Teses / dissertações sobre o tema "Plasmas (gaz ionisés) – Turbulence – Mesure"
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Vermare, Laure. "Mesures de l'activité magnétohydrodynamique et de la micro-turbulence par réflectométrie à balayage". Aix-Marseille 1, 2005. http://www.theses.fr/2005AIX11014.
Texto completo da fonteMoreau, Philippe. "Développement d'un réflectomètre micro-onde hétérodyne à balayage ultra rapide : étude de l'influence de la turbulence du plasma sur la mesure des profils de densité électronique". Aix-Marseille 1, 1997. http://www.theses.fr/1997AIX11019.
Texto completo da fonteScipioni, Angel. "Contribution à la théorie des ondelettes : application à la turbulence des plasmas de bord de Tokamak et à la mesure dimensionnelle de cibles". Electronic Thesis or Diss., Nancy 1, 2010. http://www.theses.fr/2010NAN10125.
Texto completo da fonteThe necessary scale-based representation of the world leads us to explain why the wavelet theory is the best suited formalism. Its performances are compared to other tools: R/S analysis and empirical modal decomposition method (EMD). The great diversity of analyzing bases of wavelet theory leads us to propose a morphological approach of the analysis. The study is organized into three parts. The first chapter is dedicated to the constituent elements of wavelet theory. Then we will show the surprising link existing between recurrence concept and scale analysis (Daubechies polynomials) by using Pascal's triangle. A general analytical expression of Daubechies' filter coefficients is then proposed from the polynomial roots. The second chapter is the first application domain. It involves edge plasmas of tokamak fusion reactors. We will describe how, for the first time on experimental signals, the Hurst coefficient has been measured by a wavelet-based estimator. We will detail from fbm-like processes (fractional Brownian motion), how we have established an original model perfectly reproducing fBm and fGn joint statistics that characterizes magnetized plasmas. Finally, we will point out the reasons that show the lack of link between high values of the Hurst coefficient and possible long correlations. The third chapter is dedicated to the second application domain which is relative to the backscattered echo analysis of an immersed target insonified by an ultrasonic plane wave. We will explain how a morphological approach associated to a scale analysis can extract the diameter information
Scipioni, Angel. "Contribution à la théorie des ondelettes : application à la turbulence des plasmas de bord de Tokamak et à la mesure dimensionnelle de cibles". Thesis, Nancy 1, 2010. http://www.theses.fr/2010NAN10125.
Texto completo da fonteThe necessary scale-based representation of the world leads us to explain why the wavelet theory is the best suited formalism. Its performances are compared to other tools: R/S analysis and empirical modal decomposition method (EMD). The great diversity of analyzing bases of wavelet theory leads us to propose a morphological approach of the analysis. The study is organized into three parts. The first chapter is dedicated to the constituent elements of wavelet theory. Then we will show the surprising link existing between recurrence concept and scale analysis (Daubechies polynomials) by using Pascal's triangle. A general analytical expression of Daubechies' filter coefficients is then proposed from the polynomial roots. The second chapter is the first application domain. It involves edge plasmas of tokamak fusion reactors. We will describe how, for the first time on experimental signals, the Hurst coefficient has been measured by a wavelet-based estimator. We will detail from fbm-like processes (fractional Brownian motion), how we have established an original model perfectly reproducing fBm and fGn joint statistics that characterizes magnetized plasmas. Finally, we will point out the reasons that show the lack of link between high values of the Hurst coefficient and possible long correlations. The third chapter is dedicated to the second application domain which is relative to the backscattered echo analysis of an immersed target insonified by an ultrasonic plane wave. We will explain how a morphological approach associated to a scale analysis can extract the diameter information
De, Dominici Gregory. "Understand and predict the power threshold leading to reduced turbulent transport at the edge of tokamak plasma". Electronic Thesis or Diss., Aix-Marseille, 2019. http://www.theses.fr/2019AIXM0320.
Texto completo da fonteA model based on a model which natively contained turbulence and turbulence driven flow. It has been improved to include the diamagnetic effects, the magnetic fluctuations, and in this work, we study the parametric dependencies of the observed L-H transition power threshold with respect to the ion mass. By including the diamagnetic effects in our model, we allow the competition between the drift waves and the interchange instabilities. This competition is here studied using fixed gradient simulation. We show in this work that the diamagnetic effects are stabilizing for a resistivity close to experimental conditions. Electromagnetic effects lead to more unstable modes at realistic resistivities. Moreover, a quasilinear estimation of the turbulent flux is able to qualitatively grasp the competition between the drift waves and the interchange and the behaviour of the nonlinear electrostatic turbulent flux with resistivity and plasma beta. Another parametric dependency of the turbulence is studied, by changing the mass of the isotope. This is known as the isotope effect. We show here that the turbulence is reduced when the ion mass is increased. Finally, the characteristic times of the turbulence are studied.Magnetic fluctuations have a dramatic effect on correlation times of the turbulence, by drastically reducing them. Accounting for these results, we present in this work the auto-generation of a transport barrier with electromagnetic simulations of edge turbulence, when the heat power is higher than a threshold, using flux-driven simulations. We have then changed the isotope, and correspondingly to experiments, the power threshold is lower for higher isotope mass
Fleurence, Emmanuel. "Descriptions fluide et cinétique d'une turbulence d'interchange dans un plasma magnétisé". Nancy 1, 2005. http://docnum.univ-lorraine.fr/public/SCD_T_2005_0141_FLEURENCE.pdf.
Texto completo da fonteThe kinetic and fluid descriptions of the interchange instability are compared in the linear and non-linear regimes. The prediction of the turbulent transport in tokamaks is the general framework. The kinetic model is two dimensional in space, and one dimensional in energy. The fluid analogue retains the dynamics of both density and pressure. The closure, which acts on the heat flux, is collisional. The fluid system is shown to be equivalent to two Vlasov-type equations for two distinct monokinetic distribution functions. This allows one to use the same numerical tool to run non-linear simulations in kinetic and fluid. For similar linear characteristics, the level of heat turbulent transport is larger in fluid than in kinetics, by orders of magnitudes. Zonal flows do not explain the whole discrepancy. As a matter of fact, the distribution function departs significantly from a maxwellian, so that the adopted closure is failing. An alternative collisionless closure is proposed. It aims at fitting the quasi-linear entropy production rates. In this case, the kinetic and fluid linear properties can be made similar
Futatani, Shimpei. "Etude du transport turbulent des impuretés dans les plasmas de tokamak". Aix-Marseille 1, 2009. http://www.theses.fr/2009AIX11056.
Texto completo da fonteThe aim of this thesis is to understand impurity transport dynamics in the magnetically confined fusion plasmas. This includes the influence of the turbulent field which induces an anomalous transport in the plasma. Furthermore, this work demonstrates remarkable aspects for impurity transport in two different instabilities: ``ion temperature gradient (ITG) and trapped electron mode (TEM)''. It is shown that in presence of an internal transport barrier (ITB) created by a reversed magnetic shear configuration, one can obtain the reversal of impurity pinch velocity which can change from inward direction to outward direction. This scenario is favourable for expelling impurities from the central region and decontaminating the core plasma. The mechanism of pinch reversal is attributed to a change of direction of the curvature pinch and to a modification of the dominant underlying instability caused by a change of the gradient of the ion temperature which is a consequence of the ITB formation
Zekri, Stéphane. "Approche hamiltonienne de la turbulence faible de Langmuir". Aix-Marseille 1, 1993. http://www.theses.fr/1993AIX11022.
Texto completo da fonteBrun, Cédric. "Propriétés statistiques des modes de Fourier en turbulence dévéloppée". Nice, 2001. http://www.theses.fr/2001NICE5597.
Texto completo da fonteThis thesis deals with the intermittency in fully developed turbulence. In a first part, we study the statistical properties of Fourier modes in a turbulent flow. These quantities are often evoked to describe turbulence. Recent experimental studies demonstrated that the Fourier modes of vorticity and temperature are only weakly intermittent in the inertial range of scales. We confirm these results by measurements done in direct numerical simulations. Our work proposes an explanation of the observed effects and some more general remarks. The second part of this work is devoted to the intermittency of a passive scalar. We use an approach based on the evolution equations through scales of the density probability function of the passive scalar increments. These equations are derived from first principles, under different hypothesis of isotropy ; the closure problem is reduced to the determination of some conditional averages. We carry out a study by means of direct numerical simulations in order to provide the data needed to validate this approach. This work demonstrates the influence of the large scales in the inertial statistical properties of the mixing
Tretinnikov, Pavel. "New developments on the interpretative model of ITER diagnostics using microwaves". Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0027.
Texto completo da fonteThe turbulence in thermonuclear fusion plasma can lead to increase of the plasma transport coefficient that enhances the particles and energy losses therefore deteriorates the plasma confinement. That is why measurements of the plasma turbulence is important for description of the plasma transport processes. Microwave plasma diagnostics is an efficient kind of the active plasma diagnostics to characterize the turbulence. Their principles are based on the collection of information integrated over the probed plasma volume, and printed on the reflected or scattered waves reaching the detector, for which interpretative models are required to extract the turbulence properties. The radial correlation reflectometry (RCR) and the Doppler reflectometry are widely used technics that provide information on plasma turbulence characteristics. The Collective Thomson Scattering (CTS) is a valuable microwave diagnostic that can give the information about such plasma parameters as the bulk ion temperature, bulk ion composition, the fast particles (alpha particles or ions of a heating beam) velocity distribution. The knowledge of the alpha particle velocity distribution is especially interesting and important to evaluate the ITER performance where the energy amplification of plasma heating power Q is expected to achieve the value about 10. The actual data analysis of the microwave diagnostics, or interpretations of a measured signal is fulfilled assuming a smooth Gaussian probing beam. It was demonstrated that the microwave beams can be significantly distorted due to the edge plasma turbulence, that may lead to misleading interpretations of the diagnostic signal, based on the microwaves, so the diagnostics interpretative models should be analyzed taken into account to the probing beam distortion. The improvement of the microwave diagnostics interpretative models is the main object of the study presented in this thesis.The aim of this PhD work is to include these missing effects in an interpretative model for CTS using X-mode probing beam
Marino, Raffaele. "Scaling laws in solar wind turbulence". Nice, 2009. http://www.theses.fr/2009NICE4104.
Texto completo da fonteIn my PhD thesis I re-derive the Yaglom law for the MHD, a proportionality relation between the mixed third-order moment of the longitudinal increments of the Elsässer fields and the increment scale. Using Ulysses spacecraft measurements, I evidenced for the first time the existence of this relation in the polar solar wind, proving the presence of a local energy cascade and the turbulent character of the field fluctuations in Alfvénic plasmas. The observation of the Yaglom law for MHD also permits the first direct estimation of pseudo-energy dissipation rates in solar wind turbulence. In the thesis I also explored the possibility that the dissipation of energy occurring at the end of a turbulent MHD cascade can be responsible for the solar wind heating. I found that incompressible turbulent cascade can contribute to the solar wind in situ heating from 8 % to 50 % on average, and up to 100 % in some cases. Analysis of the ecliptic wind measured by Ulysses show that while the fast ecliptic streams have similar properties as the polar fast wind, the slow streams show an highly enhanced energy transport. I also study the role of large scale solar wind density fluctuations in MHD turbulence and showed that a phenomenological compressible Yaglom-like relation is verified within the solar wind, indicating that compressible effects are an important ingredient of the turbulent cascade and permit the transfer of a considerably larger amount of energy toward the small scales, where it can be dissipated to heat the plasma and slow down the radial cooling of the wind as observed in interplanetary space
Garbet, Xavier. "Turbulence et transport anormal dans les plasmas de Tokamak". Grenoble 2 : ANRT, 1988. http://catalogue.bnf.fr/ark:/12148/cb37613789t.
Texto completo da fonteDubuit, Nicolas. "Transport turbulent d'impuretés dans un plasma magnétisé". Nancy 1, 2006. http://docnum.univ-lorraine.fr/public/SCD_T_2006_0253_DUBUIT.pdf.
Texto completo da fonteThe turbulent impurity transport is an essential issue for the achievement of magnetically confined fusion. Indeed, a build-up of impurities in the plasma core induces radiation energy losses. On the other hand, it is foreseen to inject impurities at the plasma edge in order to extract heat by radiation. These conditions impose control and therefore understanding of impurity transport. An analytical expression for the turbulent flux of impurities for a given turbulent spectrum have been derived using a quasilinear theory applied to a drift-wave turbulence. The nature and characteristics of the main accumulation (or pinch) mechanisms for impurities have been identified. A new version of the fluid turbulence code TRB has been developed. This version allows simulating the transport of impurities in electronic and ionic drift-wave turbulence. Zero-flux simulations, corresponding to a stationnary plasma whose impurity source is limited to the plasma edge, confirmed the presence of an impurity pinch. Quasilinear predictions have been numerically confirmed, and the relative importance of the various pinch mechanisms has been studied. The dominant role of curvature pinch has been shown. Finally, simulations have been compared to experimental measurements made on JET and Tore Supra tokamaks. The presence of a turbulent pinch of impurities has been confirmed, as well as the independence of transport on impurity charge
Guiziou, Laurent. "Etude du transport de l'énergie sur le tokamak TORE SUPRA". Aix-Marseille 1, 1995. http://www.theses.fr/1995AIX11053.
Texto completo da fonteSandoz, Caroline. "Structures turbulentes de petites échelles dans le problème à N corps : application aux plamas unidimensionnels et aux rotateurs couplés". Aix-Marseille 1, 1995. http://www.theses.fr/1995AIX11078.
Texto completo da fonteDif-Pradalier, Guilhem. "First-principle description of collisional gyrokinetic turbulence in tokamak plasmas". Aix-Marseille 1, 2008. http://www.theses.fr/2008AIX11048.
Texto completo da fonteAs a hot rarefied medium, a tokamak plasma is crucially affected by wave–particle resonances, trapping phenomena, orbit effects and is intrinsically low collisional. As such, any fully self-consisent description of its response is therefore kinetic, and includes collisions. Though they lead to usually smaller transport than the one generated by the turbulence, one may not conclude on collisions being negligible. The discovery of high-confinement regimes –which seem universal and reproducible features of fusion devices and as such are the foreseen operating scenari for iter– has led to a renewed interest in collisional theories. These regimes are indeed characterised by a local strong reduction of the turbulent activity; in that case, the transport processes may get very close to being collisional. Binary interactions are also important features for a kinetic theory since they alone may provide a correct description of equilibrium flows. Such large-scale sheared flows are of special relevance for the saturation mechanisms of the turbulence, potentially leading to high-confinement regimes. At last, there seems to exist a non trivial interaction between collisions and turbulence in tokamak-relevant operating regimes. Through these processes, collisions might indirectly have a strong influence on the actual level of transport; elucidating this interplay is a subject of great current focus for first-principle modeling
Girardo, Jean-Baptiste. "Control of instabilities and turbulence by fast particles in fusion plasmas". Palaiseau, Ecole polytechnique, 2015. http://www.theses.fr/2015EPXX0121.
Texto completo da fonteSalazar, Luigui. "Data-driven discovery approach to tackle turbulence in fusion plasmas". Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0327.
Texto completo da fonteOne way to achieve fusion on Earth is through toroidal magnetic confinement, in which the main devices are tokamaks and stellarators. One of the main limiting phenomena is turbulence. This is the result of a mixture of instabilities on different temporal and spatial scales. The aim of this thesis was to contribute to the understanding of turbulent transport, which is the interaction of this turbulence with the plasma properties, in magnetically confined plasmas, focusing on the characterisation of density fluctuations and their dynamics using reflectometry. From the signal spectrum provided by reflectometry, each spectral component was extracted using an algorithm developed in this thesis. It performs this spectral decomposition(Low frequency component, Broadband component, QC modes and noise) in an intelligent way (without ad hoc constraints), preserving its statistical and physical information. Particular attention has been paid to a spectral component, the so-called QC modes because of its link with TEM, an instability acting in turbulent transport. Thanks to the algorithm, the properties and dynamics of the QC modes have been studied, allowing a statistical study and a detailed analysis of the interaction with other spectral components in different magnetic configurations: ToreSupra(limiter), WEST(divertor) and W7X(island divertor). This work is qualified as exploratory because there were no methods to access these QC mode properties and dynamicsThe statistical studies carried out with the ToreSupra and WEST database provided a probability map of the occurrence of the QC modes in the ohmic confinement regime, confirming their link with the TEM,but also 2 more classes, one at low current and another at high current were found. The broadband component was also analysed as it is believed to be related to the microturbulence. Since the spectral amplitude of the BB component is found with the AGG, mainly μ and β, which correspond to the assymetry and shape of the spectrum, show a variation during the transition. On the other hand, the analysis of the dynamics of the QC modes by means of an algorithm developed in this thesis showed a recurrent interaction between the QC modes and the modes in the LF component. Finally, in order to better discriminate the QC modes, the concept of transfer entropy is used to analyse the causality in this interaction. Some cases shows the interaction between the LF component corresponding to the sawtooth instability and the QC modes. All this analysis of dynamics and also causality is then applied to ToreSupra, WEST and W7X, where there is a similar dynamic for signals with bi-directional causality, i.e. not only from the LF component to the QC modes, but also from the QC modes to the BB component. In addition, the ECRH is also examined for the TS database, which shows a decrease in the probability of QC modes as the PECRH increases. Since these QC modes appear in different magnetic configurations, the ultimate objective is to discover if all these QC modes are produced by the same mechanism, since it is believed that there must be a universal mechanism underlying the physics in fusion plasmas, this is still under investigation
Mazzi, Samuele. "Impact of fast ions on microturbulence in fusion plasmas". Electronic Thesis or Diss., Aix-Marseille, 2021. http://www.theses.fr/2021AIXM0470.
Texto completo da fonteThe exploitation of magnetically confined fusion plasmas as a sustainable and clean energy source is limited by the radially outward turbulent transport. Such transport is mainly induced by microinstabilities. Next-generation fusion devices will be mainly heated by the alpha particles born from the nuclear fusion reactions. Alpha particles must be well confined in order to transfer their energy to the bulk ions. However, very little knowledge is available regarding the interaction between alpha particles and microturbulence. Thus, unexpected turbulence and transport regimes may lead to further detrimental effects on the performance of future alpha-heated devices. The study of a tokamak scenario which can mimic the experimental conditions expected in future devices is hence crucial. Numerical investigations on the impact of highly energetic fast ions, mimicking the alpha particle dynamics, on the turbulent transport driven by Ion Temperature Gradient and Trapped Electron Mode instabilities have been carried out in a validated framework. It is shown that a suppression of the ion-scale turbulent transport may be achieved in the presence of such highly energetic ions. Alfvén Eigenmodes (AEs), destabilized by the highly energetic ions through a wave-particle interaction, play an essential role in the multi-scale mechanism leading to the turbulence suppression. Deep analyses further highlight the possibility to recognize hallmarks of the ion-scale transport reduction, regardless the dominant turbulent regime
Chôné, Laurent. "Turbulence, flows and transport barriers at the tokamak plasma edge". Electronic Thesis or Diss., Aix-Marseille, 2016. http://www.theses.fr/2016AIXM4701.
Texto completo da fonteThe topic of this thesis is the interaction between turbulence and flows at the tokamak edge, and their influence on the confinement. Turbulence is the main contribution to the outward transport in magnetic fusion devices, and a strong limiting factor for their performance. It can be stabilised by flows, through shear-mediated decorrelation of convective cells, and through non-linear coupling. Strong shear flows causing a localised reduction of transport (transport barrier) are often observed in experiments, and several regimes of improved confinement such as the High-confinement mode are accessed routinely. There is a growing body of evidence from experiments showing that the mean flow responsible for the barrier is governed by force balance, while non-linear interplay between turbulence, turbulence-driven zonal-flows, and the mean flow occurs during the transition phase. In this thesis, we extend a fluid model for plasma edge turbulence to include collisional relaxation of flows towards force. We show that accounting for a contribution of neoclassical allows for the spontaneous formation of a transport barrier to occur in flux-driven simulation. Dynamical features reminiscent of the L-H transition and H-mode are recovered, such as relaxation-oscillations of the barrier and dithering of the radial electric field during the barrier formation. An analysis is carried out to identify the roles of zonal-flows and force balance during the transition, and it is found that in our simulations that zonal flows provide temporary quenching of the turbulence via non-linear coupling, allowing for the mean flow to grow and form the barrier
Gabbaï, Philippe. "Instabilités, auto-organisation et transport dans la turbulence d'ondes de dérive dissipatives". Aix-Marseille 1, 1997. http://www.theses.fr/1997AIX11039.
Texto completo da fonteGuyomarc'h, Didier. "Un tube à onde progressive pour l'étude de la turbulence plasma". Aix-Marseille 1, 1996. http://www.theses.fr/1996AIX11040.
Texto completo da fonteSahraoui, Fouad. "Etude des fluctuations magnétiques dans la magnétogaîne terrestre : vers une interprétation dans le cadre de la théorie de la turbulence faible de la MHD-Hall". Versailles-St Quentin en Yvelines, 2003. http://www.theses.fr/2003VERS0014.
Texto completo da fonteThe thesis is dedicated to study experimentally and theoretically the magnetic turbulence in the terrestrial magnetosheath. Due to the absence of collisions, this turbulence plays an important role in the transfers between the solar wind and the magnetosphere. The experimental study is done using the Cluster multipoint data and the k-filtering technique. It is shown that the observed turbulent spectrum is supported by linear low frequency modes. These modes are not described by the ideal MHD theory. Therefore, a weak turbulence theory of the Hall-MHD system is justified for interpreting the observations. This model is developed in the framework of the Hamiltonian formalism. The most important step has been accomplished: the canonical description of the Hall-MHD system in terms of the generalized Clebsch variables. This new formalism will allow to solve the problem of diagonalizing the Hall-MHD non linear equations, and deriving the corresponding kinetic equations of waves
Bultel, Arnaud. "Analyse d'un plasma d'azote basse pression en situations de jet libre et de couche limite". Rouen, 1994. http://www.theses.fr/1994ROUE5007.
Texto completo da fonteLeconte, Michaël. "Interaction entre écoulements cisailles et transport turbulent dans les plasmas de fusion magnétique". Aix-Marseille 1, 2008. http://www.theses.fr/2008AIX11047.
Texto completo da fonteGerbaud, Thomas. "Étude de la microturbulence par réflectométrie dans un plasma de fusion sur le tokamak Tore-Supra". Thesis, Nancy 1, 2008. http://www.theses.fr/2008NAN10086/document.
Texto completo da fonteFast-sweeping reflectometry in extraordinary mode allows direct measurement of radial wavenumber local spectra S(_n/n](kr, r), and radial profiles of density fluctuations, on Tore Supra tokamak. Wavelet-based approach – a mathematical tool for position-frequency analysis – made possible to consider the strong radial variation of the measured turbulence. Special consideration was given to the validation of spectra and turbulent profiles measurements, by comparing with experimental measurements (reflectometries, probes) and numerical non-linear gyrokinetic simulations. This density fluctuations measurement method has been used to analyse the local transport, by performing a dimensionless scaling on collisionnality, __. The scaling experiments allow direct comparisons of plasmas from different tokamaks. A clear decrease of the normalized confinement time of the plasma energy with the normalized collisionnality was observed : B_E _ __-0.5±0.15. These new measurements of density fluctuations profiles have shown an intense rise of the edge turbulence (r/a > 0.8) when increasing – also observed by Doppler reflectometry diagnostic – providing a physical explanation of the loss of confinement with the normalized collisionnality. More central regions did not present apparent variations (_n/n, _eff). Core plasma simulations (linear stability code KineZero and non-linear gyrokinetic GYRO) were performed, in order to analyse the experimental behaviour of the plasma
Figarella, Charles F. "Etude des barrières de transport dans les plasmas de fusion magnétisés". Aix-Marseille 1, 2002. http://www.theses.fr/2002AIX11021.
Texto completo da fonteStrugarek, Antoine. "Turbulence, transport et confinement : des tokamaks à la tachocline solaire". Paris 7, 2012. http://www.theses.fr/2012PA077204.
Texto completo da fonteThis thesis is part of the general study of self-organization in hot and magnetized plasmas. We focus our work on two specific objects : stars and tokamaks. We use first principle numerical simulations to study turbulence, transport and confinement in these plasmas. The first part of this thesis introduces the main characteristics of stellar and tokamak plasmas. The reasons for studying them together are properly detailed. The second part is focused on stellar aspects. We study the interactions between the 3D turbulent motions in the solar convection zone with an internal magnetic field in the tachocline (the transition region between the instable and stable zones in the Sun). The tachocline is a very thin layer (less than five percent of the solar radius) that acts as a transport barrier of angular momentum. We show that such an internal magnetic field is not likely to explain the observed thickness of the tachocline and we give some insights on how to find alternative mecanisms to constrain it. We also explore the effect of the environment of star on its structure. We develop a methodology to study the influence of stellar wind and of the magnetic coupling of a star with its orbiting planets. We use the same methodology to analyse the magnetic interaction between a stellar wind and a planetary magnetosphere that acts as a transport barrier of matter. Then, the third part is dedicated to fusion oriented research. We present a numerical investigation on the experimental mecanisms that lead to the development of transport barriers in the plasma. These barriers are particularly important for the design of high performance fusion devices. The creation of transport barriers is obtained in turbulent first principle simulations for the very first time. The collaboration between the two scientific teams lead to the results presented in the fourth part of this thesis. An original spectral method is developed to analyse the saturation of stellar convective dynamos and of the ion temperature gradient instability. We also develop a reduced model that tackles the interaction between turbulent motions, waves, and zonal and mean flows. The same model is applied to both the solar tachocline and tokamak turbulence, putting forth the strong analogy that exists between the two objects we studied
Sabot, Roland. "Importance de la structure magnétique fine dans un tokamak pour le transport anormal et les disruptions internes". Châtenay-Malabry, Ecole centrale de Paris, 1996. http://www.theses.fr/1996ECAP0488.
Texto completo da fonteVillegas, Daniel. "Etude expérimentale de l'influence du gradient de température électronique sur le transport turbulent des impuretés dans un plasma de fusion". Aix-Marseille 1, 2010. http://theses.univ-amu.fr.lama.univ-amu.fr/2010AIX11055.pdf.
Texto completo da fonteUnderstanding impurity transport is a key to an optimal regime for a future fusion device. In this thesis, the theoretical and experimental influence of the electron temperature gradient R/LTe on heavy impurity transport is analyzed both in Tore Supra and ASDEX Upgrade. The electron temperature profile is modified locally by heating the plasma with little ECRH power deposited at two different radii. Experimental results have been obtained with the impurity transport code (ITC) which has been completed with a genetic algorithm allowing to determine the transport coefficient profiles with more accuracy. Transport coefficient profiles obtained by a quasilinear gyrokinetic code named QuaLiKiz are consistent with the experimental ones despite experimental uncertainties on gradients. In the core dominated by electron modes, the lower R/LTe the lower the nickel diffusion coefficient. The latter tends linearly to the neoclassical level when the instability threshold is approached. The experimental threshold is in agreement with the one computed by QuaLiKiz. Further out, where the plasma is dominated by ITG, which are independent of R/LTe , both experimental and simulated results show no modification in the diffusion coefficient profile. Furthermore, the convection velocity profile is not modified. This is attributed to a very small contribution of the thermodiffusion (1/Z dependence) in the total convection. On ASDEX, the preliminary results, very different from the Tore Supra ones, show a internal transport barrier for impurities located at the same radius as the strong ECRH power deposit
Futtersack, Romain. "Modélisation fluide du transport magnétisé dans les plasmas froids". Toulouse 3, 2014. http://thesesups.ups-tlse.fr/2537/.
Texto completo da fonteThe knowledge on the subject of magnetized transport have remained little changed in the field of cold plasmas since the 1960s and is no longer sufficient to explain the complex behavior of the plasma which is found in low-pressure magnetized sources currently under development. Theories and methods developed for the study of magnetized transport in the context of the thermonuclear fusion energy research are inadequate to describe the nonambipolar dynamics of these plasmas. Indeed, in these sources, ions are weakly magnetized, collisions with neutral significantly influence the transport whereas walls are perfect sinks for particles and control equilibrium profiles. To address this problematic, this thesis revisits the cold plasmas modelling and proposes a new model describing the magnetized fluid transport in the plane perpendicular to the magnetic field. We address the complexity of this transport through the development of a 2D+1/2 fluid model and its numerical scheme without approximation on the ordering between characteristic lengths scales of magnetized plasmas (i. E. The plasma size Lambda, the mean free path lai,e and Larmor radii of ions and electrons rho Li,e). The equations are solved in the plane perpendicular to the magnetic field where asymmetries and inhomogeneities representative of the magnetized transport appear, while the boundary conditions (parallel and transverse) are derived from the classical sheath theory. Moreover, the consideration of particles inertia makes the model able to capture the plasma transient dynamics and certain types of instabilities. The model, which supports a wide range of magnetic field strengths and topologies, is applied to the configurations of two negative ions sources. Asymmetries and inhomogeneities observed experimentally are recovered and, in a geometry representing the Scrape-Of-Layer of tokamaks, the model is able to simulate the interchange turbulence which is thought to dominate the perpendicular transport of the edge plasma
Tamain, Patrick. "Etude des flux de matière dans le plasma de bord des tokamaks : alimentation, transport et turbulence". Aix-Marseille 1, 2007. http://www.theses.fr/2007AIX11060.
Texto completo da fonteParticle transport in the edge of tokamaks plays a decisive role both in the center on plasma performances, since it governs the building of density profiles from external particle fuelling, and in the edge on the lifetime of plasma facing components, since it determines particle and energy fluxes reaching the wall. However, this subject has been little explored due to the complexity of modelling the interaction, in the same volume, of the plasma with strong particle, momentum and energy sources and sinks. In the perspective of ITER, the capability of gas puffing systems to reach required density levels without degrading the confinement, as well as the properties of density profiles and flows near the pedestal and in the SOL, remain open questions. This thesis contributes to the effort aiming at giving a better understanding of the mechanisms governing particle fluxes in the edge plasma and their impact on these questions. In a first phase of our work, we present an original approach for the modelling of fuelling by gas puffing, focusing on the thermal impact of the injection on the plasma. On the basis of analytical and numerical models with a reduced number of dimensions, we demonstrate the existence of thermal bifurcations trigerred by the injection and their importance in the dynamics of the neutral penetration and of the plasma relaxation. In the case of Tore Supra, we show that the local cooling linked to a strong injection allows a deeper penetration of particles (r/a = 1. 1 to r/a = 0. 9), but can also lead to a thermal instability of the whole plasma below a given ratio heating power / particle source. The extrapolation of this study for ITER remains pessimistic on the penetration depth of neutrals. However, the sensitivity of the results of these simplified models to the interaction between the parallel and perpendicular directions show that the developpement of numerical tools modelling coherently particle transport in both directions is necessary to progress on these questions. This led to the design of a 3D code presented in the second part of this work. This new tool is a full-torus code, including curvature effects. It solves electrostatic fluid drift equations without scale separation hypothesis, which allows to address with the same tool issues linked to large scale transport as well as micro-turbulence. Two versions of the code have been developped and validated : one treats exclusively closed field lines ; the other, more demanding from the numerical point of view, includes both the Scrape Off Layer (SOL) and the external part of the confined plasma. In a last part, the code is used to address the issue of poloidal asymmetries of parallel flows in the SOL which are observed experimentally but whose origin is not fully clear yet. Simulations reproduce the order of magnitude of measured amplitudes and evidence two different mechanisms which are likely to play a role in this phenomenon, the first at large scales, the other linked to turbulence. The former leans on a coupling between large scale drifts and curvature effects in the SOL while the latter is linked to the ballooning of the radial turbulent flux on the low field side. Finally, the importance of the plasma parallel resistivity on the characteristics of the turbulent transport is analysed as well as the impact of a localized particle injection on the local properties of turbulence
Lo-Cascio, Guillaume. "Barrières et transport des impuretés dans les plasmas de Tokamak". Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0280.
Texto completo da fonteNuclear fusion is the process that powers stars and consists in fusing light nuclei, typically hydrogen isotopes, into a heavier nucleus such as helium. The mass deficit obtained through this type of reaction yields significant amount of energy without emitting greenhouse gases. Fusion through gravitational confinement is impossible to do on Earth. Another way of confining fusion plasma, which are at several millions Kelvin, is required if we want to exploit this source of energy. Nowadays, the most widely studied way of achieving fusion is by magnetically confining the plasma. Since charged particles follow a circular trajectory around magnetic field lines. It is therefore possible to build a reactor, usually toroidal in shape, to confine a plasma that is less dense than in the cores of stars, but at temperatures 10 times higher. A magnetic field along the main axis of the toroid is generated by transverse (i.e. poloidal) coils that inject a current along the same toroidal axis. This generates a transverse magnetic field which, when combined with the first field, creates a total helical field based on interlocking toroidal surfaces. The particles evolving in these closed surfaces are then said to be confined. However, plasma confinement in a tokamak is not perfect; Coulomb collisions allow ions to drift towards the walls by crossing the last closed flux surface of the tokamak (i.e. the separator). This transport phenomenon is known as "neoclassical" transport. However, the transport coefficients calculated using neoclassical theory are 2 to 3 orders of magnitude lower than the experimental values. The transport observed in tokamaks is mainly due to instabilities developing in turbulence by drawing on the free energy stored in the temperature gradients, thus greatly increasing the transport of heat and particles towards the walls of the tokamak. Nonetheless, this transport can be limited by a state transition that takes place by injecting energy into the tokamak via fast neutrals. A very significant reduction in transport and a stiffening of the pressure profiles at the edge is then observed; a "transport barrier" has been set up. This L-H (i.e. "Low - High") transition is experimentally reproducible in many tokamaks and is the standard operating mode envisaged for ITER. However, even in H mode, large flows of particles and heat can reach the wall and pulverise it. Impurities (i.e. species not involved in the fusion reaction) can then contaminate the plasma core and dilute or even extinguish it. This is particularly the case with tungsten, which is not completely ionised under core plasma conditions and therefore re-emits much of the energy it absorbs in the form of radiation. In this thesis, we used GYSELA, a 5D gyrokinetic massively parallelized code developed by CEA Cadarache, the various mechanisms that generate transport barriers. To this end, a sheared poloidal momentum source was used to reduce turbulent intensity locally. The resulting quench of turbulent structures size led to an equivalent reduction in heat flux and associated diffusivity coefficient. Those elements led us to conclude that a transport barrier was successfully triggered in our simulations. Finally, simulations with impurities (He, Ar, W) with and without a transport barrier showed that the transport barrier successfully prevented heavy impurities from outside the barrier to penetrate and contaminate the core plasma. Thermal screening effect is mainly responsible for the resulting confinement enhancement we observed
Rebont, Cyril. "Etude d'une colonne de plasma magnétisé par fluorescence induite par laser". Aix-Marseille 1, 2010. http://theses.univ-amu.fr.lama.univ-amu.fr/2010AIX11056.pdf.
Texto completo da fonteThe experimental study of linear magnetized plasmas was previously performed using Langmuir probes, spectroscopy or more recently fast imaging. In this thesis, another diagnotic, time resolved Laser Induced Fluorescence (LIF) , was implemented in the linear magnetoplasma device MISTRAL. This diagnostic provides the instantaneous Argon ion velocity distribution function. The MISTRAL device creates a linear magnetized plasma limited by a 10 cm circular aperture. We observe, for different experimental conditions, a radial rotation of an extension of the plasma outside the area bounded by the diaphragm. This extension has the form of one or two spiral "arms". From the distribution functions, the temporal fluctuations of density, velocity and electric field are calculated. These innovative measurements show that the ion motion is more complex than the expected ExB drift. The plasma does not rotate in block as has often been assumed. One has to notice that differences exist between the two observed modes, specially on the rotation frequency as compared to cyclotronic frequency. This work opens the path to new exciting theorical analysis
Caschera, Elisabetta. "Global confinement properties in global, flux-driven, gyrokinetic simulations". Electronic Thesis or Diss., Aix-Marseille, 2019. http://www.theses.fr/2019AIXM0509.
Texto completo da fonteUnderstanding and predicting the performance of a fusion reactor in terms of confinement is one of the missing milestones for the availability of fusion energy. The predictions for the design of future reactors such as ITER are based on the extrapolation of empiricalscaling laws. We investigate global confinement properties of turbulent heat transport in a Tokamak with first principle simulations. The research is carried on two main topics: the scaling properties of plasma confinement and the effect of the plasma boundary on the turbulent transport. An important result is obtained when reproducing the global scaling for the energy confinement time with numerical simulations. However the scaling properties are found to brake at the local level. The boundary condition of the code has been modified to mimic the experimental Scrape-Off Layer at the plasma edge. Additional physics is now accessible, such as Kelvin-Helmholtz-like instability at separatrix and edge subcritical turbulence
Marradi, Luca. "Turbulence et structures dans le vent solaire". Nice, 2011. http://www.theses.fr/2011NICE4078.
Texto completo da fonteTwo specific aspects of solar wind turbulence at scales comparable with the ion Larmor radius were considered : anisotropic plasma heating, and the electromagnetic spectral properties of the turbulent cascade in the direction perpendicular to the ambient magnetic field. These problems were studied by means of one-dimensional numerical simulations with two different methods : a FLR-Landau fluid (FLR-LF) model that extends anisotropic magnetohydrodynamics by retaining low-frequency kinetic effects such as Landau damping and finite Larmor radius corrections, and a hybrid Vlasov code in 1d-3V phase space dimensions (HV). Preliminary investigations were performed in the context of compressible Hall-MHD that demonstrated a possible Alfvén wave cascade in the parallel direction at scales smaller than the ion inertial length. Simulations of the FLR-LF model in quasi-perpendicular directions with a random forcing aiming at mimicking the tail of the Alfvén cascade show a non-resonant ion perpendicular heating and the development of the mirror instability which constraints the system to remain near instability threshold. The FLR-LF model has been extended to include the effect of a weak amount of collisions described by a BGK operator. Simulations including collisions show a very good agreement with satellite data. We also performed simulations with the HV code where a forcing on the electric fields code was introduced. These simulations showed the generation of a power-law spectrum with slope -5/3 at scales larger than the ion skin depth, and slope -7/3 in the short wavelengths range, together with the formation of “perpendicular shocks” and magnetic holes
Muraglia, Magali. "Modélisation et simulation de l'interaction multi-échelle entre îlots magnétiques et la microturbulence dans les plasmas de fusion magnétisés". Aix-Marseille 1, 2009. http://www.theses.fr/2009AIX11052.
Texto completo da fonteIn a tokamak, it exists many kind of instability at the origin of a dammage of the confinement and worst of a lost of a confinement. This phd presents a study of the dynamics of a magnetic island in presence of turbulence in magnetized plasmas. More precisely, the goal is to understand the multiscales interaction between turbulence, generated by a pressure gradient and the magnetic field curvature, and a magnetic island formed thanks to a tearing mode. Thanks to the derivation of a 2D slab model takking account both tearing and interchange instabilities, theoretical and numerical linear studies show the pressure effect on the magnetic island linear formation and show interchange modes are stabilized in presence of a strong magnetic field. Then, a numerical nonlinear study is presented in order to understand how the interchange mechanism affects the nonlinear dynamics of a magnetic island. It is showned that the pressure gradient and the magnetic field curvature affect strongly the nonlinear evolution of a magnetic island through dynamics bifurcations. The nature of these bifurcations should be characterized in fonction of the linear situation. Finally, the last part of this phd is devoted to the study of the origin of the nonlinear poloidal rotation of the magnetic island. A model giving the different contributions to the rotation is derived. It is showned, thanks to the model and to the numerical studies, that the nonlinear rotation of the island is mainly governed by the E x B poloidal flow and/or by the nonlinear diamagnetic drift
Darmet, Guillaume. "Transport cinétique dans un plasma de fusion magnétique à flux force". Lyon, École normale supérieure (sciences), 2007. http://www.theses.fr/2007ENSL0424.
Texto completo da fonteMadon, Alex. "Instabilités et transition vers la turbulence faible spatio-temporelle dans une expérience de plasma". Aix-Marseille 1, 1996. http://www.theses.fr/1996AIX11045.
Texto completo da fonteDonnel, Peter. "Impurity transport in tokamak plasmas : gyrokinetic study of neoclassical and turbulent transport". Electronic Thesis or Diss., Aix-Marseille, 2018. http://www.theses.fr/2018AIXM0485.
Texto completo da fonteImpurity transport is an issue of utmost importance for tokamaks. Indeed high-Z materials are only partially ionized in the plasma core, so that they can lead to prohibitive radiative losses even at low concentrations, and impact dramatically plasma performance and stability. On-axis accumulation of tungsten has been widely observed in tokamaks.While the very core impurity peaking is generally attributed to neoclassical effects, turbulent transport could well dominate in the gradient region at ITER relevant collisionality. Up to recently, first principles simulations of corresponding fluxes were performed with different dedicated codes, implicitly assuming that both transport channels are separable and therefore additive. The validity of this assumption is questionned. Simulations obtained with the gyrokinetic code GYSELA have shown clear evidences of a neoclassical-turbulence synergy for impurity transport and allowed the identification of a mechanism that underly this synergy.An analytical work allows to compute the level and the structure of the axisymmetric part of the electric potential knowing the turbulence intensity. Two mechanisms are found for the generation of poloidal asymmetries of the electric potential: flow compressibility and the ballooning of the turbulence. A new prediction for the neoclassical impurity flux in presence of large poloidal asymmetries and pressure anisotropies has been derived. A fair agreement has been found between the new theoretical prediction for neoclassical impurity flux and the results of a GYSELA simulation displaying large poloidal asymmetries and pressure anisotropies induced by the presence of turbulence
Chôné, Laurent. "Turbulence, flows and transport barriers at the tokamak plasma edge". Thesis, Aix-Marseille, 2016. http://www.theses.fr/2016AIXM4701.
Texto completo da fonteThe topic of this thesis is the interaction between turbulence and flows at the tokamak edge, and their influence on the confinement. Turbulence is the main contribution to the outward transport in magnetic fusion devices, and a strong limiting factor for their performance. It can be stabilised by flows, through shear-mediated decorrelation of convective cells, and through non-linear coupling. Strong shear flows causing a localised reduction of transport (transport barrier) are often observed in experiments, and several regimes of improved confinement such as the High-confinement mode are accessed routinely. There is a growing body of evidence from experiments showing that the mean flow responsible for the barrier is governed by force balance, while non-linear interplay between turbulence, turbulence-driven zonal-flows, and the mean flow occurs during the transition phase. In this thesis, we extend a fluid model for plasma edge turbulence to include collisional relaxation of flows towards force. We show that accounting for a contribution of neoclassical allows for the spontaneous formation of a transport barrier to occur in flux-driven simulation. Dynamical features reminiscent of the L-H transition and H-mode are recovered, such as relaxation-oscillations of the barrier and dithering of the radial electric field during the barrier formation. An analysis is carried out to identify the roles of zonal-flows and force balance during the transition, and it is found that in our simulations that zonal flows provide temporary quenching of the turbulence via non-linear coupling, allowing for the mean flow to grow and form the barrier
Depret, Gilles. "Simulation eulérienne de Vlasov pour l'étude de la turbulence ionique dans les plasmas de tokamak". Nancy 1, 1999. http://www.theses.fr/1999NAN10245.
Texto completo da fonteNorscini, Claudia. "Self-organized turbulent transport in fusion plasmas". Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4076.
Texto completo da fonteTransport barriers (TB) are a key element in controlling turbulent transport and achieving high performance burning plasmas. Theoretical studies are addressing the turbulence self-regulation as a possible explanation for transport barrier formation but a complete understanding of such complex dynamics is still missing. In this context, we address self-organized turbulent transport in fusion plasmas with the aim of presenting a novel understanding of transport barriers dynamics. The numerical tools we use span simulations from the most complex gyrokinetic turbulence to simpler 2D fluid turbulence and predator-prey like models.Two features of self-organizations, avalanches and zonal flows (ZFs), appear to control large scale transport. In the SOL (Scrape Off Layer) , intermittent avalanche events do not allow for time or space scale separation between mean fields and fluctuation terms. In the edge, the generation of long living double shear layers in the profiles of the velocity reduces radial turbulent transport. Such radially distributed barriers govern profile corrugations. A 2D turbulent model for pedestal generation, which is not specific of Tokamak plasmas, has been developed, the pedestal being localized at the interface between regions with different zonal flow damping: the edge region, where zonal flows are weakly damped by collisions, and the SOL region characterized by zonal flow damping due to boundary conditions. Quasi-periodic relaxation events are studied reducing the model to three modes coupling to identify the interplay between streamers and ZFs and the role of Reynolds stress in the generation and saturation of TBs
Coulette, David. "Simulation numérique de modèles cinétiques réduits pour l'étude de la dynamique des plasmas de fusion par confinement magnétique". Thesis, Université de Lorraine, 2013. http://www.theses.fr/2013LORR0159/document.
Texto completo da fonteThe research exposed therein is developed in the context of the study of turbulent energy and particle transport phenomena occuring in magnetically confined fusion plasmas. A study of the ion temperature gradient instability, one of the main sources of such turbulent transport, is carried out using a gyrokinetic model. The main originality of this work lies in the use of a reduced model, the so-called Multi-Water-Bag model, which allows to reduce the problem dimension while preserving kinetic effects. The model is developed in two types of confinement field geometries. In cylindrical geometry, the growth of the instability is analysed by the mean of three dynamical models : linear, quasi-linear and non-linear. Starting from a given unstable stationary state, linear stability analysis allows one to obtain spectral and geometrical characteristics of the instability. In a second phase, comparing results of numerical simulations implementing the three dynamical models, the growth of turbulence is analysed as well as the first stages of non-linear saturation of the instability. In toroidal geometry, a linear stability analysis is performed. Two different methods, time-based and spectral, were implemented in order to obtain the spectral and geometrical characteristics of the most unstable modes. In both field geometries encompassed by this research, the numerical methods used to obtain the results are described and their performances analyzed. Throughout the work, particular care is given to the balance between the benefits and costs of the Multi-Water-Bag reduction
Peyroux, Julien. "Simulations numériques de l'équation de Vlasov à l'aide d'outils parallèles". Nancy 1, 2005. http://docnum.univ-lorraine.fr/public/SCD_T_2005_0114_PEYROUX.pdf.
Texto completo da fonteThe problems related to a laser-matter interaction or turbulence in takomak plasmas encountered in magnetic fusion call for a study of not strongly linear/relativist Vlasov plasmas. It becomes necessary to use powerful tools for simulation on powerful computers in order to better include/understand the physical mechanisms put in play in these two domains. The development of nonparticular (or more exactly semi-Lagrangian) methods for the resolution of kinetic equations and, in particular, for the study of the wave-particle interactions remain a particularly promising way, taking into account the importance of results already obtained. This project aims to make even more powerful the resolution of Vlasov codes through the various parallelisation tools (MPI, OpenMP. . . ). For our work, a simplified ``test case'' served for us as a base for constructing the parallel codes for obtaining a data-processing skeleton which, thereafter, could be re-used for increasingly complex models (more than four variables of phase space). What will thus make it possible to treat more realistic situations linked, for example, to the injection of ultra short and ultra intense impulses in inertial fusion plasmas, or the study of the instability of trapped ions now taken as being responsible for the generation of turbulence in tokamak plasmas
Brochard, Frédéric David. "Caractérisation des instabilités à basse fréquence dans une colonne de plasma magnétisé et étude de leur transition à la turbulence". Nancy 1, 2004. http://www.theses.fr/2004NAN10184.
Texto completo da fonteThis work is devoted to the experimental study of low frequency instabilities that occur in a magnetized plasma column. By using a limiter at the entry of the column, a transition between the Kelvin-Helmholtz instability and drift waves is observed when the magnetic field is increased. The Rayleigh-Taylor (or centrifugal) instability, can also be observed in between. With a view to to improve the efficiency of control methods, within the framework of controlled thermonuclear fusion research, we then studied the transitions to turbulence of these various instabilities. For each of them, a similar transition scenario is observed, in which spatio-temporal chaos results from the nonlinear interaction between two modes of incommensurable frequencies
Chasapis, Alexandros. "Étude de la reconnexion magnétique dans les plasmas turbulents à partir des données satellites". Thesis, Paris 11, 2015. http://www.theses.fr/2015PA112205/document.
Texto completo da fonteMagnetic reconnection is a fundamental energy conversion process in plasma. It occurs in thin regions of strong current known as current sheets and results in particle heating and acceleration. In turbulence, which is ubiquitous in space plasma, magnetic reconnection has been observed to occur in small scale structures that form therein, and is thought to contribute to dissipation of turbulent energy at kinetic scales. For this work we examine data from the Cluster spacecraft in the Earth's magnetosheath, downstream of the quasi-parallel shock. The detection of ion-scale current sheets was performed by implementing the PartialVariance of Increments (PVI) method for multiple spacecraft. The properties of the observed current sheets were different for high (> 3) and low (< 3) values of the PVI index. We observed a distinct population of high PVI (> 3) structures that accounted for ~ 20% of the total. Those current sheets have high magneticshear (> 90degrees). In order to estimate the local heating occurring within those current sheets, a proxy of the electron temperature was obtained at high time resolution(125ms) from the partial distributions measured by Cluster. This allowed for the first time to study the localized electron heating within ion-scale currentsheets. The observed enhancement of the estimated electron temperature withinthe high PVI current sheets suggest that they are important for local electron heating and energy dissipation. We also examined measurements inside the diffusion region of a thin reconnecting current sheet. Multi-spacecraft observationsallow as to study electron distributions and wave activity at different distances from the x-line. Significant differences were observed in the electron populations as they were heated going through the current sheet. In particular electrons were heated in the direction parallel to the magnetic field in close proximity to thex-line, whereas no significant variation was observed in the perpendicular direction. However, the distribution was more isotropic downstream of the x-line with electrons heated in the perpendicular direction
Vinogradov, Alexander. "Coherent structures from MHD to sub-ion scales in turbulent solar wind". Electronic Thesis or Diss., Sorbonne université, 2023. http://www.theses.fr/2023SORUS612.
Texto completo da fonteTurbulence is a nonlinear process of cross-scale energy transfer in a fluid. The solar wind is an example of a turbulent plasma available for investigation owing to in-situ spacecraft measurements. Solar wind fluctuations are highly irregular and chaotic. Above an incoherent background fluctuations high-amplitude coherent structures, localized in space, are present. In this thesis we brought a piece of a new knowledge about the properties of coherent structures in the solar wind at close distance from the Sun (0.17 au). We confirm, with Parker Solar Probe (PSP) data, that coherent structures are present not only at MHD and ion scales, but also at sub-ion scales. We could identify these structures thanks to Morlet wavelet transform. For the first time, we apply a multi-scale analysis in physical space from MHD down to sub-ion scales. Using plasma and magnetic field time profiles, we analyze several events in details. The amplitude of MHD scale coherent structures is high, comparable in magnitude with the local mean magnetic field. We show examples of MHD coherent structures with different geometries, such as current sheets and Alfvén vortices. Some of MHD coherent structures are located within switchbacks or their boundaries. MHD structures contain a number of embedded substructures at ion and sub-ion scales. The latter have smaller amplitudes and are not seen in the raw data, but can be observed after filtration in frequencies and zooming in time. These structures have a typical magnetic field profiles representing an incompressible Alfvén vortex at ion scales and compressible vortex at sub-ion scales. We also study the fragmentation of coherent structures, showing that the number of isolated intermittent events is larger at smaller scales, while the filling factor decrease from 12% at MHD scales to 7% at ion and 6% sub-ion scales. Another important point of this thesis is a contribution to our understanding of Alfvén vortices. We refined the derivation of an Alfvén vortex model by providing detailed discussion of the underlying assumptions. We also generalize the model to describe multipole Alfvén vortices. Finally, we explicit how the modes described multipole vortices of different order could be combined, or not. This provides more general Alfvén vortex solutions than derived before. Finally, we derive a new method of classification of coherent structures in the solar wind. This method involves the statistical comparison of the observed structures with expectations of models (using the amplitude anisotropy of magnetic fluctuations). The results with PSP data are dominantly consistent with the crossings of the Alfvén vortex model. Only a small fraction of the structures corresponds to current sheets and magnetic holes, in contrast with the previous results of visual classification. The results presented in this thesis open a new window of turbulence analysis by showing the importance of Alfvén vortices from MHD down to sub-ion scales. It will be worth to extend this study to different types of winds and to different solar distances, in particular to constrain how these vortices are created and how they evolve in the solar wind. Extending the analysis to higher frequencies will also provide information on their dissipation at electronic scales, so their contribution to solar wind heating
Donnel, Peter. "Impurity transport in tokamak plasmas : gyrokinetic study of neoclassical and turbulent transport". Thesis, Aix-Marseille, 2018. http://www.theses.fr/2018AIXM0485/document.
Texto completo da fonteImpurity transport is an issue of utmost importance for tokamaks. Indeed high-Z materials are only partially ionized in the plasma core, so that they can lead to prohibitive radiative losses even at low concentrations, and impact dramatically plasma performance and stability. On-axis accumulation of tungsten has been widely observed in tokamaks.While the very core impurity peaking is generally attributed to neoclassical effects, turbulent transport could well dominate in the gradient region at ITER relevant collisionality. Up to recently, first principles simulations of corresponding fluxes were performed with different dedicated codes, implicitly assuming that both transport channels are separable and therefore additive. The validity of this assumption is questionned. Simulations obtained with the gyrokinetic code GYSELA have shown clear evidences of a neoclassical-turbulence synergy for impurity transport and allowed the identification of a mechanism that underly this synergy.An analytical work allows to compute the level and the structure of the axisymmetric part of the electric potential knowing the turbulence intensity. Two mechanisms are found for the generation of poloidal asymmetries of the electric potential: flow compressibility and the ballooning of the turbulence. A new prediction for the neoclassical impurity flux in presence of large poloidal asymmetries and pressure anisotropies has been derived. A fair agreement has been found between the new theoretical prediction for neoclassical impurity flux and the results of a GYSELA simulation displaying large poloidal asymmetries and pressure anisotropies induced by the presence of turbulence
Benazzouz, Tewfik. "Modélisation numérique de plasmas en écoulements turbulents : application au cas de l'argon". Rouen, 1999. http://www.theses.fr/2000ROUES029.
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