the amounts of hydrogen retention are decreased. It suggests that hydrogen ion has a significant influence on the He trapping sites at the irradiation temperature up to 573 K, while the hydrogen retention is determined by the distribution of He bubbles and dislocation loops. • Helium was mainly trapped by dislocation loops in tungsten irradiated by 3 keV He+- 1 keV H 2 + at 573 K. • Helium concentration in tungsten under 3 keV He+- 1 keV H 2 + irradiation was decreased at 873 K. • Increased hydrogen isotope uptake in tungsten was found when helium was mainly trapped at dislocation loops. [ABSTRACT FROM AUTHOR]

Abstract: In order to investigate the morphology change of tungsten due to helium plasma exposure, tungsten (W) materials fabricated in different processes, ITER grade W, ultra-fine grained W (W–0.8TiC), and powder metallurgy W after helium plasma exposure were observed by scanning electron microscope. In all the three cases, pin-holes are observed on the surface; moreover, the non-uniformity of the formation of helium bubbles and fiberform nanostructure was observed on ITER grade W. From the detailed analysis of the crystal orientation with the help of focused ion beam method (FIB) indicates that the initial growth of the nanobubbles may be slow on the (110) face of tungsten. [Copyright &y& Elsevier]

Abstract: The effect of crystallographic orientation on the morphology change of tungsten (W) has been investigated in the linear plasma device, NAGDIS-I by exposing high density helium plasma to ITER grade W. After helium plasma exposure, the surface of specimens was analyzed by scanning electron microscope and orientation imaging microscope. The internal structure was analyzed with transmission electron microscope after the sample was milled by focused ion beam procedure. A clear difference in the surface morphology was observed on different crystal grains. Regular wavy structure were observed on crystal grains of {001}, {112}, {101}, and {111} surface, but there were no clear wavy structure on the crystal grains of {103}, {102}, {407} and {203}. The experimental results indicate that the angle between a slip face and face of each grains are important to determine the helium induced surface morphology change of tungsten. [Copyright &y& Elsevier]