Naoaki Yoshida

Formation process of tungsten nanostructure by the exposure to helium plasma under fusion relevant plasma conditions

Helium irradiation on tungsten changes the surface morphology dramatically by forming a nanometre-sized fibreform structure which could bring about serious problems for fusion reactors. From the experimental results in liner divertor simulators, it is revealed that the incident ion energy and surface temperature are key parameters for the formation of the structure. It is shown that the tungsten nanostructure is easily formed when the temperature is in the range 1000?2000?K, and the incident ion energy is higher than 20?eV. Furthermore, on the basis of the helium irradiation experiments performed in the divertor simulator NAGDIS-I, the initial formation process of the nanostructure is revealed. It is shown that the nanostructure formation is related to pinholes appearing on the bulk part of the material, and then, the rough structure develops to a much finer nanostructure. The nanostructure was also observed on the molybdenum surface that was exposed to the helium plasma. It increases interest in the possibility that nanostructure formation by helium irradiation is a common phenomenon that occurs on various metals.

Growth of Interstitial Type Dislocation Loops and Vacancy Mobility in Electron Irradiated Metals

Interstitial type dislocation loops formed in Al, Au, Cu, Fe and Mo by electron irradiation in a high voltage electron microscope grow proportionally to a smaller power of the irradiation time than unity at lower temperatures under constant irradiation. They grow linearly with the irradiation time at higher temperatures at which vacancies are mobile. The balance of the product of the mobility and concentration between interstitials and vacancies explains the linear growth as well as the observed square root dependence of the growth speed on the irradiation intensity. The temperature dependence of the growth speed can be used to obtain the migration activation energy of vacancies. Temperature independent growth in a very thin specimen is understood as the surface dominant case as sinks. Sink efficiency of an edge dislocation to interstitials and vacancies is discussed.

Point Defect Clusters in Electron-Irradiated Gold

Defect formation in gold by the irradiation of 2.0∼2.5 MeV electrons was studied with a high voltage electron microscope. The formation of dislocation loops by the aggregation of interstitials generally precedes the formation of the defects by vacancies. Permanent sinks, such as specimen surfaces and dislocations, prevent the formation of interstitial type of defects and enhance the formation of vacancy clustered defects. A rapid formation of the vacancy type defects and the simultaneous shrinkage of interstitial type loops after the cessation of the irradiation are observed. Kinetics analysis of the point defect behaviour, standing on the random migration of interstitials and vacancies, explains all the observed various phenomena as the competition processes between two kinds of point defects under various circumstances. The irradiation intensity dependence of the density of interstitial type defects shows that the di-interstitials are their stable nucleus.

Sub-ms laser pulse irradiation on tungsten target damaged by exposure to helium plasma

The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamura et al 2002 Plasma Sources Sci. Technol. 11 A42). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.

Electron Radiation Damage of Iron in High Voltage Electron Microscope

Aggregation process of irradiation-induced point defects in iron is examined between room temperature and 400°C with a high voltage electron microscope. In the whole temperature range examined interstitial type dislocation loops are formed. The loops formed between slightly above room temperature and 350°C have flower-like shape. Especially the petalous loops formed around 300°C are divided into many small loops at the latter stage of irradiation by absorving mobile vacancies. The formation of the loops in the pre-irradiated specimen also tells that the vacancies become mobile at about 300°C. Analyzing the nucleation of the loops by using the chemical rate theory considering the effects of impurity atoms one obtains the activation energy for interstitial migration as about 0.26 eV.

Review of recent works in development and evaluation of high-Z plasma facing materials

This paper reviews the recent activities and results in development and evaluation of new high-Z materials focusing on tungsten and its alloys. VPS-W coating on graphite and CFC showed superior high heat load properties by inserting sophisticated multilayer diffusion barriers (W/Re) between tungsten and substrate. For successful application it is necessary to know the temperatures for recrystallization and interdiffusion of rhenium and tungsten, which may degrade bonding properties. Sputtering by plasma impurities and damage by energetic charge-exchange neutrals must be considered. Retention of hydrogen isotopes strongly depends on material grade and production process. Dense bubbles and dislocations formed by helium bombardment may act as effective traps for hydrogen. It is anticipated that long range diffusion of helium induces embrittlement. Neutron irradiation is one of the major concerns for tungsten materials in fusion reactor application. The tungsten materials developed so far perform poorly under neutron irradiation and their brittle nature could limit their application as a fusion material. Further efforts are needed to improve ductility while keeping other desirable properties such as high thermal conductivity.

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.

Nanostructured Black Metal: Novel Fabrication Method by Use of Self-Growing Helium Bubbles

Black metal absorbing light can be used for the light absorber material for a solar thermophotovoltaic system, which is expected to be a photovoltaic system in the next generation. Here we show that the fiberform nanostructured tungsten formed by helium irradiation absorbs the light from all angles of interest and is virtually black for a solar spectrum, from visible to near infrared wavelength; the absorptivity of the total solar power could be 98%. It is revealed that the nanostructure is formed by a novel process, self-growth of the helium nano-bubbles.

Microstructure formation and its role on yield strength in AISI 316 SS irradiated by fission and fusion neutrons

Abstract The microstructure of AISI 316 SS irradiated by fission and fusion neutrons at 90 and 290°C was observed by weak-beam dark-field stereo technique and compared with yield strength data. In all cases, tiny (about 1 nm on average) interstitial type dislocation loops were formed. At 90°C, the density of dislocation loops was extremely high and those of fission and fusion irradiations correlate well on the basis of dpa. At 290°C, interstitial loops are formed by rearrangement of point defects formed in a subcascade. Both the loop density and yield stress change of fission neutron irradiation increased abruptly above 0.003 dpa. This peculiar behavior might be caused by undesired irradiation at lower temperatures during daily heating-up and cooling-down of the samples. Major defects which increase yield strength were the observed tiny interstitial type dislocation loops. The barrier strength, α, of the dislocation loops was determined to be 0.2.

The Japanese experimental program on RTNS-II of DT-neutron irradiation of materials☆

Research project of D-T neutron radiation damage of materials with the rotating target neutron source RTNS-II of LLNL is explained with a brief explanation of the results obtained in the first year of the project. Wide variety of materials were irradiated at three temperatures; 25°C, 200°C and 400°C up to 1 × 1018n/cm2. From the result of the observations of defect structures, discussions are made on the conditions for the defect cluster formation from cascade damage, type of defects, defects made from sub-cascade damage, correlation between defects and disordered zones, and the roles of free interstitials released from cascades. Future research plans are briefly presented.