1,020 research outputs found
Strongly and Weakly Unstable Anisotropic Quark-Gluon Plasma
Using explicit solutions of the QCD transport equations, we construct an
effective potential for an anisotropic quark-gluon plasma which under plausible
assumptions holds beyond the Hard Loop approximation. The configurations, which
are unstable in the linear response approach, are characterized by a negative
quadratic term of the effective potential. The signs of higher order terms can
be either negative or positive, depending on the parton momentum distribution.
In the case of a Gaussian momentum distribution, the potential is negative and
unbound from below. Therefore, the modes, which are unstable for gauge fields
of small amplitude, remain unstable for arbitrary large amplitudes. We also
present an example of a momentum distribution which gives a negative quadratic
term of the effective potential but the whole potential has a minimum and it
grows for sufficiently large gauge fields. Then, the system is weakly unstable.
The character of the instability is important for the dynamical evolution of
the plasma system.Comment: 13 pages, 1 figure, revised, to appear in Phys. Rev.
Laser scattering by density fluctuations of ultra-cold atoms in a magneto-optical trap
We study the spectrum of density fluctuations in the ultra-cold gas of
neutral atoms, confined in a magneto-optical trap. We determine the
corresponding amplitude and spectra of laser light scattered by this medium. We
derive an expression for the dynamical structure function, by using a test
particle method. We propose to use the collective laser scattering as a
diagnostic method for the microscopic properties of the ultra-cold matter. This
will also allow us to check on the atomic correlations which are mediated by
the collective mean field inside the gas.Comment: J. Phys. B (in press
Theoretical studies of space plasmas Summary report, 3 May 1965 - 1 May 1966
Synchrotron radiation, ionospheric currents, auroral bombardment, and plasma instabilitie
Some remarks on one-dimensional force-free Vlasov-Maxwell equilibria
The conditions for the existence of force-free non-relativistic
translationally invariant one-dimensional (1D) Vlasov-Maxwell (VM) equilibria
are investigated using general properties of the 1D VM equilibrium problem. As
has been shown before, the 1D VM equilibrium equations are equivalent to the
motion of a pseudo-particle in a conservative pseudo-potential, with the
pseudo-potential being proportional to one of the diagonal components of the
plasma pressure tensor. The basic equations are here derived in a different way
to previous work. Based on this theoretical framework, a necessary condition on
the pseudo-potential (plasma pressure) to allow for force-free 1D VM equilibria
is formulated. It is shown that linear force-free 1D VM solutions, which so far
are the only force-free 1D VM solutions known, correspond to the case where the
pseudo-potential is an attractive central potential. A general class of
distribution functions leading to central pseudo-potentials is discussed.Comment: Physics of Plasmas, accepte
Gamma-Ray Burst Afterglow: Polarization and Analytic Light Curves
GRB afterglow polarization is discussed. We find an observable, up to 10%,
polarization, if the magnetic field coherence length grows at about the speed
of light after the field is generated at the shock front. Detection of a
polarized afterglow would show that collisionless ultrarelativistic shocks can
generate strong large scale magnetic fields and confirm the synchrotron
afterglow model. Non-detection, at a 1% level, would imply that either the
synchrotron emission model is incorrect, or that strong magnetic fields, after
they are generated in the shock, somehow manage to stay un-dissipated at
``microscopic'', skin depth, scales. Analytic lightcurves of synchrotron
emission from an ultrarelativistic self-similar blast wave are obtained for an
arbitrary electron distribution function, taking into account the effects of
synchrotron cooling. The peak synchrotron flux and the flux at frequencies much
smaller than the peak frequency are insensitive to the details of the electron
distribution function; hence their observational determination would provide
strong constraints on blast wave parameters.Comment: 19 pages, submitted to Ap
A Self-Consistent Marginally Stable State for Parallel Ion Cyclotron Waves
We derive an equation whose solutions describe self-consistent states of
marginal stability for a proton-electron plasma interacting with
parallel-propagating ion cyclotron waves. Ion cyclotron waves propagating
through this marginally stable plasma will neither grow nor damp. The
dispersion relation of these waves, {\omega} (k), smoothly rises from the usual
MHD behavior at small |k| to reach {\omega} = {\Omega}p as k \rightarrow
\pm\infty. The proton distribution function has constant phase-space density
along the characteristic resonant surfaces defined by this dispersion relation.
Our equation contains a free function describing the variation of the proton
phase-space density across these surfaces. Taking this free function to be a
simple "box function", we obtain specific solutions of the marginally stable
state for a range of proton parallel betas. The phase speeds of these waves are
larger than those given by the cold plasma dispersion relation, and the
characteristic surfaces are more sharply peaked in the v\bot direction. The
threshold anisotropy for generation of ion cyclotron waves is also larger than
that given by estimates which assume bi-Maxwellian proton distributions.Comment: in press in Physics of Plasma
Mirror instability in a plasma with cold gyrating dust particles
In this work linear stability analysis of a magnetized dusty plasma with an
anisotropic dust component having transversal motions much stronger than
motions parallel to the external magnetic field, and isotropic light plasma
components is described. Such a situation presumably establishes in a shock
compressed space dusty plasma downstream the shock front. Oblique low-frequency
magneto-hydrodynamic waves (, being the
dust cyclotron frequency) are shown to be undergone to the mirror instability.
Consequences for nonthermal dust destruction behind shock fronts in the
interstellar medium are discussed.Comment: 13 pages, 5 figs, accepted to Phys. Pasma
Self-consistent nonlinear kinetic simulations of the anomalous Doppler instability of suprathermal electrons in plasmas
Suprathermal tails in the distributions of electron velocities parallel to the magnetic field are found in many areas of plasma physics, from magnetic confinement fusion to solar system plasmas. Parallel electron kinetic energy can be transferred into plasma waves and perpendicular gyration energy of particles through the anomalous Doppler instability (ADI), provided that energetic electrons with parallel velocities v ≥ (ω + Ωce )/k are present; here Ωce denotes electron cyclotron frequency, ω the wave angular frequency and k the component of wavenumber parallel to the magnetic field. This phenomenon is widely observed in tokamak plasmas. Here we present the first fully self-consistent relativistic particle-in-cell simulations of the ADI, spanning the linear and nonlinear regimes of the ADI. We test the robustness of the analytical theory in the linear regime and follow the ADI through to the steady state. By directly evaluating the parallel and perpendicular dynamical contributions to j · E in the simulations, we follow the energy transfer between
the excited waves and the bulk and tail electron populations for the first time. We find that the ratio Ωce /(ωpe + Ωce ) of energy transfer between parallel and perpendicular, obtained from linear analysis, does not apply when damping is fully included, when we find it to be ωpe /(ωpe + Ωce ); here ωpe denotes the electron plasma frequency. We also find that the ADI can arise beyond the previously expected range of plasma parameters, in particular when Ωce > ωpe . The simulations also exhibit a spectral feature which may
correspond to observations of suprathermal narrowband emission at ωpe detected from low density tokamak plasmas
Jet-induced gauge field instabilities in the quark-gluon plasma: A kinetic theory approach
We discuss the properties of the collective modes of a system composed by a
thermalized quark-gluon plasma traversed by a relativistic jet of partons. The
transport equations obeyed by the components of the plasma and of the jet are
studied in the Vlasov approximation. Assuming that the partons in the jet can
be described with a tsunami-like distribution function we derive the
expressions of the dispersion law of the collective modes. Then the behavior of
the unstable gauge modes of the system is analyzed for various values of the
velocity of the jet, of the momentum of the collective modes and of the angle
between these two quantities. We find that the most unstable modes are those
with momentum orthogonal to the velocity of the jet and that these
instabilities appear when the velocity of the jet is higher than a threshold
value, which depends on the plasma and jet frequencies. The results obtained
within the Vlasov approximation are compared with the corresponding results
obtained using a chromohydrodynamical approach.The effect we discuss here
suggests a possible collective mechanism for the description of the jet
quenching phenomena in heavy ion collisions.Comment: 13 pages, 6 figure
Dispersion Relations for Bernstein Waves in a Relativistic Pair Plasma
A fully relativistic treatment of Bernstein waves in an electron-positron
pair plasma has remained too formidable a task owing to the very complex nature
of the problem. In this article, we perform contour integration of the
dielectric response function and numerically compute the dispersion curves for
a uniform, magnetized, relativistic electron-positron pair plasma. The behavior
of the dispersion solution for several cases with different plasma temperatures
is highlighted. In particular, we find two wave modes that exist only for large
wavelengths and frequencies similar to the cyclotron frequency in a moderately
relativistic pair plasma. The results presented here have important
implications for the study of those objects where a hot magnetized
electron-positron plasma plays a fundamental role in generating the observed
radiation.Comment: 8 pages, 8 figures, Accepted for publication by Phys. Rev. E with
minor change
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