593 research outputs found
Derivation of a gyrokinetic model. Existence and uniqueness of specific stationary solutions
A finite Larmor radius approximation is derived from the classical Vlasov
equation, in the limit of large (and uniform) external magnetic field. We also
provide an heuristic derivation of the electroneutrality equation in the finite
Larmor radius setting. Existence and uniqueness of a solution is proven in the
stationary frame for solutions depending only on the direction parallel to the
magnetic field and factorizing in the velocity variables
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
Control of test particle transport in a turbulent electrostatic model of the Scrape Off Layer
The drift motion of charged test particle dynamics in
the Scrape Off Layer (SOL)is analyzed to investigate a transport control
strategy based on Hamiltonian dynamics. We model SOL turbulence using a 2D
non-linear fluid code based on interchange instability which was found to
exhibit intermittent dynamics of the particle flux. The effect of a small and
appropriate modification of the turbulent electric potential is studied with
respect to the chaotic diffusion of test particle dynamics. Over a significant
range in the magnitude of the turbulent electrostatic field, a three-fold
reduction of the test particle diffusion coefficient is achieved
Controlling chaotic transport in a Hamiltonian model of interest to magnetized plasmas
We present a technique to control chaos in Hamiltonian systems which are
close to integrable. By adding a small and simple control term to the
perturbation, the system becomes more regular than the original one. We apply
this technique to a model that reproduces turbulent ExB drift and show
numerically that the control is able to drastically reduce chaotic transport
Tailoring Phase Space : A Way to Control Hamiltonian Transport
We present a method to control transport in Hamiltonian systems. We provide
an algorithm - based on a perturbation of the original Hamiltonian localized in
phase space - to design small control terms that are able to create isolated
barriers of transport without modifying other parts of phase space. We apply
this method of localized control to a forced pendulum model and to a system
describing the motion of charged particles in a model of turbulent electric
field
Self-regulation of turbulence bursts and transport barriers
International audienceThe interplay between turbulent bursts and transport barriers is analyzed with a simplified model of interchange turbulence in the Scrape-Off Layer of magnetically confined plasmas. The turbulent bursts spread into the transport barriers, and, depending on the competing magnitude of the burst and stopping capability of the barrier can burn through. Two models of transport barriers are presented, a hard barrier where all turbulent modes are stable in a prescribed region and a soft barrier with external plasma biasing. This process can be modeled on the basis of competing stochastic processes. For classes of probability density function of these processes one can predict the heavy tail properties of the bursts downstream from the barrier, either exponential for a leaky barrier, or with power laws, for a tight barrier. The intrinsic probing of the transport barriers by the turbulent bursts thus gives access to properties of the transport barriers. The main stochastic variables of the two models addressed here are the barrier width and the spreading distance of the turbulent bursts within the barrier together with their level of correlation. One finds that in the case of a barrier located in the Scrape-Off-Layer, the stochastic model predicts a leaky barrier with an exponential probability density function of escaping turbulent bursts in agreement with the simulation data
Electrostatic transport in L-mode scrape-off layer plasmas of Tore Supra tokamak: II: transport by fluctuations
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