122 research outputs found
Control of stochasticity in magnetic field lines
We present a method of control which is able to create barriers to magnetic
field line diffusion by a small modification of the magnetic perturbation. This
method of control is based on a localized control of chaos in Hamiltonian
systems. The aim is to modify the perturbation locally by a small control term
which creates invariant tori acting as barriers to diffusion for Hamiltonian
systems with two degrees of freedom. The location of the invariant torus is
enforced in the vicinity of the chosen target. Given the importance of
confinement in magnetic fusion devices, the method is applied to two examples
with a loss of magnetic confinement. In the case of locked tearing modes, an
invariant torus can be restored that aims at showing the current quench and
therefore the generation of runaway electrons. In the second case, the method
is applied to the control of stochastic boundaries allowing one to define a
transport barrier within the stochastic boundary and therefore to monitor the
volume of closed field lines
Channeling chaotic transport in a wave-particle experiment
A numerical and experimental study of a control method aimed at channeling
chaos by building barriers in phase space is performed on a paradigm for
wave-particle interaction, i.e., a traveling wave tube. Control of chaotic
diffusion is achieved by adding small apt modifications to the system with a
low additional cost of energy. This modification is realized experimentally
through additional waves with small amplitudes. Robustness of the method is
investigated both numerically and experimentally
Local control of area-preserving maps
International audienceWe present a method of control of chaos in area-preserving maps. This method gives an explicit expression of a control term which is added to a given area-preserving map. The resulting controlled map which is a small and suitable modification of the original map, is again area-preserving and has an invariant curve whose equation is explicitly known
Reduction of the chaotic transport of impurities in turbulent magnetized plasmas
The chaotic transport of charged particles in a turbulent electrostatic potential sets the conditions of a severe limitation to the plasma confinement in devices such as tokamaks. In this chapter, we consider the motion of impurities driven by the ExB velocity where a strong magnetic field B (which allows for the guiding center approximation) is uniform and constant, and a turbulent electric field is obtained from models or from numerical fluid codes. Hamiltonian dynamics rule the transport properties of these impurities. Therefore a technique to reduce chaotic diffusion in Hamiltonian systems is able to address the issue of reducing the radial transport of impurities under some approximations. The general idea is to build barriers in phase space by a small and apt modification of the Hamiltonian. We show numerically that such perturbations are able to drastically reduce the diffusion of test-particles, and we discuss the robustness of such additional modifications
Anomalous dynamical scaling in anharmonic chains and plasma models with multiparticle collisions
We study the anomalous dynamical scaling of equilibrium correlations in one
dimensional systems. Two different models are compared: the Fermi-Pasta-Ulam
chain with cubic and quartic nonlinearity and a gas of point particles
interacting stochastically through the multiparticle collision dynamics. For
both models -that admit three conservation laws- by means of detailed numerical
simulations we verify the predictions of nonlinear fluctuating hydrodynamics
for the structure factors of density and energy fluctuations at equilibrium.
Despite this, violations of the expected scaling in the currents correlation
are found in some regimes, hindering the observation of the asymptotic scaling
predicted by the theory. In the case of the gas model this crossover is clearly
demonstrated upon changing the coupling constant.Comment: 12 pages, 8 figures. Matching the version published in Phys. Rev.
Gyromap for a two-dimensional Hamiltonian fluid model derived from Braginskii's closure for magnetized plasmas
5 pages, 2 columnsInternational audienceWe consider a plasma described by means of a two-dimensional fluid model across a constant but non-uniform magnetic field B = B(x,y) z. The dynamical evolution of the density and the vorticity takes into account the interchange instability and magnetic field inhomogeneities. First, in order to described the Finite Larmor Radius effects we apply the gyromap to build a Hamiltonian model with ion temperature from a cold-ion model. Second, we show that the gyromap is justified using Braginskii's closure for the stress tensor as well as an apt ordering on the fluctuating quantities
Multiparticle collision simulations of two-dimensional one-component plasmas: Anomalous transport and dimensional crossovers
By means of hybrid multi-particle collsion--particle-in-cell (MPC-PIC)
simulations we study the dynamical scaling of energy and density correlations
at equilibrium in moderately coupled 2D and quasi 1D plasmas. We find that the
predictions of Nonlinear Fluctuating Hydrodynamics for the structure factors of
density and energy fluctuations in 1D systems with three global conservation
laws hold true also for two dimensional systems that are more extended along
one of the two spatial dimensions. Moreover, from the analysis of the
equilibrium energy correlators and density structure factors of both 1D and 2D
neutral plasmas, we find that neglecting the contribution of the fluctuations
of the vanishing self-consistent electrostatic fields overestimates the
interval of frequencies over which the anomalous transport is observed. Such
violations of the expected scaling in the currents correlation are found in
different regimes, hindering the observation of the asymptotic scaling
predicted by the theory.Comment: 13 pages, 10 figures. Submitted to Phys.Rev.E, comments welcom
Control of the chaotic velocity dispersion of a cold electron beam interacting with electrostatic waves
International audienceIn this article we present an application of a method of control of Hamiltonian systems to the chaotic velocity diffusion of a cold electron beam interacting with electrostatic waves. We numerically show the efficiency and robustness of the additional small control term in restoring kinetic coherence of the injected electron beam
Transition to super-diffusive transport in turbulent plasmas
We investigate the motion of charged particles in a turbulent electrostatic
potential using guiding-center theory. By increasing the Larmor radius, the
dynamics exhibit close-to-ballistic transport properties. The transition from
diffusive to ballistic transport is analyzed using nonlinear dynamics. It is
found that twistless invariant tori in the guiding-center dynamics are
responsible for this transition, drastically affecting transport properties of
charged particles
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