A. Manhard

Interaction of nitrogen plasmas with tungsten

The use of nitrogen seeding to reduce the edge plasma temperature has recently been successfully applied in ASDEX Upgrade. While the plasma performance was significantly improved compared with other seeding species such as Ar or Ne, questions remained as to the interaction of nitrogen with a tungsten first wall. In particular the formation of thick tungsten nitride layers with reduced melting temperature and increased physical sputtering was a concern. Therefore dedicated laboratory experiments have been performed to investigate the interaction of W surfaces with N plasmas. Tungsten coated Si samples were exposed to N ions from plasma and ion gun sources at energies from 20?eV to 10?keV and W surface temperatures from 300 to 750?K. After exposure to the N plasma with fluences of up to several 1023?N?m?2 the N content in the samples was measured by nuclear reaction analysis. The sputter erosion was determined by measuring the thickness change of the W layer by Rutherford backscattering. The formation of W-nitride phases was investigated in separate XPS experiments where the samples were implanted in situ with kiloelectronvolt N ions. It was found that only very small fractions of N are accumulated on the W surface and that N is bound in a nitride state. At temperatures above 600?K the nitrides are no longer stable which further reduces the N uptake into the W. Moreover the accumulation of N on the surface leads to a decrease in W physical sputtering due to the lower W concentration at the surface.

Quantification of the deuterium ion fluxes from a plasma source

We present an electron-cyclotron-resonance plasma source with a biased sample holder that is well suited for the implantation of deuterium ions into tungsten. Achievable energies range from several electronvolts up to 200 eV per deuteron. The deuterium ion fluxes from this plasma source were thoroughly quantified with a retarding field analyser and a plasma monitor, i.e. an energy resolving mass spectrometer. We present the results of these measurements for different microwave input powers, D2 gas pressures and sample bias settings. Typical achievable deuteron fluxes are of the order of 1019 to 1020 D m−2 s−1 and are predominantly carried by ions. Additionally, an attempt to quantify the flux of neutral deuterium atoms to the sample surface is presented.

Surface morphology and deuterium retention of tungsten after low- and high-flux deuterium plasma exposure

The surface morphology and deuterium retention were investigated of polycrystalline tungsten targets that were exposed to deuterium plasmas at widely varying conditions. By changing only one parameter at a time, the isolated effects of flux, time and pre-damaging on surface modifications and deuterium retention were studied. The sample exposed to low-flux plasma (10 20nm −2 s −1 ) is mostly smooth with only a few areas containing very large blisters(50-500nµ m). The samples exposed to high-flux plasmas (10 24nm −2 s −1 ) show large numbers of smaller blisters(1-10nµ m) and in addition even smaller protrusions (l750nnm). The size of the blisters and their density strongly increase with fluence. Pre-damaging tungsten with MeV ions leads to less blisters but to more protrusions. In addition to these (sub-)micrometer-sized structures, all samples show formation of nanostructures. Comparison of a low-flux and high-flux sample exposed to similar fluence showed that the variation in morphology is dominated by the flux differences. It is shown that the blisters and protrusions originate in inter- and intra-granular cavities, respectively. The depth of the cavities underneath the surface correlates well with the depth distributions of the retained deuterium. Trapping of significant amounts of deuterium therefore seems to take place in and/or close to these cavities and gives rise to an additional peak in the thermal desorption spectrum at 700nK.

Statistical analysis of blister bursts during temperature-programmed desorption of deuterium-implanted polycrystalline tungsten

During temperature-programmed desorption (TPD) of stress-relieved polycrystalline tungsten samples exposed to a deuterium plasma, short, intense bursts of D2 were observed on the low-temperature flank of the main desorption peak. These bursts are attributed to the rupturing of blisters filled with high-pressure D2 gas. A statistical analysis of the size distribution and temporal correlation of the bursts is presented. The influence of different measurement intervals and TPD heating rates on the observed bursts is simulated based on these statistics and compared to the experimental results. The contribution of bursts to the total D inventory in the sample is also estimated.

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.

Sub-surface structures of ITER-grade W (Japan) and re-crystallized W after ITER-similar low-energy and high-flux D plasma loadings

Tungsten is a promising candidate for plasma-facing materials in fusion reactors. In this work, two types of W materials were investigated: (i) sintered and forged tungsten (ITER-grade Japan, grain sizes 2–100 μm, elongated normal to the surface) and (ii) the same W grade, but after additional re-crystallization (at 2073 K, grain size ~50 μm). The samples were exposed to deuterium with an ion energy of 38 eV D−1, a fluence of 1027 D m−2 and a flux of ~1022 D m−2 s−1 in a plasma generator at elevated temperatures (320–700 K). The D retention (determined by thermal desorption spectroscopy and nuclear reaction analysis) of both sample types is compared. The samples were analysed with scanning electron microscopy combined with a focused ion beam for iterative cross-sectioning to obtain three-dimensional (3D) data of the sub-surface. Electron backscattered diffraction was applied to determine the grain orientation and deformation. First nano-secondary ion mass spectroscopy investigations were performed on a D-loaded sample to analyse the lateral accumulation of H/D on the surface.

Quantitative Microstructure and Defect Density Analysis of Polycrystalline Tungsten Reference Samples after Different Heat Treatments

Abstract In order to provide a solid basis for the correlation of microstructure and hydrogen isotope retention in tungsten, reference samples with different microstructures were prepared from a single batch of polycrystalline tungsten by standardised polishing and heat treatment procedures. Representative samples were analysed by scanning electron microscopy and scanning transmission electron microscopy as well as by electron backscatter diffraction. We show that if the annealing temperature is increased from 1 200 to 1 500 K, practically only the density of dislocations and grain boundaries with very small misorientations of less than 2° is reduced, while for annealing at 1 700 and 2 000 K, also the density of high-angle grain boundaries is reduced due to grain growth. Furthermore, the dislocation density is reduced by nearly two orders of magnitude compared to tungsten annealed at 1 200 K. We also comment on two different textures on the front and rear side of the samples that were observed both by X-ray diffraction and EBSD.

Depth profiling of the modification induced by high-flux deuterium plasma in tungsten and tungsten-tantalum alloys

The present work reports the results of an experimental study of the depth distribution and fluence dependence of deuterium plasma-induced material modification of tungsten and tungsten‐tantalum alloys. Plasma-induced damage was created by exposure to high-flux deuterium plasma in the plasma generator Pilot-PSI, followed by the degassing and subsequent decoration of created defects with deuterium by another plasma exposure. The depth distribution of deuterium from the decorating exposure reflects the distribution of plasma-induced defects. Depth profiling of this decorating deuterium, was performed by nuclear reaction analysis. It was found that plasma-induced material modification, which manifested itself as an increase of the deuterium concentration in the samples pre-exposed with high-flux plasma in comparison to the samples without such pre-exposure extends down to more than 5 µm from the surface. This increase features a tendency to saturation with increasing fluence of the damaging high-flux plasma. Over the entire probing range, with the exception of the narrow surface region and the deep region beyond 5 µm, the deuterium content is lower in pre-exposed W‐Ta than in similarly pre-exposed W. Sub-surface features formed as a result of high-flux plasma exposure were studied with the help of focused ion beam cross-sectioning. W was found to contain plasma-induced cavities down to much larger depth than W‐Ta.

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.