Miyuki Yajima

Helium plasma implantation on metals: Nanostructure formation and visible-light photocatalytic response

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.

Molecular dynamics and Monte Carlo hybrid simulation for fuzzy tungsten nanostructure formation

For the purposes of long-term use of tungsten divertor walls, the formation process of the fuzzy tungsten nanostructure induced by exposure to the helium plasma was studied. In the present paper, the fuzzy nanostructures formation has been successfully reproduced by the new hybrid simulation method in which the deformation of the tungsten material due to pressure of the helium bubbles was simulated by the molecular dynamics and the diffusion of the helium atoms was simulated by the random walk based on the Monte Carlo method. By the simulation results, the surface height of the fuzzy nanostructure increased only when helium retention was under the steady state. It was proven that the growth of the fuzzy nanostructure was brought about by bursting of the helium bubbles. Moreover, we suggest the following key formation mechanisms of the fuzzy nanostructure: (1) lifting in which the surface lifted up by the helium bubble changes into a convexity, (2) bursting by which the region of the helium bubble changes into a concavity, and (3) the difference of the probability of helium retention by which the helium bubbles tend to appear under the concavity. Consequently, the convex-concave surface structure was enhanced and grew to create the fuzzy nanostructure.

Comparison of Damages on Tungsten Surface Exposed to Noble Gas Plasmas

Tungsten was exposed to pure Ar or Ne plasmas over 1550 K at several incident ion energies. Even under the irradiation condition that the tungsten nanostructure is formed by He plasma irradiation, holes/bubbles and fiberform nanostructures were not formed on the surface by exposure to Ar or Ne plasmas. In addition, the results from energy dispersive X-ray spectroscopy supported the facts that Ar and Ne did not remain in the sample. We will discuss the reason for the differences in the damage to the tungsten surface exposed to noble gas plasmas.

Development of Nanostructured Black Metal by Self-Growing Helium Bubbles for Optical Application

When tungsten is exposed to a helium plasma under specific conditions, fiberform nanostructures grow from the surface, and optical properties significantly alter. It is shown that the optical absorptivity increases significantly by the helium irradiation from visible to near-infrared wavelength range. The optical absorptivity became almost 100% from visible to near infrared wavelength range after the nanostructure was formed on the surface. The result indicates that the emissivity of radiation in visible and near infrared wavelength range also increases significantly by the helium irradiation. The emissivity around 1550 nm is 4.4 times higher than the initial value, while it is twice at the visible wavelength range. The helium fluence dependences of the specular and total optical reflectances and the morphology change observed by transmission electron microscope (TEM) are revealed.

Effect of Damage Introduction and He Existence on D Retention in Tungsten by High Flux D Plasma Exposure

Both of radiation-induced damages and helium (He) existence effects on deuterium (D) retention in tungsten (W) by D plasma exposure were evaluated using high flux divertor plasma exposure device, called Compact Divertor Plasma Simulator (CDPS). The results were compared with 3 keV D2+ implanted W with low flux and fluence. The thermal desorption spectra were consisted of three desorption stages at 400, 600, 780 K. Comparing to the undamaged W, the D desorption stages were shifted towards higher temperature side and the values of D retention increased. It can be said that the formation of stable trapping sites by damage introduction enhances the D trapping in the damaged W. For He+ irradiation, D desorption at lower temperature was enhanced, due to the formation of dense dislocation loops. In case of sequential Fe2+ and He+ implantation, D desorption at higher temperature was reduced, comparing to that for only Fe2+ damaged W. These facts show that the accumulation of He near surface region reduces D diffusion toward bulk, leading to the reduction of D trapping by voids.

Development of H, D, T Simultaneous TDS Measurement System and H, D, T Retention Behavior for DT Gas Exposed Tungsten Installed in LHD Plasma Campaign

Abstract All the hydrogen isotope (H, D, T) simultaneous TDS (Thermal desorption spectroscopy) measurement system (HI-TDS system) was newly designed to evaluate all hydrogen isotope desorption behavior in materials. The present HI-TDS system was operated under Ar purge gas and the H and D desorptions were observed by a quadruple mass spectrometer equipped with an enclosed ion source, although T desorption was evaluated by an ionization chamber or proportional counters. Most of the same TDS spectra for D and T were derived by optimizing the heating rate of 0.5 K s−1 with Ar flow rate of 13.3 sccm. Using this HI-TDS system, D and T desorption behaviors for implanted or DT gas exposed tungsten samples installed in LHD (Large Helical Device) at NIFS (National Institute for Fusion Science) was evaluated. It was found that major hydrogen desorption stages consisted of two temperature regions, namely 700 K and 900 K, which was consistent with the previous hydrogen plasma campaign and most of hydrogen would be trapped by the carbon-dominated mixed-material layer. By implantation, major D desorption was found at ~900 K with a narrow peak due to energetic ion implantation. For gas exposure, H was preferentially replaced by D and T with a lower trapping energy. In addition, T replacement rate by additional H2 gas exposure was evaluated. This fact indicates that the hydrogen replacement mechanism would be clearly changed by exposure methods.