G. Van Oost

Pacific Northwest National Laboratory, Richland, Washington, United States

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Publications (243)281.83 Total impact

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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. · 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. · 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. · 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. · 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. · 2.02 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). · 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;
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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. · 1.14 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; · 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. · 1.52 Impact Factor
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    ABSTRACT: A method for the destruction of solid and liquid organic toxic waste in a three-jet plasma reactor with typical reaction temperatures 3000–5000 K has been developed. The high temperature of the process allows the destruction of the complex molecules of toxic substances. As a result simple substances and acids are produced. Acids are neutralized in the alkaline environment of a wet filter.
    Vacuum. 02/2013; 88:165–168.
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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 01/2013; · 2.02 Impact Factor
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    ABSTRACT: Samples of tungsten and tungsten–tantalum alloy (with 5 mass per cent of Ta) were exposed to high-flux deuterium plasma at different fluences. The surface modification was studied with scanning electron microscopy, and deuterium retention was measured by thermal desorption spectroscopy (TDS). In the high fluence range of ∼3.5 × 10 26 –10 27 m -2 , multiple large-size blisters are formed on the W surface, while blisters on the W–Ta surface are considerably smaller in size and number. Deuterium retention in this fluence range was found to be systematically higher in W than in W–Ta. Correlation between the evolution of the blistering patterns and the TDS spectra as a function of fluence suggests that trapping in the sub-surface cavities associated with blisters is the predominant trapping mechanism in tungsten in the case of high fluence exposures. We attribute the lower retention in W–Ta under the investigated conditions to the weaker blistering.
    Nuclear Fusion 01/2013; 53(1):013013. · 2.73 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 01/2013; · 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 01/2013; · 0.84 Impact Factor
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    Geert Verdoolaege, Guido Van Oost
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    ABSTRACT: Pattern recognition is becoming an important tool in fusion data analysis. However, fusion diagnostic measurements are often affected by considerable statistical uncertainties, rendering the extraction of useful patterns a significant challenge. Therefore, we assume a probabilistic model for the data and perform pattern recognition in the space of probability distributions. We show the considerable advantage of our method for identifying confinement regimes and edge localized mode behavior, and we discuss the potential for scaling laws.
    The Review of scientific instruments 10/2012; 83(10):10D715. · 1.52 Impact Factor
  • G. van Oost, M. Tendler, J. Stöckel
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    ABSTRACT: Experimental and theoretical aspects of the role of electric fields in plasma confinement and exhaust are overviewed. The history of the discovery of the importance of electric fields in plasmas is depicted. The correlation between radial electric fields and plasma rotation is outlined. The concept ofE ×B flow velocity shear suppression is explained and modelled. Edge polarization results on the CASTOR tokamak are presented. Recent results in the formation of internal transport barriers are summarized.
    Czechoslovak Journal of Physics 04/2012; 50:11-24. · 0.42 Impact Factor
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    ABSTRACT: Predictions of observable properties by density-functional theory calculations (DFT) are used increasingly often in experimental condensed-matter physics and materials engineering as data. These predictions are used to analyze recent measurements, or to plan future experiments. Increasingly more experimental scientists in these fields therefore face the natural question: what is the expected error for such an ab initio prediction? Information and experience about this question is scattered over two decades of literature. The present review aims to summarize and quantify this implicit knowledge. This leads to a practical protocol that allows any scientist - experimental or theoretical - to determine justifiable error estimates for many basic property predictions, without having to perform additional DFT calculations. A central role is played by a large and diverse test set of crystalline solids, containing all ground-state elemental crystals (except most lanthanides). For several properties of each crystal, the difference between DFT results and experimental values is assessed. We discuss trends in these deviations and review explanations suggested in the literature. A prerequisite for such an error analysis is that different implementations of the same first-principles formalism provide the same predictions. Therefore, the reproducibility of predictions across several mainstream methods and codes is discussed too. A quality factor Delta expresses the spread in predictions from two distinct DFT implementations by a single number. To compare the PAW method to the highly accurate APW+lo approach, a code assessment of VASP and GPAW with respect to WIEN2k yields Delta values of 1.9 and 3.3 meV/atom, respectively. These differences are an order of magnitude smaller than the typical difference with experiment, and therefore predictions by APW+lo and PAW are for practical purposes identical.
    Critical Reviews in Solid State and Material Sciences 04/2012; · 5.95 Impact Factor
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    Geert Verdoolaege, Guido Van Oost
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    ABSTRACT: We present an integrated framework for (real-time) pattern recognition in fusion data. The main premise is the inherent probabilistic nature of measurements of plasma quantities. We propose the geodesic distance on probabilistic manifolds as a similarity measure between data points. Substructure induced by data dependencies may further reduce the dimensionality and redun-dancy of the data set. We present an application to confinement mode classification, showing the distinct advantage obtained by considering the measurement uncertainty and its geometry.
    Fusion Engineering and Design 01/2012; 87(12):2068. · 0.84 Impact Factor

Publication Stats

1k Citations
281.83 Total Impact Points

Institutions

  • 2014
    • Pacific Northwest National Laboratory
      Richland, Washington, United States
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Jülich, North Rhine-Westphalia, Germany
  • 2000–2014
    • Ghent University
      • • Department of Applied Physics
      • • Department of Data Analysis
      Gand, Flanders, Belgium
  • 2007
    • Belgian Nuclear Research Centre
      Moll, Flanders, Belgium
  • 2004–2006
    • Heinrich-Heine-Universität Düsseldorf
      Düsseldorf, North Rhine-Westphalia, Germany
  • 2002
    • Laboratory of Plasma Physics
      Paliseau, Île-de-France, France
  • 1999
    • University of California, San Diego
      • Department of Mechanical and Aerospace Engineering (MAE)
      San Diego, California, United States