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Teaca B.,Ecole Polytechnique Federale de Lausanne | Navarro A.B.,Laboratory for Plasma Physics Brusells | Jenko F.,Max Planck Institute for Plasma Physics (Garching) | Brunner S.,Ecole Polytechnique Federale de Lausanne | Villard L.,Ecole Polytechnique Federale de Lausanne
Physical Review Letters | Year: 2012

The nature of nonlinear interactions in gyrokinetic turbulence, driven by the ion-temperature gradient instability, is investigated using direct numerical simulations in toroidal flux tube geometry. To account for the level of separation existing between scales involved in an energetic interaction, the degree of locality of the free energy scale flux is analyzed employing Kraichnan's infrared (IR) and ultraviolet locality functions. Because of the nontrivial dissipative nature of gyrokinetic turbulence, an asymptotic level for the locality exponents, indicative of a universal dynamical regime for gyrokinetics, is not recovered and an accentuated nonlocal behavior of the IR interactions is found instead, in spite of the local energy cascade observed. © 2012 American Physical Society.

Verdoolaege G.,Ghent University | Verdoolaege G.,Laboratory for Plasma Physics Brusells
Entropy | Year: 2015

In regression analysis for deriving scaling laws that occur in various scientific disciplines, usually standard regression methods have been applied, of which ordinary least squares (OLS) is the most popular. In many situations, the assumptions underlying OLS are not fulfilled, and several other approaches have been proposed. However, most techniques address only part of the shortcomings of OLS. We here discuss a new and more general regression method, which we call geodesic least squares regression (GLS). The method is based on minimization of the Rao geodesic distance on a probabilistic manifold. For the case of a power law, we demonstrate the robustness of the method on synthetic data in the presence of significant uncertainty on both the data and the regression model. We then show good performance of the method in an application to a scaling law in magnetic confinement fusion. © 2015 by the authors; licensee MDPI, Basel, Switzerland.

Tokar M.Z.,EURATOM | Moradi S.,Laboratory for Plasma Physics Brusells
Nuclear Fusion | Year: 2011

The radial profiles of deuteron and triton densities in a fusion tokamak reactor are computed by taking into account anomalous transport due to ion temperature gradient and trapped electron drift instabilities in the plasma core and neoclassical transport in plateau-banana regimes, residual anomalous diffusion and convection triggered by edge localized modes in the edge transport barrier. The charged particle sources due to influx of neutral particles through the separatrix and ablation of frozen pellets in the plasma interior are optimized by searching for conditions providing the minimal outflow of unburned tritons to divertor target plates and their exhaust by pumping systems. It is demonstrated that such conditions correspond to fuelling of deuterium plasma component with gas puffing and of tritium with pellets and a ratio of deuteron/triton densities close to 2/1 at the separatrix. Effects of the absolute level and the ion mass dependence of neoclassical and anomalous transport contributions, of the position where neutrals enter the confined volume through the separatrix on the radial profiles of the ion densities are investigated. © 2011 IAEA, Vienna.

Haudin F.,Free University of Colombia | Riolfo L.A.,Free University of Colombia | Knaepen B.,Laboratory for Plasma Physics Brusells | Homsy G.M.,University of British Columbia | de Wit A.,Free University of Colombia
Physics of Fluids | Year: 2014

When a given fluid displaces another less viscous miscible one in a horizontal Hele- Shaw cell, the displacement is stable from the viscous point of view. Nevertheless, thin stripes perpendicular to the moving interface can be observed in the mixing zone between the fluids both in rectilinear and radial displacements. This instability is due to buoyancy effects within the gap of the cell which develop because of an unstable density stratification associated with the underlying concentration profile. To characterize this buoyancy-driven instability and the related striped pattern, we perform a parametric experimental study of viscously stable miscible displacements in a horizontal Hele-Shaw cell with radial injection. We analyze the influence of the flow rate, the thickness of the gap, and the relative physical fluid properties on the development and characteristics of the instability. © 2014 AIP Publishing LLC.

Ongena J.,Laboratory for Plasma Physics Brusells | Ogawa Y.,University of Tokyo
Energy Policy | Year: 2016

The paper gives an overview of fusion research in the world. The prospects for fusion as an energy source for the future are reviewed. Environmental compatibility, safety and resources are discussed. © 2016 Elsevier Ltd.

Kazakov Y.O.,Laboratory for Plasma Physics Brusells | Fulop T.,Chalmers University of Technology
Physical Review Letters | Year: 2014

A Reply to the Comment by Kim and Johnson. © 2014 American Physical Society.

Koch R.,Laboratory for Plasma Physics Brusells
Fusion Science and Technology | Year: 2010

This lecture complements the three previous lectures1,2,3 on waves by addressing, on the basis of elementary and intuitive treatment, the process of coupling of electromagnetic power to plasma. Coupling is here meant in a broad sense. It consists of four different steps, (i) The first one is the coupling of vacuum electromagnetic power to plasma waves. An elementary antenna coupling theory is given. The state of the art in coupling models and status of comparisons with experiments are briefly discussed. (ii) The second is the transfer of plasma wave energy to particle energy. The resonant processes leading to this transfer are described in a heuristic way. (iii) The third one is the build-up of fast particle populations. It will be outlined through a sketch of quasilinear diffusion for the simple case of Landau damping, (iv) The last step is the conversion of power through the resonant particle population to bulk plasma heating by collisions, which will be briefly addressed.

Koch R.,Laboratory for Plasma Physics Brusells
Fusion Science and Technology | Year: 2010

The heating of plasmas by fast ions, with a focus on Neutral Beam Injection (NBI), is reviewed. First, the need of auxiliary heating and current drive systems in fusion machines is outlined. For the particular case of tokamaks, the limitations of ohmic heating are discussed. The different ways of generating fast particles in plasmas are presented. The principle of operation of neutral beam injectors is explained. Positive-ion (PNBI) and negativeion (NNBI) based concepts are discussed. Next, the physical processes by which the beam transfers energy to the plasma, namely ionisation and slowing-down are described. For both, an elementary theory is given, whereby simple approximations to the distribution functions of beam injected ions and of alpha particles in reactors are obtained. Applications of NBI to heating, current drive and rotation drive are reviewed and the prospects of NBI for ITER are commented.

Vantieghem S.,Laboratory for Plasma Physics Brusells | Knaepen B.,Laboratory for Plasma Physics Brusells
International Journal of Heat and Fluid Flow | Year: 2011

We present numerical simulation results of the quasi-static magnetohydrodynamic (MHD) flow in a toroidal duct of square cross-section with insulating Hartmann walls and conducting side walls. Both laminar and turbulent flows are considered. In the case of steady flows, we present a comprehensive analysis of the secondary flow. It consists of two counter-rotating vortex cells, with additional side wall vortices emerging at sufficiently high Hartmann number. Our results agree well with existing asymptotic analysis. In the turbulent regime, we make a comparison between hydrodynamic and MHD flows. We find that the curvature induces an asymmetry between the inner and outer side of the duct, with higher turbulence intensities occurring at the outer side wall. The magnetic field is seen to stabilize the flow so that only the outer side layer remains unstable. These features are illustrated both by a study of statistically averaged quantities and by a visualization of (instantaneous) coherent vortices. © 2011 Elsevier Inc.

Louche F.,Laboratory for Plasma Physics Brusells | Dumortier P.,Laboratory for Plasma Physics Brusells | Messiaen A.,Laboratory for Plasma Physics Brusells | Durodie F.,Laboratory for Plasma Physics Brusells
Nuclear Fusion | Year: 2011

In the framework of the ion cyclotron resonance heating (ICRH) antenna development for ITER, a design based on an external matching concept has been proposed [1]. We present in this work a series of electromagnetic simulations of this design performed with the commercial code CST Microwave Studio [2]. On the one hand, we explore how various geometrical modifications of some parts of the antenna (the straps and the four-port junction) can practically double the RF power coupled to the plasma. This optimization is supported by transmission line analysis. On the other hand, we treat the important question of the opportunity to tilt the straps in the toroidal direction to follow the plasma curvature as close as possible. We show that a configuration with two toroidal segments is sufficient and that further segmentation is not necessary. This work also underlines significant progress in the realism of ICRH antenna modelling and the importance of considering realistic load shaping in the models. © 2011 IAEA, Vienna.

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