181 research outputs found
Instabilités et transport turbulent dans les plasmas de fusion
La production d'Ă©nergie par fusion
thermonucléaire contrôlée est un des grands défis scientifiques de ce siècle.
L'utilisation d'un plasma chaud confiné par un champ magnétique est une voie prometteuse
qui franchira une étape décisive avec le projet ITER. Parmi les grands thèmes de la
problématique scientifique des plasmas de fusion; les questions de stabilité
magnétohydrodynamique (MHD) et de transport turbulent suscitent un intérêt particulier.
La stabilité MHD contrôle en effet le domaine opérationnel d’un tokamak. Ce domaine peut
être étendu via une stratégie d’évitement ou le contrôle actif des instabilités
macroscopiques. Un élément important de cette stratégie est la modélisation des
instabilités en régime non linéaire, et la validation par l’expérience. Le point sera
fait sur l’état de l’art dans ce domaine. Par ailleurs, le transport de chaleur et des
particules dans un plasma magnétisé est contrôlé par une turbulence, qui résulte
d'instabilités à petite échelle. La compréhension des mécanismes régissant le transport
turbulent est un objectif majeur en vue d’optimiser le confinement dans un plasma de
tokamak. Cet exposé fera le point sur le sujet, en s'appuyant sur des résultats de
modélisation et d'expérience. La conclusion portera sur les perspectives à l'horizon de
la mise en service d'ITER, en 2018
Te/Ti effects on JET energy confinement properties
Lately the question has been raised if a modification of the
energy-confinement scaling law with respect to the electron to ion temperature
ratio, Te/Ti, is required. Theoretically, like in e.g. the Weiland model, the
confinement is thought to degrade with Te/Ti and studies of the hot-ion
(Ti>/Te) mode seems to corroborate this. In this paper, it is shown that due to
a number of effects that cancel each other out, the energy confinement time
remains constant for Te/Ti>~1. The numerical study relies on a series of JET
shots specifically designed to reveal an effect of Te/Ti in the hot-electron
(Te>Ti) mode. A distinct effort was made to keep all current scaling-law
parameters constant, including the total heating power. The effects that
provide the constant confinement times have therefore nothing to do with the
global properties of the plasma, but are rather due to variations in the
temperature gradients which affects the transport locally.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Full particle orbit effects in regular and stochastic magnetic fields
We present a numerical study of charged particle motion in a time-independent
magnetic field in cylindrical geometry. The magnetic field model consists of an
unperturbed reversed-shear helical part and a perturbation consisting of a
superposition of modes. Contrary to most of the previous studies, the particle
trajectories are computed by directly solving the full Lorentz force equations
of motion in a six-dimensional phase space using a sixth-order, implicit,
symplectic Gauss-Legendre method. The level of stochasticity in the particle
orbits is diagnosed using averaged, effective Poincare sections. It is shown
that when only one mode is present the particle orbits can be stochastic even
though the magnetic field line orbits are not stochastic. The lack of
integrability of the particle orbits in this case is related to separatrix
crossing and the breakdown of the global conservation of the magnetic moment.
Some perturbation consisting of two modes creates resonance overlapping,
leading to Hamiltonian chaos in magnetic field lines. Then, the particle orbits
exhibit a nontrivial dynamics depending on their energy and pitch angle. It is
shown that the regions where the particle motion is stochastic decrease as the
energy increases. The non-monotonicity of the -profile implies the existence
of magnetic ITBs which correspond to shearless flux surfaces located in the
vicinity of the -profile minimum. It is shown that depending on the energy,
these magnetic ITBs might or might not confine particles. That is, magnetic
ITBs act as an energy-dependent particle confinement filter. Magnetic field
lines in reversed-shear configurations exhibit topological bifurcations due to
separatrix reconnection. We show that a similar but more complex scenario
appears in the case of particle orbits that depends in a non-trivial way on the
energy and pitch angle of the particles.Comment: 25 pages, accepted for publication in Phys. Plasma
Chaotic motion of charged particles in toroidal magnetic configurations
We study the motion of a charged particle in a tokamak magnetic field and
discuss its chaotic nature. Contrary to most of recent studies, we do not make
any assumption on any constant of the motion and solve numerically the
cyclotron gyration using Hamiltonian formalism. We take advantage of a
symplectic integrator allowing us to make long-time simulations. First
considering an idealized magnetic configuration, we add a non generic
perturbation corresponding to a magnetic ripple, breaking one of the invariant
of the motion. Chaotic motion is then observed and opens questions about the
link between chaos of magnetic field lines and chaos of particle trajectories.
Second, we return to a axi-symmetric configuration and tune the safety factor
(magnetic configuration) in order to recover chaotic motion. In this last
setting with two constants of the motion, the presence of chaos implies that no
third global constant exists, we highlight this fact by looking at variations
of the first order of the magnetic moment in this chaotic setting. We are
facing a mixed phase space with both regular and chaotic regions and point out
the difficulties in performing a global reduction such as gyrokinetics
Dynamics of heavy impurities in non-linear MHD simulations of sawtoothing tokamak plasmas
The effect of sawteeth on impurity dynamics is studied with the XTOR-2F code. Non-linear full 3D MHD simulations including appropriate fluid equations for heavy impurities show that the presence of regular sawtooth crashes affects the impurity behaviour. A spatial non-uniformity of 5% in post-crash impurity density profiles persists due to 2D structures of impurity density which appear during sawtooth crashes. They are shown to be mainly driven by the E Ă— B velocity, and are responsible for the sudden impurity transport in the core plasmas
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