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In this work, a kinetic model is presented to describe hydrogen absorption and desorption from tungsten at different surface coverages. Activation energies for hydrogen absorption into the bulk and desorption from the surface of tungsten are modelled by functions that depend explicitly and continuously on the hydrogen surface coverage. A steady-state model is developed to derive these activation energies from experimental data. The newly developed coverage dependent activation energies are then implemented in the non steady-state rate-equation code MHIMS. Published experimental results on D uptake and retention of self-damaged tungsten exposed to 0.28 eV deuterium atoms at different temperatures ranging from 450 K to 1000 K can be successfully described with this approach. Finally, the steady-state model is applied to determine surface concentration, bulk concentration and migration depths of hydrogen isotopes in tungsten exposed to various atomic fluxes and temperatures ranging from milder conditions in laboratory experiments to divertor strike point conditions in tokamaks. [ABSTRACT FROM AUTHOR]

The simultaneous interaction of methyl radicals (CH[sub 3]) and atomic hydrogen (H) with the surface of amorphous hydrogenated carbon (a-C:H) films is investigated by using quantified radical beam sources. The growth and/or erosion of the films during the interaction of the H and CH[sub 3] radical beam with the surface is monitored by means of in situ real-time ellipsometry at a substrate temperature of 320 K. Interaction with the CH[sub 3] beam alone results in a slow growth rate corresponding to a sticking coefficient for CH[sub 3] of ∼10[sup -4]. Simultaneous interaction of an atomic hydrogen beam and the CH[sub 3] radical beam with the surface results in a sticking coefficient for CH[sub 3] of ∼10[sup -2]. A microscopic modeling of this synergistic growth yields a cross section of 3.8 Å2 for CH[sub 3] adsorption at a dangling bond, created by abstraction of surface bonded hydrogen due to impinging atomic hydrogen. The cross section for the abstraction of surface bonded hydrogen by impinging CH[sub 3] radicals is 1.5x10[sup -3] Å[sup 2]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new algorithm for the diffusion in 2D and 3D discrete simple cubic lattices based on a recently proposed technique, Green-functions or first-passage kinetic Monte Carlo has been developed. It is based on the solutions of appropriately chosen Greens functions, which propagate the diffusing atoms over long distances in one step (superhops). The speed-up of the new approach over standard kinetic Monte Carlo techniques can be orders of magnitude, depending on the problem. Using this new algorithm we simulated recent hydrogen isotope exchange experiments in recrystallized tungsten at 320 K, initially loaded with deuterium. It was found that the observed depth profiles can only be explained with ‘active’ traps, i.e. traps capable of exchanging atoms with activation energies significantly lower than the actual trap energy. Such a mechanism has so far not been considered in the modeling of hydrogen transport. [ABSTRACT FROM AUTHOR]

The simultaneous interaction of methyl radicals (CH[sub 3]) and atomic hydrogen (H) with the surface of amorphous hydrogenated carbon (a-C:H) film is investigated. Two identical quantified beam sources for H and CH[sub 3] are used. The growth and/or erosion during the simultaneous interaction of the two beams with an amorphous hydrogenated carbon film is monitored by using in situ real-time ellipsometry at a substrate temperature of 320 K. Interaction with the CH[sub 3] beam alone causes slow growth, corresponding to a sticking coefficient for CH[sub 3] of ∼3x10[sup -5]. Simultaneous interaction of the atomic hydrogen beam and the CH[sub 3] radical beam yields a sticking coefficient for CH[sub 3] of 3x10[sup -3], which is two orders of magnitude larger than for CH[sub 3] alone. From a microscopic modeling of this synergistic growth, the reaction probability for CH[sub 3] adsorbing at an adsorption site, which is created by atomic hydrogen at the surface, is derived to be 0.14. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Trapping at irradiation-induced defects significantly enhances hydrogen-isotope retention in metals with low hydrogen solubility. Here, we present the first microscopic observation of nanocavity formation on tungsten surfaces exposed to low-dose hydrogen plasma at room temperature. The nanocavities exhibit very high aspect ratios between their lateral size L and thickness ω (L/ω : 60-90) and represent the predominant microstructural defects in the early development stage of the hydrogen supersaturated surface layer on hydrogen plasma-irradiated tungsten (see Nucl Fusion 57 (2017) 016026, Acta Mater 201 (2020) 55). We also quantify the vacancy yield in tungsten under hydrogen irradiation with ion energies below the threshold to create stable Frenkel pairs. Such defect production at sub-threshold energy is conventionally deemed impossible, but we consider that the exposed surface acts as a defect sink with strong absorption bias for mobile self-interstitial atoms from temporarily created Frenkel pairs. This results in a high excess of immobile vacancies in the nanometer-thin surface layer and their agglomeration causes the evolution of large, disc-like nanocavities. [Display omitted] [ABSTRACT FROM AUTHOR]

We discuss the surface morphology modification of W samples observed after simultaneous heat and particle loading using a mixed H/He particle beam with a He concentration of 1 at.%. The applied heat flux of 10 MW/m 2 is representative for the normal operation of the divertor of DEMO or a power plant. The long pulse high heat flux experiments on actively water-cooled W samples were performed in the GLADIS facility at surface temperatures between 600 °C and 2000 °C. This allows together with the applied total fluences between 1 × 10 24 m −2 and 1 × 10 26 m −2 the temperature- and fluence dependent study of the growing nano-structures. We analyse in detail the surface modifications up to a depth of several μm by scanning electron microscopy combined with focussed ion beam preparation. The hydrogen and helium release of the samples is analysed by long term thermal desorption spectroscopy and compared with the prediction of a diffusion trapping model. [ABSTRACT FROM AUTHOR]

Abstract: In this paper well defined test samples are used to show how the analysis depth and the matrix material influence the depth resolution of nuclear reaction analysis. The reaction D(3He,p)α is used to detect deuterium. By covering 10nm thin deuterated amorphous carbon (a–C:D) films on silicon with tungsten (Z W =74) and titanium (Z Ti =22) of various thicknesses between 500nm and 8μm the influence of the atomic number and the overlayer thickness on the depth resolution is studied. The most probable depth profiles are calculated from the experimental data with the program NRADC, which implements Bayesian statistics. The resulting apparent layer width of the deuterium containing layer broadens with increasing thickness of the coating and this broadening is more pronounced for coatings with higher Z. These apparent layer widths are a measure for the experimentally achievable depth resolution. Their absolute values are in the same range as the theoretical optimum calculated with RESOLNRA. To investigate the depth resolution of the reaction, a 12nm thin hydrogenated amorphous carbon (a–C:H) film on silicon and a pure tungsten sample are analysed. The width of the instrument function of this method is deduced from the surface hydrogen peak of the pure tungsten sample. The two methods are compared. [Copyright &y& Elsevier]

Abstract: Hydrogen gas-driven permeation through graphite coated with different types of tungsten coatings with thicknesses up to 200μm has been investigated. The substrate material was fine-grain graphite R5710 and R6710, which properties with respect to hydrogen transport are well known . Magnetron-sputtered W coatings of thicknesses 1 and 3μm and two coatings of ASDEX Upgrade were investigated: A 3μm thick layer of tungsten deposited by physical vapor deposition (PVD-W) and 200μm thick layers of tungsten deposited by plasma-spaying in vacuum (VPS-W). The gas-driven permeation was investigated at a pressure gradient of 10−2–150Pa. The gas-driven permeation occurs through the carbon base-materials by hydrogen molecular gas flow through the internal porosity network rather than hydrogen atom diffusion through the graphite lattice. It was found that W coatings with thicknesses up to 3μm are transparent for hydrogen gas penetration and do not influence the permeability of coated fine-grain graphite, because the open porosity system of graphite remains open. Even a 200μm thick layer of VPS-W has an open system of connected pores, which connects the front and rear surfaces of the deposited layer. [Copyright &y& Elsevier]