18,271 research outputs found
Anomalous Viscosity of the Quark-Gluon Plasma
The shear viscosity of the quark-gluon plasma is predicted to be lower than
the collisional viscosity for weak coupling. The estimated ratio of the shear
viscosity to entropy density is rather close to the ratio calculated by N = 4
super Yang-Mills theory for strong coupling, which indicates that the
quark-gluon plasma might be strongly coupled. However, in presence of momentum
anisotropy, the Weibel instability can arise and drive the turbulent transport.
Shear viscosity can be lowered by enhanced collisionality due to turbulence,
but the decorrelation time and its relation to underlying dynamics and
color-magnetic fields have not been calculated self-consistently. In this
paper, we use resonance broadening theory for strong turbulence to calculate
the anomalous viscosity of the quark-gluon plasma for nonequilibrium. For
saturated Weibel instability, we estimate the scalings of the decorrelation
rate and viscosity and compare these with collisional transport. This
calculation yields an explicit connection between the underlying momentum space
anisotropy and the viscosity anomaly.Comment: 16 pages, 2 figure
Theory of Two Dimensional Mean Field Electron Magnetohydrodynamics
The theory of mean field electrodynamics for diffusive processes in Electron
Magnetohydrodynamic (EMHD) model is presented. In contrast to
Magnetohydrodynamics (MHD) the evolution of magnetic field here is governed by
a nonlinear equation in the magnetic field variables. A detailed description of
diffusive processes in two dimensions are presented in this paper. In
particular, it has been shown analytically that the turbulent magnetic field
diffusivity is suppressed from naive quasilinear estimates. It is shown that
for complete whisterlization of the spectrum, the turbulent diffusivity
vanishes. The question of whistlerization of the turbulent spectrum is
investigated numerically, and a reasonable tendency towards whisterlization is
observed. Numerical studies also show the suppression of magnetic field
diffusivity in accordance with the analytical estimates.Comment: 18 pages, 6 figure
Turbulence model reduction by deep learning
A central problem of turbulence theory is to produce a predictive model for
turbulent fluxes. These have profound implications for virtually all aspects of
the turbulence dynamics. In magnetic confinement devices, drift-wave turbulence
produces anomalous fluxes via cross-correlations between fluctuations. In this
work, we introduce a new, data-driven method for parameterizing these fluxes.
The method uses deep supervised learning to infer a reduced mean-field model
from a set of numerical simulations. We apply the method to a simple drift-wave
turbulence system and find a significant new effect which couples the particle
flux to the local \emph{gradient} of vorticity. Notably, here, this effect is
much stronger than the oft-invoked shear suppression effect. We also recover
the result via a simple calculation. The vorticity gradient effect tends to
modulate the density profile. In addition, our method recovers a model for
spontaneous zonal flow generation by negative viscosity, stabilized by
nonlinear and hyperviscous terms. We highlight the important role of symmetry
to implementation of the new method.Comment: To be published in Phys. Rev. E Rap. Comm. 6 pages, 7 figure
Suppression of Cross-Field Transport of a Passive Scalar in Two-Dimensional Magnetohydrodynamic Turbulence
The theory of passive scalar transport in two dimensional turbulent fluids is
generalized to the case of 2D MHD. Invariance of the cross correlation of
scalar concentration and magnetic potential produces a novel contribution to
the concentration flux. This pinch effect is proportional to the mean potential
gradient, and is shown to drastically reduce transport of the passive scalar
across the mean magnetic field when . Transport parallel to the mean magnetic
field is unchanged. Implications for models of transport in turbulent
magnetofluids are discussed.
PAC NOS. 47.25.Jn, 47.65.+aComment: uuencoded compressed postscript fil
Potential Vorticity Mixing in a Tangled Magnetic Field
A theory of potential vorticity (PV) mixing in a disordered (tangled)
magnetic field is presented. The analysis is in the context of -plane
MHD, with a special focus on the physics of momentum transport in the stably
stratified, quasi-2D solar tachocline. A physical picture of mean PV evolution
by vorticity advection and tilting of magnetic fields is proposed. In the case
of weak-field perturbations, quasi-linear theory predicts that the Reynolds and
magnetic stresses balance as turbulence Alfv\'enizes for a larger mean magnetic
field. Jet formation is explored quantitatively in the mean field-resistivity
parameter space. However, since even a modest mean magnetic field leads to
large magnetic perturbations for large magnetic Reynolds number, the physically
relevant case is that of a strong but disordered field. We show that numerical
calculations indicate that the Reynolds stress is modified well before
Alfv\'enization -- i.e. before fluid and magnetic energies balance. To
understand these trends, a double-average model of PV mixing in a stochastic
magnetic field is developed. Calculations indicate that mean-square fields
strongly modify Reynolds stress phase coherence and also induce a magnetic drag
on zonal flows. The physics of transport reduction by tangled fields is
elucidated and linked to the related quench of turbulent resistivity. We
propose a physical picture of the system as a resisto-elastic medium threaded
by a tangled magnetic network. Applications of the theory to momentum transport
in the tachocline and other systems are discussed in detail.Comment: 17 pages, 10 figures, 2 table
Impact of Resonant Magnetic Perturbations on Zonal Modes, Drift-Wave Turbulence and the L-H Transition Threshold
We study the effects of Resonant Magnetic Perturbations (RMPs) on turbulence,
flows and confinement in the framework of resistive drift-wave turbulence. This
work was motivated, in parts, by experiments reported at the IAEA 2010
conference [Y. Xu {\it et al}, Nucl. Fusion \textbf{51}, 062030] which showed a
decrease of long-range correlations during the application of RMPs. We derive
and apply a zero-dimensional predator-prey model coupling the Drift-Wave Zonal
Mode system [M. Leconte and P.H. Diamond, Phys. Plasmas \textbf{19}, 055903] to
the evolution of mean quantities. This model has both density gradient drive
and RMP amplitude as control parameters and predicts a novel type of transport
bifurcation in the presence of RMPs. This model allows a description of the
full L-H transition evolution with RMPs, including the mean sheared flow
evolution. The key results are: i) The L-I and I-H power thresholds \emph{both}
increase with RMP amplitude |\bx|, the relative increase of the L-I threshold
scales as \Delta P_{\rm LI} \propto |\bx|^2 \nu_*^{-2} \gyro^{-2}, where
is edge collisionality and \gyro is the sound gyroradius. ii) RMPs
are predicted to \emph{decrease} the hysteresis between the forward and
back-transition. iii) Taking into account the mean density evolution, the
density profile - sustained by the particle source - has an increased turbulent
diffusion compared with the reference case without RMPs which provides one
possible explanation for the \emph{density pump-out} effect.Comment: 30 pages, IAEA-based articl
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