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

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

Fluid Models for Kinetic Effects on Coherent Nonlinear Alfven Waves. I. Fundamental Theory

Collisionless regime kinetic models for coherent nonlinear Alfven wave dynamics are studied using fluid moment equations with an approximate closure anzatz. Resonant particle effects are modelled by incorporating an additional term representing dissipation akin to parallel heat conduction. Unlike collisional dissipation, parallel heat conduction is presented by an integral operator. The modified derivative nonlinear Schrodinger equation thus has a spatially nonlocal nonlinear term describing the long-time evolution of the envelope of parallel-propagating Alfven waves, as well. Coefficients in the nonlinear terms are free of the 1/(1-beta) singularity usually encountered in previous analyses, and have very a simple form which clarifies the physical processes governing the large amplitude Alfvenic nonlinear dynamics. The nonlinearity appears via coupling of an Alfvenic mode to a kinetic ion-acoustic mode. Damping of the nonlinear Alfven wave appears via strong Landau damping of the ion-acoustic wave when the electron-to-ion temperature ratio is close to unity. For a (slightly) obliquely propagating wave, there are finite Larmor radius corrections in the dynamical equation. This effect depends on the angle of wave propagation relative to B_0 and vanishes for the limit of strictly parallel propagation. Explicit magnetic perturbation envelope equations amenable to further analysis and numerical solution are obtained. Implications of these models for collisionless shock dynamics are discussed.Comment: 34 pages (including 6 figures

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 $\nu_*$ 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

Nonlinear parallel momentum transport in strong turbulence

Most existing theoretical studies of momentum transport focus on calculating the Reynolds stress based on quasilinear theory, without considering the \emph{nonlinear} momentum flux-$$. However, a recent experiment on TORPEX found that the nonlinear toroidal momentum flux induced by blobs makes a significant contribution as compared to the Reynolds stress [Labit et al., Phys. Plasmas {\bf 18}, 032308 (2011)]. In this work, the nonlinear parallel momentum flux in strong turbulence is calculated by using three dimensional Hasegawa-Mima equation. It is shown that nonlinear diffusivity is smaller than quasilinear diffusivity from Reynolds stress. However, the leading order nonlinear residual stress can be comparable to the quasilinear residual stress, and so could be important to intrinsic rotation in tokamak edge plasmas. A key difference from the quasilinear residual stress is that parallel fluctuation spectrum asymmetry is not required for nonlinear residual stress

A long-term VLBA monitoring campaign of the v=1, J=1-0 SiO masers toward TX Cam - I. Morphology and Shock Waves

We present the latest and final version of the movie of the SiO masers toward the Mira variable TX Cam. The new version consists of 112 frames (78 successfully reduced epochs) with data covering almost three complete stellar cycles between 24 May 1997 to 25 January 2002, observed with the VLBA. In this paper we examine the global morphology, kinematics and variability of the masering zone. The morphology of the emission is confined in a structure that usually resembles a ring or an ellipse, with occasional deviations due to localised phenomena. The ring appears to be contracting and expanding, although for the first cycle contraction is not observed. The width and outer boundary of the masering zone follow the stellar pulsation. Our data seem to be consistent with a shock being created once per stellar cycle at maximum that propagates with a velocity of ~7 km/s. The difference in velocities along different axes strongly suggests that the outflow in TX Cam is bipolar. The contribution of projection is examined and our results are compared with the latest theoretical model

Breaking Kelvin: Circulation conservation and vortex breakup in MHD at low Magnetic Prandtl Number

In this paper we examine the role of weak magnetic fields in breaking Kelvin's circulation theorem and in vortex breakup in two-dimensional magnetohydrodynamics for the physically important case of a low magnetic Prandtl number (low $Pm$) fluid. We consider three canonical inviscid solutions for the purely hydrodynamical problem, namely a Gaussian vortex, a circular vortex patch and an elliptical vortex patch. We examine how magnetic fields lead to an initial loss of circulation $\Gamma$ and attempt to derive scaling laws for the loss of circulation as a function of field strength and diffusion as measured by two non-dimensional parameters. We show that for all cases the loss of circulation depends on the integrated effects of the Lorentz force, with the patch cases leading to significantly greater circulation loss. For the case of the elliptical vortex the loss of circulation depends on the total area swept out by the rotating vortex and so this leads to more efficient circulation loss than for a circular vortex.Comment: 21 pages, 12 figure

Plus Charge Prevalence in Cosmic Rays: Room for Dark Matter in the Positron Spectrum

The unexpected energy spectrum of the positron/electron ratio is interpreted astrophysically, with a possible exception of the 100-300 GeV range. The data indicate that this ratio, after a decline between $0.5-8$ GeV, rises steadily with a trend towards saturation at 200-400GeV. These observations (except for the trend) appear to be in conflict with the diffusive shock acceleration (DSA) mechanism, operating in a \emph{single} supernova remnant (SNR) shock. We argue that $e^{+}/e^{-}$ ratio can still be explained by the DSA if positrons are accelerated in a \emph{subset} of SNR shocks which: (i) propagate in clumpy gas media, and (ii) are modified by accelerated CR \emph{protons}. The protons penetrate into the dense gas clumps upstream to produce positrons and, \emph{charge the clumps positively}. The induced electric field expels positrons into the upstream plasma where they are shock-accelerated. Since the shock is modified, these positrons develop a harder spectrum than that of the CR electrons accelerated in other SNRs. Mixing these populations explains the increase in the $e^{+}/e^{-}$ ratio at $E>8$ GeV. It decreases at $E<8$ GeV because of a subshock weakening which also results from the shock modification. Contrary to the expelled positrons, most of the antiprotons, electrons, and heavier nuclei, are left unaccelerated inside the clumps. Scenarios for the 100-300 GeV AMS-02 fraction exceeding the model prediction, including, but not limited to, possible dark matter contribution, are also discussed.Comment: 36 pages, 6 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

Spontaneous Transport Barriers Quench Turbulent Resistivity in 2D MHD

This Letter identifies the physical mechanism for the quench of turbulent resistivity in 2D MHD. Without an imposed, ordered magnetic field, a multi-scale, blob-and-barrier structure of magnetic potential forms spontaneously. Magnetic energy is concentrated in thin, linear barriers, located at the interstices between blobs. The barriers quench the transport and kinematic decay of magnetic energy. The local transport bifurcation underlying barrier formation is linked to the inverse cascade of $\langle A^2\rangle$ and negative resistivity, which induce local bistability. For small scale forcing, spontaneous layering of the magnetic potential occurs, with barriers located at the interstices between layers. This structure is effectively a magnetic staircase