G. Van Oost

Ghent University, Gand, Flemish, Belgium

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Publications (294)338.77 Total impact

  • Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 06/2015; 352:96-99. DOI:10.1016/j.nimb.2014.11.103 · 1.19 Impact Factor
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    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.
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    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.
    Journal of Nuclear Materials 04/2015; 464(2015):69-72. DOI:10.1016/j.jnucmat.2015.04.028 · 2.02 Impact Factor
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    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.
    Journal of Applied Physics 02/2015; 117(8):083302. DOI:10.1063/1.4913478 · 2.19 Impact Factor
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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; 50. 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: 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.
    IEEE Transactions on Antennas and Propagation 12/2014; 62(12):6307-6316. DOI:10.1109/TAP.2014.2361902 · 2.46 Impact Factor
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    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.
    Journal of Nuclear Materials 12/2014; DOI:10.1016/j.jnucmat.2014.12.006 · 2.02 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 · 1.30 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 · 1.30 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 · 1.30 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 radio-frequency (RF) plasma production technique in the ion cyclotron range of frequency (ICRF) attracts growing attention among fusion experts because of its high potential for solving several basic problems of reactor-oriented superconducting fusion machines, such as ICRF wall conditioning in tokamaks and stellarators (Te =3-5eV, ne <1012 cm􀀀3), ICRF-assisted tokamak start-up and target plasma production (ne = 1013 cm􀀀3) in stellarators. Plasma initiation by ICRF has been studied intensively using single particle descriptions and basic analytic models. To further improve the present understanding on plasma production employing the vacuum RF field of ICRF antennas in toroidal devices in presence of the toroidal magnetic field, and its parametric dependencies a Monte Carlo code has been developed. The 1D code RFdinity1D describes the motion of electrons, accelerated by the RF field in front of the ICRF antenna, along one toroidal magnetic field line. Dependent on their individual energies and the related electron collision cross sections (ionisation, excitation and dissociation) weighted by a Monte Carlo procedure, an electron avalanche may occur. Breakdown conditions are discussed as function of RF discharge parameters (i) RF vacuum electric field strength, (ii) RF frequency and (iii) neutral pressure (H2). The slope of the exponential density increase, taken as measure for the breakdown speed, shows qualitative agreement to experimental breakdown times as found in literature and experimental data of the ASDEX upgrade and TEXTOR tokamak, and is interpreted by studying the characteristic electron velocity distribution functions.
    20th Topical Conference on Radio Frequency Power in Plasmas, Sorrento 2013; 01/2014
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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

Publication Stats

2k Citations
338.77 Total Impact Points

Institutions

  • 2000–2015
    • Ghent University
      • Department of Applied Physics
      Gand, Flemish, Belgium
    • University of California, San Diego
      • Department of Mechanical and Aerospace Engineering (MAE)
      San Diego, California, United States
  • 2007
    • Belgian Nuclear Research Centre
      Moll, Flanders, Belgium
  • 2002
    • Laboratory of Plasma Physics
      Paliseau, Île-de-France, France
  • 1993
    • Princeton University
      Princeton, New Jersey, United States
  • 1990
    • University of California, Los Angeles
      • Department of Mechanical and Aerospace Engineering
      Los Ángeles, California, United States