A review of late-stage tungsten fuzz growth

Tungsten will be used as the plasma-facing divertor material in the International Thermonuclear Experimental Reactor (ITER) fusion reactor. Under high temperatures and high ion fluxes, a ‘fuzz’ nanostructure forms on the tungsten surface with dramatically different properties and could contaminate the plasma. Although simulations and experimental observations have provided understanding of the initial fuzz formation process, there is debate over whether tungsten or helium migration is rate-limiting during late-stage growth, and the mechanisms by which tungsten and helium migrations occur. Here, the proposed mechanisms are considered in turn. It is concluded that tungsten migration occurs by adatom diffusion along the fuzz surface. Continual helium migration through the porous fuzz to the tungsten bulk is also required for fuzz growth, for continued bubble growth and rupture. Helium likely migrates due to ballistic penetration, although diffusion may contribute. It is difficult to determine the limiting process, which may switch from helium penetration to tungsten adatom diffusion above a threshold flux. Areas for further research to clarify the mechanisms are then considered. A greater understanding of the fuzz formation mechanism is key to the successful design of plasma-facing tungsten components, and may have applications in forming porous tungsten catalysts.

INVESTIGATION OF TUNGSTEN SURFACE CARBIDIZATION UNDER PLASMA IRRADIATION

This paper presents the results of a study of the formation of a carbidized layer under various experimental conditions and the choice of optimal parameters for carbidization of a tungsten surface under plasma irradiation. To study the effect of the surface temperature of a tungsten sample and the duration of plasma irradiation, experiments were carried out at a sample surface temperature of 1300 °C and 1700 °C with an irradiation duration of 300–2400 s. Analysis of the research results showed that the maximum formation of W2C on the surface is observed at a test temperature of 1700 °C. At a temperature of 1300 °C, the phase composition of the carbidized layer depends on the duration of plasma irradiation. According to the literature analysis, the formation of WC occurs on the surface of tungsten, from which C diffuses into the particle and forms the underlying layer of W2C. With an increase in the ion fluence, depending on the irradiation time and the temperature of the sample surface, the diffusion of C into W accelerates, the WC content decreases, and W2C becomes the dominant carbide compound.

Study of the Temperature Dependence of a Carbidized Layer Formation on the Tungsten Surface Under Plasma Irradiation

The paper considers a method of tungsten surface carbidization using a beam-plasma discharge (BPD), which was implemented in a plasma-beam installation (PBI). The advantage of this method is to create conditions for chemical reactions and physical processes as close as possible to those possible in thermonuclear installations. The BPD makes it possible to generate plasma using different working gases. Methane was used as a plasma-forming gas. The working gas pressure in a chamber was (1,3-1,4)·10-1 Pa. The temperature dependence of the carbidized layer formation on the tungsten surface under plasma irradiation was determined in the temperature range of 700-1700 °C. The formation of tungsten carbides in surface layers was confirmed by SEM and X-ray diffraction analysis. It was found that interaction between tungsten and methane in a wide temperature range can proceed with simultaneous or sequential formation of the carbide phases W2C and WC.

ELECTRON-STIMULATED DESORPTION OF LITHIUM ATOMS ADSORBED ON THE SURFACE OF GOLD-LITHIUM INTERMETALLIC

Получена тонкая пленка интерметаллида LiAu при комнатной температуре при напылении атомов Li на слой адсорбированного золота на поверхности вольфрама. Исследованы процессы электронно-стимулированной десорбции (ЭСД) атомов Li с поверхности LiAu. Показано, что ЭСД атомов Li может наблюлаться только в том случае, когда напылено не менее одного монослоя атомов Li и с Au . Зависимость выхода ЭСД атомов Li от количества напыленного Li и Au имеет максимум при напылении двух монослоев атомов Li на Au. Полученные результаты свидетельствуют о формировании LiAu различной стехиометрии: от LiAu с дефицитом атомов лития до LiAu при напылении двух монослоев атомов Li. В энергетическом распределении по кинетическим энергиям десорбирующихся атомов Li обнаружено два пика: высокоэнергетический и низкоэнергетический. Первый из них связан с десорбцией атомов Li из LiAu, а второй в десорбцией атомов Li из верхнего монослоя атомов Li. A thin film of the intermetallic compound LiAu was obtained at room temperature by deposition Li atoms onto a layer of adsorbed gold on the tungsten surface. The processes of electron-stimulated desorption of Li atoms from the surface are investigated. It is shown that electron-stimulated desorption of Li atoms can be observed only in the case when at least one monolayer of Li and Au atoms is deposited. The dependence of the electron-stimulated desorption yield of Li atoms on the amount of deposited has a maximum when two monolayers of atoms Li are deposited on Au . The results obtained indicate the LiAu formation of various stoichiometry: from with a deficit of lithium atoms to the deposition of two monolayers of atoms. In the kinetic energy distribution of desorbed Li atoms, two peaks were found: high-energy and low-energy. The first of them is associated with the desorption of Li atoms from LiAu, and the second with the desorption of Li atoms from the upper monolayer of Li atoms.

Nanostructuring of the tungsten surface under the action of femtosecond laser radiation

In the paper, we show the possibility of nanostructured tungsten surface by femtosecond laser radiation. The features of the LIPPS formation are also shown. The dependence of the formation of the structuring region on the number of femtosecond laser pulses is shown. Based on the analysis of experimental data, the formation mechanisms of nanostructures are discussed.

Investigation of tungsten surface carbidization under plasma irradiation

This paper presents the results of studying the formation of a carbide layer under various experimental conditions and the choice of the optimal parameters for carbidization of the tungsten surface under plasma irradiation in methane. Therefore, to assess the effect of the surface temperature of a tungsten sample during experimental work, the surface temperature of the sample was 1300-1700°C, and to assess the effect of the irradiation time, a range within 300-2400 s was selected. From the results of X-ray phase analysis of the surface of tungsten samples, concluded that the temperature of the sample surface and the duration of irradiation have a significant effect on the formation of a carbidized layer on the tungsten surface during plasma irradiation on plasma-beam installation.

Field electron emission from a nanostructured tungsten surface

In this paper, the electron emission from a nanostructured tungsten surface was investigated. A method for measuring an extremely low current (10−12 – 10−14 A) has been tested. It made possible to reduce the effect of the electric field on the sample surface and to minimize the probability of spontaneous breakdowns. For a detailed study of tungsten fuzz, a point tungsten anode (diameter 90 μm) was used. Field enhancement factor (β = 2000 – 3000) and effective emission area were calculated using the Fowler–Nordheim plots. The pre-breakdown current rise was studied. The emission current waveforms suggest the formation of several emission structures before the breakdown.

Influence of temperature on tungsten carbide formation in a beam plasma discharge

This paper presents a method of tungsten surface carbidization using a plasma-beam setup to assess the effect of temperature on the formation of tungsten carbides. Methane was used as a plasma-forming gas. The working gas pressure in the chamber was (1.3–1.4)10−1 Pa. Experiments on the formation of carbides were carried out at different temperatures (700-1000°C). It was recorded that at a temperature of 700°C, crystallization centers of a carbon film appear on the sample surface. With an increase in the irradiation temperature to 800°C and 900°C, the surface of the samples is covered with a continuous carbon film. As a result of the experimental work carried out, it was found that the formation of tungsten semicarbide occurs at 900°C. A further increase in temperature leads to the formation of tungsten monocarbide.

Insights into the Nature of the Active Sites of Pt-WOx/Al2O3 Catalysts for Glycerol Hydrogenolysis into 1,3-Propanediol

The chemo-selective hydrogenolysis of secondary hydroxyls is an important reaction for the production of biomass-derived α,ω-diols. This is the case for 1,3-propanediol production from glycerol. Supported Pt-WOx materials are effective catalysts for this transformation, and their activity is often related to the tungsten surface density and Brönsted acidity, although there are discrepancies in this regard. In this work, a series of Pt-WOx/γ-Al2O3 catalysts were prepared by modifying the pH of the solutions used in the active metal impregnation step. The activity–structure relationships, together with the results from the addition of in situ titrants, i.e., 2,6-di-tert-butyl-pyridine or pyridine, helped in elucidating the nature of the bifunctional active sites for the selective production of 1,3-propanediol.