U. von Toussaint

Chemical sputtering of carbon films by simultaneous irradiation with argon ions and molecular oxygen

The erosion of hard amorphous hydrocarbon films by bombardment with argon ions and simultaneous exposure to thermal molecular oxygen is studied as a function of oxygenflux density (0-11400 times the ionflux density), ion energy (20-800eV), and surface temperature (110-875K). While erosion due to Ar + ions only is dominated by physical sputtering, the additional presence of molecular oxygen leads to a marked increase of erosion, indicating chemical sputtering. The erosion yield increases with both ion energy and oxygen flux density. Starting from about 700K thermal chemical erosion (combustion) by O2 is observed even without ion bombardment. Additional ion bombardment in this temperature range causes an increase of the erosion rate over the sum of thermal chemical erosion and the rate observed at room temperature. Below 300K, the rate increases with decreasing temperature. We explain the latter behavior by the ion-induced reaction of adsorbed oxygen which constitutes a significant surface coverage only at low temperatures. A rate equation model is presented, which incorporates the mechanisms of physical sputtering, chemical reaction of O2 at reactive sites created by ion bombardment, the ion-induced reaction of adsorbed oxygen and ion-enhanced thermal chemical erosion. The models nine free parameters are optimized by fitting 68 experimental data points. The model yields good agreement in all investigated dependences.

Synergistic erosion process of hydrocarbon films: a molecular dynamics study

Fundamental processes leading to the erosion of hydrocarbon films due to energetic argon ions and hydrogen atoms have been investigated using molecular dynamics simulations. A generic mechanism has been identified for carbon erosion due to energetic (150 eV) argon ions in the presence of sub-eV hydrogen atoms. This surface erosion process, which we call hydrogen enhanced physical sputtering (HEPS), is primarily a physical sputtering mechanism, enhanced due to the screening effect of hydrogen atoms. The energetic argon ions create open bonds within their penetration range. The hydrogen atoms passivate the open bonds created within the first few atomic layers. Subsequent ion bombardment causes the breaking of C–C bonds within and beyond the H penetration range. The steric effect of H atoms bound to the top layer of carbon atoms prevents the re-attachment of the broken bonds, and this leads to unsaturated molecule emission from the surface. The kinetic energy of the emitted molecules is above thermal energy and the emission takes place within 5 ps after the ion impact.

Transport of hydrogen in metals with occupancy dependent trap energies

Common diffusion trapping models for modeling hydrogen transport in metals are limited to traps with single de-trapping energies and a saturation occupancy of one. While they are successful in predicting typical mono isotopic ion implantation and thermal degassing experiments, they fail at describing recent experiments on isotope exchange at low temperatures. This paper presents a new modified diffusion trapping model with fill level dependent de-trapping energies that can also explain these new isotope exchange experiments. Density function theory (DFT) calculations predict that even mono vacancies can store between 6 and 12 H atoms with de-trapping energies that depend on the fill level of the mono vacancy. The new fill level dependent diffusion trapping model allows to test these DFT results by bridging the gap in length and time scale between DFT calculations and experiment.

Molecular dynamics study of grain boundary diffusion of hydrogen in tungsten

Understanding the influence of the microstructure of tungsten on hydrogen transport is crucial for the use of tungsten as first-wall material in fusion reactors. Here, we report the results of molecular dynamics and transition state studies on the influence of grain boundaries in tungsten on the transport of hydrogen. An exhaustive mapping of possible minimum activation energy migration trajectories for hydrogen as the trace impurity reveals a strongly modified activation energy distribution in the neighborhood of grain boundaries together with an altered connectivity matrix. The results indicate that grain boundaries in polycrystalline tungsten may provide an important transport channel, especially for neutron-damaged tungsten.

Influence of nitrogen pre-implantation on deuterium retention in tungsten

The influence of nitrogen (N) pre-implantation on the deuterium (D) retention in tungsten (W) at different temperatures was investigated. Bulk W samples were exposed to D plasma with a fluence of 110 24 D/m 2 with or without nitrogen pre-implantation at 300 K and 500 K, respectively. Nuclear reaction analysis was applied for the determination of N content and D retention in the near surface. Optical microscopy was used to investigate the surface modification by blistering after implantation. It is shown that, the W:N layers formed during the N preimplantation play very different roles on D retention and blistering in the samples at different temperatures. At 500 K, the W:N layer seems to enhance D diffusion into the bulk by suppressing D loss from the surface, which results in a much higher D concentration in the bulk and larger blisters than without N pre-implantation. At 300 K, the effect of this layer is much less pronounced than that at 500 K.

Molecular dynamics simulations of amorphous hydrogenated carbon under high hydrogen fluxes.

We study the flux dependence of the carbon erosion yield and the hydrogen enrichment of the surface in the high flux regime at 10(28) ions per m(2) s and higher by using molecular dynamics (MD). We simulate an amorphous hydrogenated carbon sample exposed to high flux hydrogen bombardment with a hydrogen energy of 10 eV at surface temperatures of 700 and 1000 K. As interaction potential the reactive empirical bond order potential of Brenner-Beardmore is taken and energy dissipation is simulated with the Berendsen thermostat. The simulation results show that the carbon erosion yield is higher for higher sample temperatures but does not show a strong dependence on the hydrogen flux. Hence, the hydrogen enrichment in the upper surface layer observed in the simulations most likely does not contribute to the erosion yield reduction in the experiments. Furthermore, the composition of the eroded material shows a slight increase in CH, C(2)H and C(2)H(2) for higher fluxes, whereas species with more hydrogen, C atoms and C(2) are decreased. However, the H : C ratio in the eroded material shows no flux dependence.

Deuterium supersaturation in low-energy plasma-loaded tungsten surfaces

Fundamental understanding of hydrogen–metal interactions is challenging due to a lack of knowledge on defect production and/or evolution upon hydrogen ingression, especially for metals undergoing hydrogen irradiation with ion energy below the displacement thresholds reported in literature. Here, applying a novel low-energy argon-sputter depth profiling method with significantly improved depth resolution for tungsten (W) surfaces exposed to deuterium (D) plasma at 300 K, we show the existence of a 10 nm thick D-supersaturated surface layer (DSSL) with an unexpectedly high D concentration of ~10 at.% after irradiation with ion energy of 215 eV. Electron back-scatter diffraction reveals that the W lattice within this DSSL is highly distorted, thus strongly blurring the Kikuchi pattern. We explain this strong damage by the synergistic interaction of energetic D ions and solute D atoms with the W lattice. Solute D atoms prevent the recombination of vacancies with interstitial W atoms, which are produced by collisions of energetic D ions with W lattice atoms (Frenkel pairs). This proposed damaging mechanism could also be active on other hydrogen-irradiated metal surfaces. The present work provides deep insight into hydrogen-induced lattice distortion at plasma–metal interfaces and sheds light on its modelling work.

Surface layer destruction during ion beam analysis

Abstract In ion beam analysis the decrease of the measuring signal with a number of incident ions, due to a destruction of the surface layer being analysed, depends critically on the lateral intensity distribution in the analysing ion beam. For the assumption of destruction in one step, the decrease was calculated and the obtained analytical formulae was fitted to the decrease as measured in ERDA and PIXE analyses. This allows to obtain values for the destruction cross sections for the ions and the samples in the analysis, as well as information about the lateral intensity distribution in the analysing ion beam.

Interaction of atomic and low-energy deuterium with tungsten pre-irradiated with self-ions

Polycrystalline tungsten (W) specimens were pre-irradiated with self-ions to create identical samples with high density of defects up to ∼2.5 μm near the surface. Then, W specimens were exposed to either thermal atomic deuterium (D) beam with an incident energy of ∼0.2 eV or low energy D plasma with the incident energy varied between 5 and 200 eV at different sample temperatures. Each sample was exposed once at certain temperature and fluence. The D migration and accumulation in W were studied post-mortem by nuclear reaction method. It was shown that the rate of the D to occupy radiation-induced defects increases with increasing the incident energy, ion flux, and temperature. Experimental investigation was accompanied by modelling using the rate-equation model. Moreover, the analytical model was developed and benchmarked against numerical model. The calculations of the deuterium diffusion with trapping at radiation-induced defects in tungsten by analytical model are consistent with numerical calculations ...

Comparative study of the dust particle population sampled during four consecutive campaigns in full-tungsten ASDEX Upgrade

Scanning electron microscopy images and energy-dispersive x-ray spectra were recorded for a total of about 4×104 dust particles collected on the same position within the vacuum vessel via silicon wafers during four consecutive full-tungsten first wall campaigns of ASDEX Upgrade between 2007 and 2009. By careful analysis of the elemental composition and shape of the sampled particles, seven statistically relevant classes of dust were identified. The particle flux and area coverage of each class were normalized to the total plasma duration of each sampling period, revealing a high sensitivity of the dust composition to device conditioning. According to the present results, particles produced by arcing on divertor tiles with delaminated coatings were transported to the main chamber first wall.