[Show abstract][Hide abstract] ABSTRACT: The interaction of interstitial hydrogen with a dislocation and point defects in tungsten is studied by means of atomistic simulations. Two different types of interatomic potentials were tested by comparing their results with available ab initio data. The recently developed embedded atom method potential showed a better agreement with ab initio results than the bond order potential. Static calculations involving screw and edge dislocations showed that hydrogen is attracted to the dislocation core in both cases. It is also found that hydrogen atoms prefer to arrange themselves as elongated clusters on dislocation lines. Molecular dynamics simulations of hydrogen migration along the edge dislocation core confirmed the results of the static calculations and demonstrated a strong attraction to the dislocation core and one-dimensional migration along it.
[Show abstract][Hide abstract] ABSTRACT: To investigate synergistic effects of high heat flux loading on H and H/He loaded tungsten surfaces, specimens were exposed to a 30 keV steady-state H or H/He beam and subsequently loaded with an electron beam to simulate ELMs. The heat flux during the H and H/He loading was 10.5 MW m−2, while a 2 × 1025 m−2 fluence was reached. After exposure, all specimens exhibited an altered surface morphology. The H/He samples with a surface temperature of 1000 °C and 1500 °C had a multitude of surface extrusions. Afterwards the particle loaded samples were exposed to 100 ELM-like pulses around the material's damage threshold. Transient heat fluxes of 190 MW m−2 and 380 MW m−2 were applied at room temperature and 400 °C for a duration of 1 ms. Post-mortem analysis showed no deterioration of thermal shock resistance in comparison with polished material. For some tests the reference specimens roughened or cracked while the H or H/He exposed material had no damage. The H-content and the H/He-induced cavities and/or extrusions are suggested as two potential causes for this change in material behaviour.
No preview · Article · Oct 2015 · Fusion Engineering and Design
[Show abstract][Hide abstract] ABSTRACT: The paper presents objectives and activities of IAEA Coordinated Research Projects 'Conceptual development of steady-state compact fusion neutron sources' and 'Utilisation of a network of small magnetic confinement fusion devices for mainstream fusion research'. The background and main projects of the CRP on FNS are described in detail, as this is a new activity at IAEA. Recent activities of the second CRP, which continues activities of previous CRPs, are overviewed.
[Show abstract][Hide abstract] ABSTRACT: The renewable evolution in the energy industry and the depletion of natural resources are putting pressure on the waste industry to shift towards flexible treatment technologies with efficient materials and/or energy recovery. In this context, a thermochemical conversion method of recent interest is plasma gasification, which is capable of producing syngas from a wide variety of waste streams. The produced syngas can be valorized for both energetic (heat and/or electricity) and chemical (ammonia, hydrogen or liquid hydrocarbons) end-purposes. This paper evaluates the performance of experiments on a single-stage plasma gasification system for the treatment of refuse-derived fuel (RDF) from excavated waste. A comparative analysis of the syngas characteristics and process yields was done for seven cases with different types of gasifying agents (CO2+O2, H2O, CO2+H2O and O2+H2O). The syngas compositions were compared to the thermodynamic equilibrium compositions and the performance of the single-stage plasma gasification of RDF was compared to that of similar experiments with biomass and to the performance of a two-stage plasma gasification process with RDF. The temperature range of the experiment was from 1400 to 1600K and for all cases, a medium calorific value syngas was produced with lower heating values up to 10.9MJ/Nm(3), low levels of tar, high levels of CO and H2 and which composition was in good agreement to the equilibrium composition. The carbon conversion efficiency ranged from 80% to 100% and maximum cold gas efficiency and mechanical gasification efficiency of respectively 56% and 95%, were registered. Overall, the treatment of RDF proved to be less performant than that of biomass in the same system. Compared to a two-stage plasma gasification system, the produced syngas from the single-stage reactor showed more favourable characteristics, while the recovery of the solid residue as a vitrified slag is an advantage of the two-stage set-up.
[Show abstract][Hide abstract] ABSTRACT: A new mechanism for the nucleation and growth of hydrogen (H) bubbles on dislocations under plasma exposure of tungsten was recently proposed on the basis of direct ab initio calculations. Density functional theory calculations demonstrated that H atoms are strongly bound to a screw dislocation core and exhibit fast one-dimensional migration along its line. Once the number of hydrogen atoms trapped on a dislocation segment exceeds eight, the emission of a jog occurs thereby converting a pure HN cluster into a HN+1-jog configuration. On the basis of these results a kinetic model was formulated to evaluate the conditions (i.e., range of temperature and flux exposure) for the transformation of pure H clusters into supercritical hydrogen-vacancy clusters attached to the dislocation line. In this work, a parametric study employing the kinetic nucleation model was performed to derive the hydrogen bubble formation energy function that offers the best agreement with available experimental results. The obtained results allow one to rationalize the depth and temperature dependence of the experimentally observed hydrogen deposition after high flux low energy plasma exposure for ITER relevant conditions.
Full-text · Article · Jun 2015 · Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms
[Show abstract][Hide abstract] ABSTRACT: High-resolution spectroscopy is a powerful tool for the measurement of plasma rotation as well as ion temperature using the Doppler shift of the emitted spectral lines and their Doppler broadening, respectively. Both passive and active diagnostic variants for the COMPASS tokamak are introduced. The passive diagnostic focused on the C III lines at about 465 nm is utilized for the observation of the poloidal plasma rotation. The current set-up of the measuring system is described, including the intended high-throughput optics upgrade. Different options to increase the fiber collection area are mentioned, including a flower-like fiber bundle, and the use of micro-lenses or tapered fibers. Recent measurements of poloidal plasma rotation of the order of 0–6 km/s are shown. The design of the new active diagnostic using a deuterium heating beam and based on charge exchange recombination spectroscopy (C VI line at 529 nm) is introduced. The tool will provide both space (0.5–5 cm) and time (10 ms) resolved toroidal plasma rotation and ion temperature profiles. The results of the Simulation of Spectra code used to examine the feasibility of charge exchange measurements on COMPASS are shown and connected with a selection of the spectrometer coupled with the CCD camera.
No preview · Article · Apr 2015 · Fusion Engineering and Design
[Show abstract][Hide abstract] ABSTRACT: The paper presents the results of an experimental study of deuterium retention in W and W–Ta alloy that were exposed to first-wall relevant low flux ($10 20 m À2 s À1) deuterium plasma in the ECR plasma generator PlaQ. Subsequent analysis included surface imaging by optical microscopy, deuterium depth profiling by nuclear reaction analysis (NRA) and measurements of deuterium content by thermal desorption spec-troscopy (TDS). It was found that under investigated exposure conditions the deuterium content was higher in W–Ta alloy than in W. Combined with the previously reported results showing that under high-flux ($10 24 m À2 s À1) retention is higher in W instead, this gives rise to a peculiar flux effect – dependence of relative retention between different materials on exposure flux. We interpret this effect as evidence that at different flux ranges different populations of trapping sites determine the retention, namely pre-existing microstructural traps at low-flux exposure and plasma-induced ones at high-flux exposure.
[Show abstract][Hide abstract] ABSTRACT: The effect of severe plastic deformation on the deuterium retention in tungsten exposed to high-flux low-energy plasma (flux ∼ 1024 D/m2/s, energy ∼ 50 eV, and fluence up to 3 × 1026 D/m2) at the plasma generator Pilot-PSI was studied by thermal desorption spectroscopy and scanning electron microscopy. The desorption spectra in both reference and plastically deformed samples were deconvolved into three contributions attributed to the detrapping from dislocations, deuterium-vacancy clusters, and pores, respectively. The plastically induced deformation, resulting in high dislocation density, does not change the positions of the three peaks, but alters their amplitudes as compared to the reference material. The appearance of blisters detected by scanning electron microscopy and the desorption peak attributed to the release from pores (i.e., deuterium bubbles) were suppressed in the plastically deformed samples but only up to a certain fluence. Beyond 5 × 1025 D/m2, the release from the bubbles in the deformed material is essentially higher than in the reference material. Based on the presented results, we suggest that a dense dislocation network increases the incubation dose needed for the appearance of blisters, associated with deuterium bubbles, by offering numerous nucleation sites for deuterium clusters eventually transforming into deuterium-vacancy clusters by punching out jogs on dislocation lines.
Full-text · Article · Feb 2015 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: In order to improve the performance of tungsten, a basic understanding of the microstructure-property relationships is essential. In the present study, a newly developed double forged pure tungsten grade from Plansee SE was investigated. By analysing the mechanical properties and microstructures in well-defined directions in the double forged tungsten, their relationships could be successfully correlated. A large amount of sub-grains with a typical size below 5 μm were observed in the as-received double-forged tungsten. After thermally treating the double forged tungsten up to 2000 °C, microstructural recovery was observed with the onset of recrystallization. Meanwhile, the sub-grain misorientation angle increased accompanied by sub-grain growth. The deformation temperature and the strain rate considerably influenced the final microstructure. The higher the temperature, the lower the amount of sub-grain boundaries due to sub-grain coarsening and the clearer the grain boundaries. The higher the deformation strain rate during tensile testing, the higher the grain orientation spread and the larger the sub-grain misorientation, but the smaller the grain size due to a lower extend of crystallization. This matched well with the mechanical testing data.
No preview · Article · Jan 2015 · International Journal of Refractory Metals and Hard Materials
[Show abstract][Hide abstract] ABSTRACT: The effect of calculating thermodynamic and transport properties of a gas mixture with mixing rules on the flow field in the modeling of a thermal plasma jet was studied. A 3D large eddy simulation model of a non-transferred direct current hybrid water/argon plasma torch issuing in nitrogen atmosphere at 400 K was developed to compare three different models for the calculation of transport and thermodynamic properties of the ternary gas mixture. In the first model, thermodynamic and transport properties of the pure gases are used with mixing rules to estimate the mixture properties. In the second model, the properties of plasma gas (Ar/H2O) are calculated rigorously and mixing rules are used for estimating the properties of the mixture of plasma gas and nitrogen. In the third model, the thermodynamic and transport properties of the ternary gas mixture are calculated rigorously without any mixing rules. From numerical results, the error introduced by using mixing rules was evaluated through comparison of calculated temperature, velocity and concentration profiles of the flow field at different positions downstream of the torch exit nozzle. It was found that the differences in transport properties between the exact solutions and the results from calculation with mixing rules can yield significantly different flow fields.
No preview · Article · Jan 2015 · Plasma Chemistry and Plasma Processing
[Show abstract][Hide abstract] ABSTRACT: Taking the example of tungsten, we demonstrate that high-flux plasma exposure of recrystallized and plastically deformed samples leads to principal differences in the gas trapping and associated surface modification. Surface of the exposed pre-deformed samples exhibits ruptured µm-sized blisters, a signature of bubbles nucleated close to the surface on the plastically induced dislocation network. Contrary to the recrystallized samples, no stage attributable to gas bubbles appeared in the desorption spectrum of the deformed samples demonstrating the strong impact of dislocations on hydrogen retention.
[Show abstract][Hide abstract] ABSTRACT: The finite-difference time-domain (FDTD) method in cylindrical coordinates is used to describe electromagnetic wave propagation in a cold magnetized plasma. This enables us to study curvature effects in toroidal plasma. We derive the discrete dispersion relation of this FDTD scheme and compare it with the exact solution. The accuracy analysis of the proposed method is presented. We also provide a stability proof for nonmagnetized uniform plasma, in which case the stability condition is the vacuum Courant condition. For magnetized cold plasma we investigate the stability condition numerically using the von Neumann method. We present some numerical examples which reproduce the dispersion relation, wave field structure and steady state condition for typical plasma modes.
Full-text · Article · Dec 2014 · IEEE Transactions on Antennas and Propagation
[Show abstract][Hide abstract] ABSTRACT: The influence of surface temperature, particle flux density and material microstructure on the surface morphology and deuterium retention was studied by exposing tungsten targets (20 μm and 40 μm grain size) to deuterium plasma at the same particle fluence (1026 m−2) and incident ion energy (40 eV) to two different ion fluxes (low flux: 1022 m−2 s−1, high flux: 1024 m−2 s−1). The maximum of deuterium retention was observed at ∼630 K for low flux density and at ∼870 K for high flux density, as indicated from the thermal desorption spectroscopy data (TDS). Scanning electron microscopy observations revealed the presence of blisters with a diameter of up to 1 μm which were formed at high flux density and high temperature (1170 K) contrasting with previously reported surface modification results at such exposure conditions.
No preview · Article · Dec 2014 · Journal of Nuclear Materials
[Show abstract][Hide abstract] ABSTRACT: Systematic study of deuterium irradiation effects on tungsten was done under ITER - relevant high particle flux density, scanning a broad surface temperature range. Polycrystalline ITER - like grade tungsten samples were exposed in linear plasma devices to two different ranges of deuterium ion flux densities (high: 3.5-7 · 1023 D+/m2 s and low: 9 · 1021 D+/m2 s). Particle fluence and ion energy, respectively 1026 D+/m2 and ∼38 eV were kept constant in all cases.The experiments were performed at three different surface temperatures 530 K, 630 K and 870 K. Experimental results concerning the deuterium retention and surface modifications of low flux exposure confirmed previous investigations. At temperatures 530 K and 630 K, deuterium retention was higher at lower flux density due to the longer exposure time (steady state plasma operation) and a consequently deeper diffusion range. At 870 K, deuterium retention was found to be higher at high flux density according to the thermal desorption spectroscopy (TDS) measurements. While blisters were completely absent at low flux density, small blisters of about 40-50 nm were formed at high flux density exposure. At the given conditions, a relation between deuterium retention and blister formation has been found which has to be considered in addition to deuterium trapping in defects populated by diffusion.
Full-text · Article · Nov 2014 · Journal of Nuclear Materials
[Show abstract][Hide abstract] ABSTRACT: 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 5µm 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 5µm, 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.
[Show abstract][Hide abstract] ABSTRACT: We have developed a new theoretical model for deuterium (D) retention in tungsten-based alloys on the basis of its being trapped at dislocations and transported to the surface via the dislocation network with parameters determined by ab initio calculations. The model is used to explain experimentally observed trends of D retention under sub-threshold implantation, which does not produce stable lattice defects to act as traps for D in conventional models. Saturation of D retention with implantation dose and effects due to alloying of tungsten with, e.g. tantalum, are evaluated, and comparison of the model predictions with experimental observations under high-flux plasma implantation conditions is presented.
Full-text · Article · Aug 2014 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: To explore the behavior of electromagnetic waves in cold magnetized plasma, a three-dimensional cylindrical hybrid finite-difference time-domain model is developed. The full discrete dispersion relation is derived and compared with the exact solutions. We establish an analytical proof of stability in the case of nonmagnetized plasma. We demonstrate that in the case of nonmagnetized cold plasma the maximum stable Courant number of the hybrid method coincides with the vacuum Courant condition. In the case of magnetized plasma the stability of the applied numerical scheme is investigated by numerical simulation. In order to determine the utility of the applied difference scheme we complete the analysis of the numerical method demonstrating the limit of the reliability of the numerical results.
[Show abstract][Hide abstract] ABSTRACT: An estimation of the contribution of gaps to beryllium deposition and resulting tritium retention in the divertor of ITER is presented. Deposition of beryllium layers in gaps of the full tungsten divertor is simulated with the 3D-GAPS code. For gaps aligned along the poloidal direction, non-shaped and shaped solutions are compared. Plasma and impurity ion fluxes from Schmid (2008 Nucl. Fusion 48 105004) are used as input. Ion penetration into gaps is considered to be geometrical along magnetic field lines. The effect of realistic ion penetration into gaps is discussed. In total, gaps in the divertor are estimated to contribute about 0.3 mgT s−1 to the overall tritium retention dominated by toroidal gaps, which are not shaped. This amount corresponds to about 7800 ITER discharges up to the safety limit of 1 kg in-vessel tritium; excluding, however, tritium release during wall baking and retention at plasma-wetted and remote areas.