G. Van Oost

Ghent University, Gand, Flemish, Belgium

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Publications (284)322.65 Total impact

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    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.
    International Journal of Refractory Metals and Hard Materials 01/2015; DOI:10.1016/j.ijrmhm.2015.01.008 · 1.86 Impact Factor
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    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.
    Plasma Chemistry and Plasma Processing 01/2015; 35(2). DOI:10.1007/s11090-014-9605-6 · 1.60 Impact Factor
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    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.
    Nuclear Fusion 01/2015; 55(1). DOI:10.1088/0029-5515/55/1/013007 · 3.24 Impact Factor
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    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.
    Journal of Nuclear Materials 11/2014; 455(1-3). DOI:10.1016/j.jnucmat.2014.06.059 · 2.02 Impact Factor
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    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.
    Nuclear Fusion 11/2014; 54:123013. DOI:10.1088/0029-5515/54/12/123013 · 3.24 Impact Factor
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    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.
    Journal of Physics Condensed Matter 08/2014; 26(39):395001. DOI:10.1088/0953-8984/26/39/395001 · 2.22 Impact Factor
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    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.
    Physica Scripta 05/2014; 2014(T161):014014. DOI:10.1088/0031-8949/2014/T161/014014 · 1.03 Impact Factor
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    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.
    Physica Scripta 04/2014; 2014(T159):014063. DOI:10.1088/0031-8949/2014/T159/014063 · 1.03 Impact Factor
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    ABSTRACT: Spark plasma sintered tungsten grades, with an yttrium content varying between 0.25 and 1 wt%, were characterized and exposed to transient thermal loads. The samples were cyclic tested at room temperature applying 1 ms long heat pulses using a Nd:YAG laser beam and the electron beam facility JUDITH 1. The absorbed power density of these pulses varied between 0.37 and 1.14 GW m−2. The material modifications were analysed with scanning electron microscopy, optical microscopy and laser profilometry. Comparison showed an improvement of the thermal shock resistance with increasing yttrium content. Additionally, three samples were tested at an elevated base temperature at 400 °C. The two materials with highest yttrium content cracked, indicating still brittle behaviour at the elevated base temperature when adding yttrium.
    Physica Scripta 04/2014; 2014(T159):014035. DOI:10.1088/0031-8949/2014/T159/014035 · 1.03 Impact Factor
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    ABSTRACT: The strain-hardening behavior of two recently developed double forged and K-doped tungsten grades in the 300–2000 °C range was analyzed applying a phenomenological model describing the evolution of the flow stress as a function of the dislocation density. The applied model allowed establishing a correlation between the strain hardening curvature and the size of microstructural features controlling the dislocation multiplication. The obtained results demonstrated that plastic deformation was controlled by the resistance of the low angle grain boundaries below 1000 °C and the high angle grain boundaries at 1500 °C and above. The experimental results obtained at different loading rates showed that thermal activation was essential for the passage of dislocations through grain boundary interfaces at 1000 °C and above. The limitations of the applied model and need for further development of the physical model accounting for stress- and temperature-induced grain growth are discussed.
    Journal of Nuclear Materials 01/2014; 444(s 1–3):214–219. DOI:10.1016/j.jnucmat.2013.09.057 · 2.02 Impact Factor
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    ABSTRACT: The VISIONI plasmatron is a plasma simulator dedicated to thermonuclear fusion material studies. The plasma is confined by means of a strong multipole cusp magnetic field produced by rectangular permanent samarium–cobalt magnets positioned just outside of the plasma chamber. The magnitude of this field as a function of the location in the plasma chamber is an important input for the ongoing plasma and material migration computer simulations for VISIONI. In this work the magnetic field in the VISIONI plasma chamber was calculated analytically. The calculated results were benchmarked and verified experimentally by Hall probe measurements. The analytical formulas are very convenient for use in different simulation codes.
    Fusion Engineering and Design 10/2013; DOI:10.1016/j.fusengdes.2013.01.071 · 1.15 Impact Factor
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    ABSTRACT: Tungsten and its alloys are currently considered as prime candidates for plasma facing components in many fusion experimental devices and power plants, but some issues regarding the activation of these materials remain to be clarified. This study has addressed the activation concerns of seven candidate W-alloys (W-W composites, W-La2O3, W-TiC, W-Ta, W-K, VM tungsten and W-Re) that could be used for W-based divertors and W-armors under the operating conditions of the ITER experimental facility and the PPCS power plant (EU power plant design studies). More specifically, the three radioactive waste management options (disposal, recycling, and clearance) as well as the transmutation of W have been examined. The latter is quite sensitive to the divertor and blanket materials. About 5-8 at.% of the W-armor transmute at the end of the PPCS blanket lifetime while the transmutation in the W-based divertor is <1%. Such a low transmutation level may not impair the physical properties of divertor W-alloys.
    Fusion Engineering and Design 10/2013; 88(9-10):2674-2678. DOI:10.1016/j.fusengdes.2013.02.042 · 1.15 Impact Factor
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    ABSTRACT: Dedicated experiments have been carried out at the TEXTOR tokamak in biasing-induced improved confinement regimes using Langmuir probe and GPI measurements. The results show the first evidence for the intimate interaction among sheared flows, eddy structures, Reynolds stress, zonal flows, and ambient fluctuations during a transition to an improved confinement. The sheared flows impose dynamic effects on turbulence structures, resulting in generation of Reynolds stress and zonal flows and eventually the suppression of background fluctuation levels via nonlinear energy transfer processes. The latter may therefore trigger a transition to the H mode.
    Physical Review Letters 08/2013; 111(5). DOI:10.1103/PhysRevLett.111.055006 · 7.73 Impact Factor
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    ABSTRACT: Pattern recognition is becoming an increasingly important tool for making inferences from the massive amounts of data produced in fusion experiments. The purpose is to contribute to physics studies and plasma control. In this work, we address the visualization of plasma confinement data and their dynamics, the identification of confinement regimes and the establishment of a scaling law for the energy confinement time. We take an intrinsically probabilistic approach, modeling data from the International Global H-mode Confinement Database with Gaussian distributions. We show that pattern recognition operations working in the associated probability space are considerably more powerful than their counterparts in a Euclidean data space. This opens up new possibilities for analyzing confinement data and for fusion data processing in general.
    08/2013; DOI:10.1063/1.4819987
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    ABSTRACT: Pattern recognition for fusion data greatly contributes to a better understanding of the measurements and the physics of fusion plasmas. Through a geometric description of probability it is shown that consideration of the inherent uncertain nature of the data significantly improves the visualization of global confinement data and the identification of confinement regimes. The framework can be extended to the development of scaling laws for ITER.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 08/2013; 720:11–13. DOI:10.1016/j.nima.2012.12.047 · 1.32 Impact Factor
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    ABSTRACT: Results of a new dedicated experiment addressing the problem of impurity deposition at the bottom in gaps are presented along with modelling. A test limiter with an isolated gap was exposed to the scrape-off layer plasma in TEXTOR. The exposure was accompanied by injection of 13C-marked methane in the vicinity of the gap. Deposition at the bottom of the gap was monitored in situ with Quartz-Microbalance diagnostics. The 13C deposition efficiency of about 2.6 × 10−5 was measured. Post mortem analysis of resulting deposited layers performed with SIMS and EPMA techniques yields about a factor 2 smaller value corresponding to approximately 10% contribution of the gap bottom to the total 13C deposition in the gap. This measured contribution is effectively much smaller than observed earlier in TEXTOR, taking the difference in geometry into account, and is in reasonable agreement with modelling performed with ERO and 3D-GAPS codes.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.166 · 2.02 Impact Factor
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    ABSTRACT: Material migration is a crucial issue in thermonuclear fusion devices. To study carbon migration, 13CH4 has been injected through a polished graphite roof-like test limiter in the TEXTOR scrape-off layer. The interpretation of the experimental 13C deposition patterns on the roof limiter surface has been done with the ERO impurity transport code. To reproduce the very low experimental 13C deposition efficiencies with ERO, an enhanced re-erosion mechanism for re-deposited carbon had to be assumed in previous studies. However, erosion by hydrogenic species produced during dissociation of injected 13CH4 was not taken into account by ERO in these studies. This additional erosion could maybe explain the very low experimental 13C deposition efficiencies. Therefore, it is now taken into account in ERO. Also more realistic physical sputtering yields and hydrocarbon reflection probabilities have been implemented in ERO. The simulations with these improvements included clearly confirm the need for enhanced re-erosion of re-deposited carbon.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.193 · 2.02 Impact Factor
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    ABSTRACT: ICRF heated L-mode pulses of JET with the new ITER like wall are simulated for the first time with the transport code COREDIV. The model, which couples self-consistently the core with the SOL as well as the main plasma with impurities, outputs, from a limited number of inputs as the heating power, the average density and the confinement time, the core temperatures and densities profiles, the effective ion charge Zeff, the power radiated, the W fluxes and concentration and the plasma parameters on the divertor. Due to the slab geometry of the SOL, the Be flux cannot be calculated self-consistently, but it is given as an input, according to spectroscopic measurements in the divertor. Comparison of simulations with experimental data both of the core and of the SOL is generally satisfactorily, with the exception of Zeff, possibly due to radiofrequency-specific effects, which are not accounted for in the model.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.118 · 2.02 Impact Factor
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    ABSTRACT: Gas puff imaging (GPI) [S. J. Zweben, D. P. Stotler et al., Phys. Plasmas 9, 1981 (2002); R. J. Maqueda, G. A. Wurden et al., Rev. Sci. Instrum. 74, 2020 (2003)] is a powerful diagnostic that permits a two-dimensional measurement of turbulence in the edge region of a fusion plasma and is based on the observation of the local emission of a neutral gas, actively puffed into the periphery of the plasma. The developed in-vessel GPI telescope observes the emission from the puffed gas along local (at the puff) magnetic field lines. The GPI telescope is specially designed to operate in severe TEXTOR conditions and can be treated as a prototype for the GPI systems on next generation machines. Also, the gas puff nozzle is designed to have a lower divergence of the gas flow than previous GPI diagnostics. The resulting images show poloidally and radially propagating structures, which are associated with plasma blobs. We demonstrate that the local gas puff does not disturb plasma properties. Our results indicate also that the neutral gas emission intensity is more sensitive to the electron density than the electron temperature. Here, we present implementation details of the GPI system on TEXTOR and discuss some design and diagnostic issues related to the development of GPI systems in general.
    The Review of scientific instruments 05/2013; 84(5):053501. DOI:10.1063/1.4803934 · 1.58 Impact Factor

Publication Stats

2k Citations
322.65 Total Impact Points


  • 2000–2015
    • Ghent University
      • Department of Applied Physics
      Gand, Flemish, Belgium
  • 2014
    • Pacific Northwest National Laboratory
      Richland, Washington, United States
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Jülich, North Rhine-Westphalia, Germany
  • 2007
    • Belgian Nuclear Research Centre
      Moll, Flanders, Belgium
  • 2002
    • Laboratory of Plasma Physics
      Paliseau, Île-de-France, France