950 research outputs found
A naked singularity stable under scalar field perturbations
We prove the stability of a spacetime with a naked singularity under scalar
field perturbations, where the perturbations are regular at the singularity.
This spacetime, found by Janis, Newman and Winicour, and independently by
Wyman, is sourced by a massless scalar field and also arises as a certain limit
of a class of charged dilatonic solutions in string theory. This stability
result opens up specific questions for investigation related to the cosmic
censorship conjecture and the mechanism by which it is implemented in nature.Comment: 19 pages, version to appear in IJMPD, references adde
Nonlinear Electron Oscillations in a Viscous and Resistive Plasma
New non-linear, spatially periodic, long wavelength electrostatic modes of an
electron fluid oscillating against a motionless ion fluid (Langmuir waves) are
given, with viscous and resistive effects included. The cold plasma
approximation is adopted, which requires the wavelength to be sufficiently
large. The pertinent requirement valid for large amplitude waves is determined.
The general non-linear solution of the continuity and momentum transfer
equations for the electron fluid along with Poisson's equation is obtained in
simple parametric form. It is shown that in all typical hydrogen plasmas, the
influence of plasma resistivity on the modes in question is negligible. Within
the limitations of the solution found, the non-linear time evolution of any
(periodic) initial electron number density profile n_e(x, t=0) can be
determined (examples). For the modes in question, an idealized model of a
strictly cold and collisionless plasma is shown to be applicable to any real
plasma, provided that the wavelength lambda >> lambda_{min}(n_0,T_e), where n_0
= const and T_e are the equilibrium values of the electron number density and
electron temperature. Within this idealized model, the minimum of the initial
electron density n_e(x_{min}, t=0) must be larger than half its equilibrium
value, n_0/2. Otherwise, the corresponding maximum n_e(x_{max},t=tau_p/2),
obtained after half a period of the plasma oscillation blows up. Relaxation of
this restriction on n_e(x, t=0) as one decreases lambda, due to the increase of
the electron viscosity effects, is examined in detail. Strong plasma viscosity
is shown to change considerably the density profile during the time evolution,
e.g., by splitting the largest maximum in two.Comment: 16 one column pages, 11 figures, Abstract and Sec. I, extended, Sec.
VIII modified, Phys. Rev. E in pres
High Energy Electron Confinement in a Magnetic Cusp Configuration
We report experimental results validating the concept that plasma confinement
is enhanced in a magnetic cusp configuration when beta (plasma
pressure/magnetic field pressure) is order of unity. This enhancement is
required for a fusion power reactor based on cusp confinement to be feasible.
The magnetic cusp configuration possesses a critical advantage: the plasma is
stable to large scale perturbations. However, early work indicated that plasma
loss rates in a reactor based on a cusp configuration were too large for net
power production. Grad and others theorized that at high beta a sharp boundary
would form between the plasma and the magnetic field, leading to substantially
smaller loss rates. The current experiment validates this theoretical
conjecture for the first time and represents critical progress toward the
Polywell fusion concept which combines a high beta cusp configuration with an
electrostatic fusion for a compact, economical, power-producing nuclear fusion
reactor.Comment: 12 pages, figures included. 5 movies in Ancillary file
Space-filter techniques for quasi-neutral hybrid-kinetic models
The space-filter approach has proved a fundamental tool in studying
turbulence in neutral fluids, providing the ability to analyze scale-to-scale
energy transfer in configuration space. It is well known that turbulence in
plasma presents challenges different from neutral fluids, especially when the
scale of interests include kinetic effects. The space-filter approach is still
largely unexplored for kinetic plasma. Here we derive the space-filtered (or,
equivalently "coarse-grained") equations in configuration space for a
quasi-neutral hybrid-kinetic plasma model, in which ions are fully kinetic and
electrons are a neutralizing fluid. Different models and closures for the
electron fluid are considered, including finite electron-inertia effects and
full electrons' pressure-tensor dynamics. Implications for the cascade of
turbulent fluctuations in real space depending on different approximations are
discussed.Comment: 43 pages, 2 figure
Thermalization vs. Isotropization & Azimuthal Fluctuations
Hydrodynamic description requires a local thermodynamic equilibrium of the
system under study but an approximate hydrodynamic behaviour is already
manifested when a momentum distribution of liquid components is not of
equilibrium form but merely isotropic. While the process of equilibration is
relatively slow, the parton system becomes isotropic rather fast due to the
plasma instabilities. Azimuthal fluctuations observed in relativistic heavy-ion
collisions are argued to distinguish between a fully equilibrated and only
isotropic parton system produced in the collision early stage.Comment: 12 pages, presented at `Correlations and Fluctuations in Relativistic
Nuclear Collisions', MIT, April 05, minor correction
A Coronal Hole's Effects on CME Shock Morphology in the Inner Heliosphere
We use STEREO imagery to study the morphology of a shock driven by a fast
coronal mass ejection (CME) launched from the Sun on 2011 March 7. The source
region of the CME is located just to the east of a coronal hole. The CME ejecta
is deflected away from the hole, in contrast with the shock, which readily
expands into the fast outflow from the coronal hole. The result is a CME with
ejecta not well centered within the shock surrounding it. The shock shape
inferred from the imaging is compared with in situ data at 1 AU, where the
shock is observed near Earth by the Wind spacecraft, and at STEREO-A. Shock
normals computed from the in situ data are consistent with the shock morphology
inferred from imaging.Comment: to appear in The Astrophysical Journa
Parameter dependence of magnetized CMB observables
Pre-decoupling magnetic fields affect the scalar modes of the geometry and
produce observable effects which can be constrained also through the use of
current (as opposed to forthcoming) data stemming from the Cosmic Microwave
Background observations. The dependence of the temperature and polarization
angular power spectra upon the parameters of an ambient magnetic field is
encoded in the scaling properties of a set of basic integrals whose derivation
is simplified in the limit of small angular scales. The magnetically-induced
distortions patterns of the relevant observables can be computed analytically
by employing scaling considerations which are corroborated by numerical
results.Comment: 48 pages, 11 figures; corrected minor typos; discussions added; to
appear in Physical Revie
Effective dynamics of a nonabelian plasma out of equilibrium
Starting from kinetic theory, we obtain a nonlinear dissipative formalism
describing the nonequilibrium evolution of scalar colored particles coupled
selfconsistently to nonabelian classical gauge fields. The link between the
one-particle distribution function of the kinetic description and the variables
of the effective theory is determined by extremizing the entropy production.
This method does not rely on the usual gradient expansion in fluid dynamic
variables, and therefore the resulting effective theory can handle situations
where these gradients (and hence the momentum-space anisotropies) are expected
to be large. The formalism presented here, being computationally less demanding
than kinetic theory, may be useful as a simplified model of the dynamics of
color fields during the early stages of heavy ion collisions and in phenomena
related to parton energy loss.Comment: 20 two-column pages, 2 figures. v3: minor changes. Accepted for
publication in Phys. Rev.
Free streaming in mixed dark matter
Free streaming in a \emph{mixture} of collisionless non-relativistic dark
matter (DM) particles is studied by implementing methods from the theory of
multicomponent plasmas. The mixture includes Fermionic, condensed and non
condensed Bosonic particles decoupling in equilibrium while relativistic, heavy
non-relativistic thermal relics (WIMPs), and sterile neutrinos that decouple
\emph{out of equilibrium} when they are relativistic. The free-streaming length
is obtained from the marginal zero of the gravitational
polarization function, which separates short wavelength Landau-damped from long
wavelength Jeans-unstable \emph{collective} modes. At redshift we find ,where are the \emph{fractions} of the respective DM components of mass
that decouple when the effective number of ultrarelativistic degrees of
freedom is , and only depend on the distribution functions at
decoupling, given explicitly in all cases. If sterile neutrinos produced either
resonantly or non-resonantly that decouple near the QCD scale are the
\emph{only} DM component,we find (non-resonant), (resonant).If WIMPs with
decoupling at are present in the mixture with
, is \emph{dominated} by CDM. If a Bose Einstein condensate is a DM
component its free streaming length is consistent with CDM because of the
infrared enhancement of the distribution function.Comment: 19 pages, 2 figures. More discussions same conclusions and results.
Version to appear in Phys. Rev.
- …