73 research outputs found
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Erratum: Time-step Considerations in Particle Simulation Algorithms for Coulomb Collisions in Plasmas [B.I. Cohen, A. M. Dimits, A. Friedman, and R. E. Caflisch, IEEE Trans. Plasma Sci. 38, 2394 (2010)]
Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmas
Differential rotation is known to suppress linear instabilities in fusion
plasmas. However, even in the absence of growing eigenmodes, subcritical
fluctuations that grow transiently can lead to sustained turbulence. Here
transient growth of electrostatic fluctuations driven by the parallel velocity
gradient (PVG) and the ion temperature gradient (ITG) in the presence of a
perpendicular ExB velocity shear is considered. The maximally simplified case
of zero magnetic shear is treated in the framework of a local shearing box.
There are no linearly growing eigenmodes, so all excitations are transient. The
maximal amplification factor of initial perturbations and the corresponding
wavenumbers are calculated as functions of q/\epsilon (=safety factor/aspect
ratio), temperature gradient and velocity shear. Analytical results are
corroborated and supplemented by linear gyrokinetic numerical tests. For
sufficiently low values of q/\epsilon (<7 in our model), regimes with fully
suppressed ion-scale turbulence are possible. For cases when turbulence is not
suppressed, an elementary heuristic theory of subcritical PVG turbulence
leading to a scaling of the associated ion heat flux with q, \epsilon, velocity
shear and temperature gradient is proposed; it is argued that the transport is
much less stiff than in the ITG regime.Comment: 36 pages in IOP latex style; 12 figures; submitted to PPC
An Asymptotic Preserving Scheme for the Euler equations in a strong magnetic field
This paper is concerned with the numerical approximation of the isothermal
Euler equations for charged particles subject to the Lorentz force. When the
magnetic field is large, the so-called drift-fluid approximation is obtained.
In this limit, the parallel motion relative to the magnetic field direction
splits from perpendicular motion and is given implicitly by the constraint of
zero total force along the magnetic field lines. In this paper, we provide a
well-posed elliptic equation for the parallel velocity which in turn allows us
to construct an Asymptotic-Preserving (AP) scheme for the Euler-Lorentz system.
This scheme gives rise to both a consistent approximation of the Euler-Lorentz
model when epsilon is finite and a consistent approximation of the drift limit
when epsilon tends to 0. Above all, it does not require any constraint on the
space and time steps related to the small value of epsilon. Numerical results
are presented, which confirm the AP character of the scheme and its Asymptotic
Stability
Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the BES system on MAST
The mean motion of turbulent patterns detected by a two-dimensional (2D) beam
emission spectroscopy (BES) diagnostic on the Mega Amp Spherical Tokamak (MAST)
is determined using a cross-correlation time delay (CCTD) method. Statistical
reliability of the method is studied by means of synthetic data analysis. The
experimental measurements on MAST indicate that the apparent mean poloidal
motion of the turbulent density patterns in the lab frame arises because the
longest correlation direction of the patterns (parallel to the local background
magnetic fields) is not parallel to the direction of the fastest mean plasma
flows (usually toroidal when strong neutral beam injection is present). The
experimental measurements are consistent with the mean motion of plasma being
toroidal. The sum of all other contributions (mean poloidal plasma flow, phase
velocity of the density patterns in the plasma frame, non-linear effects, etc.)
to the apparent mean poloidal velocity of the density patterns is found to be
negligible. These results hold in all investigated L-mode, H-mode and internal
transport barrier (ITB) discharges. The one exception is a high-poloidal-beta
(the ratio of the plasma pressure to the poloidal magnetic field energy
density) discharge, where a large magnetic island exists. In this case BES
detects very little motion. This effect is currently theoretically unexplained.Comment: 28 pages, 15 figures, submitted to PPC
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Verification of Gyrokinetic (delta)f Simulations of Electron Temperature Gradient Turbulence
The GEM gyrokinetic {delta}f simulation code [Chen, 2003] [Chen, 2007] is shown to reproduce electron temperature gradient turbulence at the benchmark operating point established in previous work [Nevins, 2006]. The electron thermal transport is within 10% of the expected value, while the turbulent fluctuation spectrum is shown to have the expected intensity and two-point correlation function
ELM triggering conditions for the integrated modeling of H-mode plasmas
Recent advances in the integrated modeling of ELMy H-mode plasmas are
presented. A model for the H-mode pedestal and for the triggering of ELMs
predicts the height, width, and shape of the H-mode pedestal and the frequency
and width of ELMs. Formation of the pedestal and the L-H transition is the
direct result of ExB flow shear suppression of anomalous transport. The
periodic ELM crashes are triggered by either the ballooning or peeling MHD
instabilities. The BALOO, DCON, and ELITE ideal MHD stability codes are used to
derive a new parametric expression for the peeling-ballooning threshold. The
new dependence for the peeling-ballooning threshold is implemented in the ASTRA
transport code. Results of integrated modeling of DIII-D like discharges are
presented and compared with experimental observations. The results from the
ideal MHD stability codes are compared with results from the resistive MHD
stability code NIMROD.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Uncertainty quantification for kinetic models in socio-economic and life sciences
Kinetic equations play a major rule in modeling large systems of interacting
particles. Recently the legacy of classical kinetic theory found novel
applications in socio-economic and life sciences, where processes characterized
by large groups of agents exhibit spontaneous emergence of social structures.
Well-known examples are the formation of clusters in opinion dynamics, the
appearance of inequalities in wealth distributions, flocking and milling
behaviors in swarming models, synchronization phenomena in biological systems
and lane formation in pedestrian traffic. The construction of kinetic models
describing the above processes, however, has to face the difficulty of the lack
of fundamental principles since physical forces are replaced by empirical
social forces. These empirical forces are typically constructed with the aim to
reproduce qualitatively the observed system behaviors, like the emergence of
social structures, and are at best known in terms of statistical information of
the modeling parameters. For this reason the presence of random inputs
characterizing the parameters uncertainty should be considered as an essential
feature in the modeling process. In this survey we introduce several examples
of such kinetic models, that are mathematically described by nonlinear Vlasov
and Fokker--Planck equations, and present different numerical approaches for
uncertainty quantification which preserve the main features of the kinetic
solution.Comment: To appear in "Uncertainty Quantification for Hyperbolic and Kinetic
Equations
Multicomponent theory of buoyancy instabilities in magnetized plasmas: The case of magnetic field parallel to gravity
We investigate electromagnetic buoyancy instabilities of the electron-ion
plasma with the heat flux based on not the magnetohydrodynamic (MHD) equations,
but using the multicomponent plasma approach when the momentum equations are
solved for each species. We consider a geometry in which the background
magnetic field, gravity, and stratification are directed along one axis. The
nonzero background electron thermal flux is taken into account. Collisions
between electrons and ions are included in the momentum equations. No
simplifications usual for the one-fluid MHD-approach in studying these
instabilities are used. We derive a simple dispersion relation, which shows
that the thermal flux perturbation generally stabilizes an instability for the
geometry under consideration. This result contradicts to conclusion obtained in
the MHD-approach. We show that the reason of this contradiction is the
simplified assumptions used in the MHD analysis of buoyancy instabilities and
the role of the longitudinal electric field perturbation which is not captured
by the ideal MHD equations. Our dispersion relation also shows that the medium
with the electron thermal flux can be unstable, if the temperature gradients of
ions and electrons have the opposite signs. The results obtained can be applied
to the weakly collisional magnetized plasma objects in laboratory and
astrophysics.Comment: Accepted for publication in Astrophysics & Space Scienc
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