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From the helium (He) plasma irradiations to tungsten performed in the Magnum-PSI device, the effects of deposition of metals on the helium-plasma induced fiberform nanostructures (fuzz) are discussed. It was found that fuzz was not formed at the center of the plasma cylinder if there were significant metallic impurities from the source. Deposition of metallic impurities (mainly molybdenum and copper) counteracted the growth of fuzz. In addition to the effects of metals from the source, we installed a sputtering source near the sample to replicate the deposition environment in fusion devices. The thickness of fuzzy layer was ～7µm, which was about five times greater than that without deposition, at the He flux of 1.3×10^26 m^2 suggesting that the growth rate of fuzz layer was significantly accelerated due to the deposition of tungsten.

4 MeV Ni ion irradiation was performed for pure Fe up to 70 dpa between 350℃ and 450℃ with and without simultaneous He injection to Simulate and understand the irradiation behaviour of ferritic steels in fusion environment. Results were compared with those from electron (HVEM) and proton irradiation. Dislocation density in Ni ion irradiated Fe was 3times higher than others and dislocations were surrounded by clouds of small defect clusters below 400℃, but at 450℃ no such clouds were observed and dislocation density quickly decreased. Voids were observed at 450℃ above 20 dpa, showing 0.29 and 0.56% swelling at 70 dpa in dual (Ni and He) and single beam irradiation, respectively. Void density in dual beam irradiation was about twice that in single beam irradiation. Incubation dose for void formation in Ni ion irradiation was about 5 times higher than that in electron irradiation. Attempts were made to interpret these phenomena taking into account heterogeneous production of defects characteristic of ion irradiation, i.e., effects of PKA spectrum and cascades of defects.

Recent experiments have revealed that accelerated tungsten (W) nanostructure growth occurs during the exposure to helium (He) plasmas with an additional W deposition, and mm-thick large-scale fiberform nanostructures (LFNs) are formed when certain condition is satisfied. In order to reveal whether accelerated growth occurs in fusion device, it is important to understand the annealing process in addition to the growth process. In this study, we will perform thermal treatment of W LFNs. Detailed observation reveals that the LFNs shrink at a faster rate than that of conventional fuzz, though it takes longer time to fully be reintegrated to the surface because the thickness is orders of magnitude greater.

A fiberform nanostructured layer is formed on a tungsten (W) surface using helium (He) plasma irradiation. The behavior of the nanostructures and He bubbles were observed during annealing via in situ transmission electron microscopy. Notable changes in the nanostructures occurred at 1223 K. A collapse of the nanostructures was observed along with a collapse of the He bubbles during annealing., source:https://doi.org/10.1585/pfr.11.1206125, identifier:0000-0002-2023-9786

We have developed a compact divertor plasma simulator (CDPS) that can produce steady-state deuterium and/or helium plasmas with densities above ∼ 1018 m−3 for Plasma-Wall Interaction (PWI) studies of neutron-irradiated materials. The maximum particle flux is about 1022 m−2s−1. The CDPS was installed and is being operated in the radiation-controlled area of the International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University. We are able to control sample temperature within uncertainty of 5 °C during plasma exposure by adjusting the cooling air flow rate to the sample holder. The CDPS has a sample-carrier system, which makes it possible to transfer a plasma-irradiated sample from the sample holder to an infrared heater for analysis using thermal desorption spectroscopy (TDS) without exposing it to the air. This avoids the oxidation of the sample and minimizes the time between the end of plasma exposure and TDS analysis. An ITER-like tungsten (W) sample (A.L.M.T. Corp.), which has been irradiated by neutrons to 0.06 dpa in a fission reactor, was exposed to a deuterium plasma in the CDPS. The experimental results clearly show that the total deuterium retention in the neutron-irradiated W sample increases significantly in comparison with a pristine W, as demonstrated by broadening of the TDS spectrum at high temperatures., source:https://doi.org/10.1585/pfr.12.1405040, identifier:0000-0001-7826-0046

Rhodium and ruthenium thin film coatings were conducted on tungsten samples by using a magnetron sputtering device; then, we irradiated He plasma to the samples in the linear plasma device NAGDIS-II (Nagoya Divertor Simulator). Fuzzy nanostructures were formed on rhodium and ruthenium samples when the surface temperatures were ∼950 and ∼1200 K, respectively. When the surface temperature was high, i.e., >1200 K, it was found that tungsten atoms diffused across the rhodium film and reached the film surface, and tungsten-fuzz was formed over the rhodium layer. From TEM analysis of ruthenium fibers, it was identified that there were thin parts in the fibers, and some fibers had no He bubbles. It was likely that fiber growth mechanism on ruthenium was different from the other metals., source:https://doi.org/10.1585/pfr.13.3406065

A tungsten (W) deposition layer is formed while a sample is exposed to a helium (He) plasma at the surface temperature of 473 or 573 K. The formed He-W co-deposition layer was composed of fine grains, the size of which increased with the surface temperature. The samples with the co-deposition layer was exposed to a deuterium (D2) plasma, and the D retention was investigated using thermal desorption spectroscopy. It was found that the co-deposition layer has quite different D retention characteristics compared with bulk W., source:https://doi.org/10.1585/pfr.15.1201004

Arcing experiments were conducted in the linear plasma device Pilot-PSI, where a pulsed plasma was superimposed to a steady state plasma. The arcing was observed by a fast framing camera, and the sample was analyzed with a transmission electron microscope. Observations of glowing objects released from the sample in response to the arcing and destruction of the fuzzy layer at the edge of the arc trail without significant melting suggested that dust was formed and released from the surface in response to arcing., source:https://doi.org/10.1585/pfr.15.1205061, identifier:0000-0002-2023-9786

An innovative color measurement technique is employed in the Large Helical Device (LHD). This study provides a method for obtaining in broad spatial extent and in great detail the color information of the first wall relating to the thickness of the deposition layer. The RGB (Red, Green, and Blue) value, mainly of the stainless steel plates on the helically twisted coil, is measured by a color analyzer equipped with an integrated sphere light source. On the outer torus side, the colors of almost all stainless steel plates are close to carbon black, which suggests that deposition is dominant. On the inner torus side, all plates except for those neighboring the carbon divertor plates are almost white, as in the case of the stainless steel substrate of the first wall, which suggests that erosion is dominant. The relationship between the color and the distance from the stainless steel plates to the plasma is investigated., source:https://doi.org/10.1585/pfr.10.1202074, identifier:0000-0001-5522-3082

Nanostructured tungsten formed by the exposure to helium plasma in a linear plasma device was installed in the large helical device (LHD). After the exposure in a series of experiments in the 2012 fiscal year campaign in LHD, the samples were analyzed by scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersion x-ray spectroscopy (EDX). It was found that part of the nanostructures was totally covered with carbon based material probably from divertor, while some other parts were eroded by sputtering. On the erosion dominant region, it was revealed that the head part of nanostructures was sputtered and the surface became rounded, but the nanostructures still remained near the surface. Optical reflectance of the material was measured, and it was found that the morphology changes increased the optical reflectivity up to ∼10% from typically less than 1%. The possibility and limitation of the nanostructured tungsten as a light absorber (viewing dump) are discussed., source:https://doi.org/10.1585/pfr.10.1402083, identifier:0000-0003-0543-5241

The key to improving the heat load of vacuum plasma sprayed tungsten coatings on low activation ferritic steel maintained at low temperatures is elimination of stratified low-density layers with many large pores, in which thermal cracks propagate preferentially. The low-density layers are formed owing to the deposition of large solidified tungsten particles, which remain mainly at the periphery of the spray stream. In this study, by shading this periphery, partially homogeneous tungsten coatings without large pores were successfully obtained. The coatings are expected to show good heat load, which is feasible for nuclear fusion applications., source:https://doi.org/10.1585/pfr.8.1405111

To evaluate Helium (He) effect on hydrogen isotope retention in tungsten (W), He⁺ was introduced into W bulk by 201 – 1000 keV He⁺, or W surface by 3 keV He⁺ for 6 MeV Fe ion damaged W at room temperature. The deuterium (D) retention behavior was evaluated by thermal desorption spectroscopy (TDS). In addition, the amount of tritium (T) at surface and bulk were separately evaluated by beta-ray induced X-ray spectroscopy (BIXS). The experimental results indicated that the formation of He-void complexes reduced the D trapping in vacancies and voids which have higher trapping energy by the bulk He retention. The BIXS measurement also supported the He enhanced the D reduction in the W bulk region. On the other hand, the He ion irradiation near the surface region enhanced D trapping by dislocation loops or surface, indicating the existence of He near surface interfered the D diffusion toward the bulk. It was concluded that the He existence in bulk or surface will significantly change the D trapping and diffusion behavior in damaged W.