Clean Tungsten Surface Research Articles

Numerical estimation of the atomic fraction during laser-induced desorption of hydrogen from tungsten and beryllium

Monitoring of the DT-fuel retention in plasma-facing components (PFCs) is vital for long-term operations of future fusion devices. Laser-induced desorption (LID) is a proposed technique for the systematic in-situ control of hydrogen isotope content in surface layers of PFCs. An accurate interpretation of LID measurements requires information on the composition of released particles. Under some conditions, both atoms and molecules can be desorbed. The present study aims to verify the role of hydrogen atomic release under nanosecond and millisecond laser pulses. After investigation of general trends, we carried out the detailed analysis of hydrogen desorption from clean tungsten and beryllium surfaces, assuming a uniform distribution of hydrogen in a 10 μm layer with 1 at.% of initial content. Calculations were based on a common reaction-diffusion model supplemented by rate equations of molecular and atomic desorption. As a result, the atomic fraction is estimated to reach ∼40% during ms-heating of tungsten to near-melting temperatures, and less than 10% in the ns-case. Regarding beryllium, molecular recombination is shown to be the main process of hydrogen desorption in both heating scenarios.

First-principles study of clean tungsten surface work function under electric field

The effect of an external electric field on the work functions of clean tungsten (W) surfaces, W (100), W (110), and W (111) has been investigated using first-principles calculations based on density functional theory. By applying an electric field from 0 up to 0.3 V/Å, the effective and local work functions can be determined for comparison after employing five different pseudopotentials. It is found that as the electric field increases, the work functions of tungsten surfaces reduce accordingly. A reduction of work function can be as large as ∼0.35 eV. Based on these calculations, a new scaling law of work function reduction due to the charge transfer near the metal/vacuum interface caused by an external electric field is obtained. In addition, the local work function is found to be closely related to the charge density distribution. With this approach, field emission properties of metals can be better understood and described.

Efficient emission of positronium atoms from an Na-coated polycrystalline tungsten surface

Time-of-flight spectra for the ortho-positronium emitted from clean and Na-coated tungsten surfaces have been measured using the pulsed slow positron beam at KEK-IMSS slow positron facility. Emission efficiency of positronium from the Na-coated sample was found to be several times greater than that from uncoated tungsten surfaces.

Contribution from recoiling atoms in secondary electron emission induced by slow highly charged ions from tungsten surface

AbstractThe electron emission yield γ induced by Ne2+ and O2+ impacting on a clean tungsten surface has been measured. The range of projectile energy is from 3 keV/u to 14 keV/u. The total electron yield gradually increases with the projectile velocity. It is found simultaneously that the total electron yield for O2+ is larger than the total electron yield for Ne2+, which is opposite to the results for higher projectile velocity. After considering the contribution from recoiling atoms to the energy distribution and electron emission yield, we find that recoiling atoms are of crucial importance in electron emission in our energy range. Thus, the unexpected results in our experiment can be explained successfully.

The influence of adsorption coating on molecular heat transfer in the system ‘rarefied gas — metal’

Based on the classical concept of atomic motion and the Goodman and Wachman lattice theory, we have developed a computer program to model the equilibrium and non-equilibrium scattering of helium atoms by 3D tungsten crystal lattice with allowance for adsorption surface coating. Within the concept of energy accommodation coefficient, we have calculated the rate of molecular heat transfer of helium to clean tungsten surface or to tungsten surface partially covered with an adsorbate. The calculations were performed for various surface temperatures. The calculated dependences were compared to test data obtained in experiments with surfaces controlled in terms of their chemical composition. Within the developed approach, the simulations proved capable of providing an adequate description to experimental data obtained for the equilibrium energy accommodation coefficient on the clean surface, and also for the non-equilibrium energy accommodation coefficient for the surface partially covered with adsorbate.

Sticking of atomic hydrogen on the tungsten (0 0 1) surface

The sticking of hydrogen atoms with kinetic energies in the range 0.003–10 eV on a clean (0 0 1) tungsten surface has been investigated using molecular dynamics simulations. The atoms are found to stick to the surface at 0 and 300 K, with a sticking coefficient smaller than 0.6 for kinetic energies higher than 3 meV. The adsorption sites for H on the W(0 0 1) surface are also presented. The dominant site is in perfect agreement with the experimentally found bridge site.

Plasma–wall interaction at the ASDEX Upgrade tungsten heat shield

The suitability of tungsten as plasma facing material for main chamber and baffle components was investigated in ASDEX Upgrade. In an initial step four W-coated graphite test tiles were employed at different poloidal positions of the inner column heat shield structure for a full experimental campaign. Tungsten erosion rates were determined by ion beam analysis of the test tiles before and after the campaign. The results are compared to predictions assuming CX sputtering as main erosion mechanism. In a second step the machine was operated with two toroidal rows of W-coated graphite tiles. The experimental campaign was started without wall conditioning by siliconization to ensure a clean tungsten surface exposed to the plasma. Tungsten influx and plasma concentration were measured spectroscopically. Tungsten migration was determined by ion beam analysis of collector probes exposed in the outer midplane and the outer divertor plate. In all important discharge scenarios the W-concentration stayed below the detection limit of ≈5×10 −6. Consequently no influence on plasma performance parameters was found, which encourages the use of W for even larger surface areas in the ASDEX Upgrade main chamber.

Calculations of the energy accommodation coefficient using classical scattering theory

The theory of the energy accommodation coefficient for exchange of energy between a rarefied gas and a clean surface is developed in terms of the differential reflection coefficient for state-to-state scattering of an incoming atomic projectile and a surface. This theory is applied to classical models which have been shown to accurately predict the measured scattered distributions in numerous state-to-state experiments of monoenergetic beams of atoms scattering from clean single crystal surfaces and from clean liquid surfaces. Full three-dimensional calculations are carried out and compared with available experimental data for the accommodation of rare gases at a clean tungsten surface. Good agreement with the experimental measurements is obtained for the heavier mass rare gases where classical theory is expected to be most valid at all measured temperatures.

Cu wetting and interfacial stability on clean and nitrided tungsten surfaces

Cu growth/nucleation behavior and thermal stability on clean and nitrided tungsten foil have been characterized by Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) under controlled ultra high vacuum (UHV) conditions. At room temperature, Auger intensity ratio versus time plots demonstrate layer by layer Cu growth for the clean tungsten surface (W) and three-dimensional nucleation for the nitride overlayer (7.5–10 Å, WN x /W). Auger intensity ratio versus temperature measurements for the Cu (1 monolayer)/W system indicate a stable interface up to 1000 K. For the Cu (1 ML)/WN x /W system, initial Cu diffusion into the nitride overlayer is observed at 550 K. Maximum diffusion of the Cu occurs at 750 K. The driving force for diffusion is due to an effective Cu–nitride repulsion and a more thermodynamically favorable Cu–W interaction. TDS measurements of the nitride overlayer demonstrate N 2 decomposition from 800–1400 K. The addition of Cu (1 ML) to the nitride overlayer lowers the decomposition temperature range of 750–1350 K. The enhancement of N 2 recombination is attributed to the perturbing effect of Cu on W–N bonding driven by a Cu–W surface alloy formation.

Origin of the field-stimulated exoelectron emission from tungsten tip surfaces

Emission characteristics of the field-stimulated exoelectron emission (FSEE) from tungsten surfaces and the effect of gas adsorption on the FSEE have been investigated. The clean tungsten tip surfaces were prepared using the field evaporation technique. The clean tungsten surfaces thus prepared did not liberate FSEE. Physical adsorption of the residual gas atoms at the tungsten surface resulted in FSEE. An obvious enhancement of the FSEE signal due to gas adsorption was observed in which no significant change of the field emission pattern was caused. Therefore, FSEE is thus revealed the high sensitivity to the surface conditions.

The interaction of ethylene, acetylene and butadiene with a clean tungsten (100) surface

The interactions of ethylene, acetylene and butadiene with a W(100) surface at 300 K have been investigated using photo electron spectroscopy, work function measurement and thermal desorption techniques. The course of the interaction is shown to be coverage dependant. At low coverage, acetylene is initially decomposed on the surface to its constituent atoms carbon and hydrogen, followed by the formation of a molecular species (I) as the coverage is increased. Further exposure results in the displacement of adsorbed hydrogen atoms by (I) and then the formation of a second adsorbed species (II) at the expense of (I). At saturation, all four species are present to some extent. With butadiene, all three adsorption regimes are again observed, but with ethylene only the first two are apparent. The nature and bonding of the adsorbed species (I and II) are discussed.

Spin-polarized low-energy electron microscopy study of the magnetic microstructure of ultra-thin epitaxial cobalt films on W(110)

Magnetic domain formation in ultra-thin Co films on W(110) was studied with spin-polarized low-energy electron microscopy (SPLEEM). Films with the expected hexagonal structure were grown on the clean tungsten surface, but also fcc structured films with a (100) plane parallel to the surface could be prepared on a stabilizing tungsten carbide layer. Generally, fcc Co films showed very irregular in-plane magnetization between 3 and 11 ML thickness. We were able to record subtle changes in the magnetic domain structure with increasing film thickness. For films with hexagonal structure, first in-plane contrast was observed at 3 ML. Rotation of the substrate crystal established the presence of a uniaxial in-plane anisotropy with the easy axis along the W[1 1 0] direction for a 6 ML film.

Effective thermionic work function measurements of zirconium carbide using a computer‐processed image of a thermionic projection microscope pattern

We report on a unique experimental method to determine thermionic work functions of major crystal planes of single crystal zirconium carbide. Applications for transition metal carbides could include cathodes for advanced thermionic energy conversion, radiation immune microcircuitry, β‐SiC substrates or high current density field emission cathodes. The primary emphasis of this paper is the analytical method used, that of computer processing a digitized image. ZrC single crystal specimens were prepared by floating zone arc refinement from sintered stock, yielding an average bulk stoichiometry of C/Zr=0.92. A 0.075 cm hemispherical cathode was prepared and mounted in a thermionic projection microscope (TPM) tube. The imaged patterns of thermally emitted electrons taken at various extraction voltages were digitized and computer analyzed to yield currents and corresponding emitting areas for major crystallographic planes. These data were taken at pyrometrically measured temperatures in the range 1700<T<2200 K. Schottky plots were then used to determine effective thermionic work functions as a function of crystallographic direction and temperature. Work function ordering for various crystal planes is reported through the TPM image processing method. Comparisons are made with effective thermionic and absolute (FERP) work function methods. To support the TPM image processing method, clean tungsten surfaces were examined and results are listed with accepted values.

Electron emission induced by multiply charged Ar ions impinging on a tungsten surface

We have investigated collisions of Ar q+ ions on a clean polycrystalline tungsten surface at collision energies between 35 eV and 250 keV by analysis of the ejected secondary electrons. We report total electron yields as well as highly resolved energy spectra of electrons for different primary projectile charge states up to q = 12, for different collision energies and for different ejection angles. Special emphasis is given to the production mechanisms of electrons with energies above ≈ 100 eV. A relatively sharp structure near 212 eV, occurring for q ⩾ 8 predominantly at collision energies below ≈ 2 keV, is ascribed to Auger transitions in the neutralized projectile on its way to the surface. A broad structure between 125 and 175 eV, occurring predominantly at collision energies above ≈ 2 keV, can be interpreted as being due to target Auger processes, induced by vacancy transfer from the projectiles to the target atoms.

The vacuum properties of doped tungsten filaments

Incandescent filaments have been examined by thermal desorption. Desorption activation energies have been calculated by two methods and compared with data for clean tungsten surfaces. Distributions of adsorbed molecules with different binding energies are also given for H 2, CO, N 2, CO 2 and H 2O. It has been observed that diffusion and chemical reactions occur at temperatures higher than 1000 K. These processes affect the thermal desorption spectra.

Electron emission in collisions of Ar 9+ on a tungsten surface

Energy distributions of secondary electrons resulting from multiply charged ion-surface interaction have been measured. A clean polycrystalline tungsten surface was bombarded with 500 eV and 20 keV Ar 9+ ions. At low primary energy (500 eV) a pronounced Ar LMM Auger line is observed at an electron energy of 207 eV, whereas it is absent at high primary energy (20 keV). The measurements indicate that at low primary energy the filling of the L-shell vacancy, originally present in the projectile, can occur before the projectile actually hits the surface. This is supported by the observation of a small energy shift of the LMM peak when varying the detection geometry.

The role of oxygen in the low work function surface complex of barium on oxygen on tungsten (substrate)

The barium on oxygen on tungsten (Ba-O-W) surface complex is responsible for the high thermionic emission properties of the barium impregnated tungsten cathode used in high frequency microwave tubes. The investigation reported here is a continuation of a study of the physical and chemical properties of the Ba-O-W surface, and in particular, of the role of surface oxygen. Desorption characteristics of a barium monolayer on an oxygenated tungsten substrate were measured at elevated temperatures as a function of time and initial oxygen concentration using a computerized scanning Auger microprobe. The oxygen concentration was varied by exposing the clean tungsten surface to oxygen levels ranging from 1.4 to 126 L. Barium desorption measurements were made at elevated temperatures, and the surface concentrations of barium, oxygen and tungsten were monitored with time. The results showed that the most stable Ba-O-W surface complex has an oxygen concentration of approximately 5×10 14 atoms/cm 2. This surface has the lowest work function and shows least barium desorption at a given temperature; the major barium evaporation loss mechanism is given by the surface chemical reaction Ba-O-W→Ba↑+(O-W). At higher oxygen concentrations the Ba-O-W complex is less stable, has a higher work function and, at elevated temperatures, barium desorbs more rapidly and in two different modes. The first is similar to that described above and the second is by means of the surface reaction Ba-O-W→ BaO↑+W. These results were confirmed by experimental line of sight measurements made with a mass spectrometer.

The inhibition of carbon deposition on stainless steel by prior selective oxidation

Techniques, developed to determine the surface concentration of monolayer adsorbates on metal substrates by means of chemical mapping Auger spectrometry, have been used to measure desorption of barium from Ba-W, Ba-C-W, and Ba-O-W surfaces. Desorption data were obtained by measuring the change in surface concentration with time at a constant temperature. By making these measurements over the temperature range 900 to 1400 K, activation energies for desorption were acquired. Higher barium desorption rates were obtained from Ba-W and Ba-C-W surfaces than from Ba-O-W surfaces. The activation energy for barium desorption from a Ba-O-W surface with about a monolayer of oxygen on the tungsten substrate was 4.8 eV and was about 1 eV lower for the other two types studied (Ba-W, Ba-C-W). To correlate electron emission with surface chemistry, time sequential measurements of both electron emission and barium (and oxygen) desorption were made on both barium-covered oxygenated and clean tungsten surfaces. The most stable thermionic emission and minimum desorption rate were obtained from Ba-O-W surfaces with a chemisorbed monolayer of oxygen. Exposing the tungsten surface to higher oxygen concentrations before monolayer barium deposition created an apparent subsurface oxygen concentration which led to poor electron emission characteristics. This effect increased with oxygen content on the Ba-O-W surfaces. Similar measurements of desorption and electron emission were made on an unimpregnated tungsten cathode, covered by a barium monolayer, and these results were collated with those obtained from Ba-O-W surfaces.

Correlation of electron emission with changes in the surface concentration of barium and oxygen on a tungsten surface

Techniques, developed to determine the surface concentration of monolayer adsorbates on metal substrates by means of chemical mapping Auger spectrometry, have been used to measure desorption of barium from Ba-W, Ba-C-W, and Ba-O-W surfaces. Desorption data were obtained by measuring the change in surface concentration with time at a constant temperature. By making these measurements over the temperature range 900 to 1400 K, activation energies for desorption were acquired. Higher barium desorption rates were obtained from Ba-W and Ba-C-W surfaces than from Ba-O-W surfaces. The activation energy for barium desorption from a Ba-O-W surface with about a monolayer of oxygen on the tungsten substrate was 4.8 eV and was about 1 eV lower for the other two types studied (Ba-W, Ba-C-W). To correlate electron emission with surface chemistry, time sequential measurements of both electron emission and barium (and oxygen) desorption were made on both barium-covered oxygenated and clean tungsten surfaces. The most stable thermionic emission and minimum desorption rate were obtained from Ba-O-W surfaces with a chemisorbed monolayer of oxygen. Exposing the tungsten surface to higher oxygen concentrations before monolayer barium deposition created an apparent subsurface oxygen concentration which led to poor electron emission characteristics. This effect increased with oxygen content on the Ba-O-W surfaces. Similar measurements of desorption and electron emission were made on an unimpregnated tungsten cathode, covered by a barium monolayer, and these results were collated with those obtained from Ba-O-W surfaces.

Theoretical models of the negative ionization of hydrogen on clean tungsten, cesiated tungsten and cesium surfaces at low energies

Theoretical models of the negative ionization of hydrogen on clean tungsten, cesiated tungsten and cesium surfaces at low energies