2,395,909 research outputs found
Pressure anisotropy generation in a magnetized plasma configuration with a shear flow velocity
The nonlinear evolution of the Kelvin Helmholtz instability in a magnetized
plasma with a perpendicular flow close to, or in, the supermagnetosonic regime
can produce a significant parallel-to-perpendicular pressure anisotropy. This
anisotropy, localized inside the flow shear region, can make the configuration
unstable either to the mirror or to the firehose instability and, in general,
can affect the development of the KHI. The interface between the solar wind and
the Earth's magnetospheric plasma at the magnetospheric equatorial flanks
provides a relevant setting for the development of this complex nonlinear
dynamics.Comment: 11 pages, 7 figures, submitted to Plasma Phys. Control. Fusio
Faceted anomalous scaling in the epitaxial growth of semiconductor films
We apply the generic dynamical scaling theory (GDST) to the surfaces of CdTe
polycrystalline films grown in glass substrates. The analysed data were
obtained with a stylus profiler with an estimated resolution lateral resolution
of m. Both real two-point correlation function and power spectrum
analyses were done. We found that the GDST applied to the surface power spectra
foresees faceted morphology in contrast with the self-affine surface indicated
by the local roughness exponent found via the height-height correlation
function. This inconsistency is explained in terms of convolution effects
resulting from the finite size of the probe tip used to scan the surfaces. High
resolution AFM images corroborates the predictions of GDST.Comment: to appear in Europhysics Letter
Kinetic cascade in solar-wind turbulence: 3D3V hybrid-kinetic simulations with electron inertia
Understanding the nature of the turbulent fluctuations below the ion
gyroradius in solar-wind turbulence is a great challenge. Recent studies have
been mostly in favor of kinetic Alfv\'en wave (KAW) type of fluctuations, but
other kinds of fluctuations with characteristics typical of magnetosonic,
whistler and ion Bernstein modes, could also play a role depending on the
plasma parameters. Here we investigate the properties of the sub-proton-scale
cascade with high-resolution hybrid-kinetic simulations of freely-decaying
turbulence in 3D3V phase space, including electron inertia effects. Two proton
plasma beta are explored: the "intermediate" and "low"
regimes, both typically observed in solar wind and corona. The
magnetic energy spectum exhibits and power laws
at , while they are slightly steeper at . Nevertheless,
both regimes develop a spectral anisotropy consistent with at , and pronounced small-scale intermittency.
In this context, we find that the kinetic-scale cascade is dominated by
KAW-like fluctuations at , whereas the low- case presents a
more complex scenario suggesting the simultaneous presence of different types
of fluctuations. In both regimes, however, a non-negligible role of ion
Bernstein type of fluctuations at the smallest scales seems to emerge.Comment: 6 pages, 5 figures, final version published in The Astrophysical
Journal Letters: Cerri, Servidio & Califano, ApJL 846, L18 (2017
Coordinate representation for non Hermitian position and momentum operators
In this paper we undertake an analysis of the eigenstates of two non
self-adjoint operators and similar, in a suitable sense, to
the self-adjoint position and momentum operators and
usually adopted in ordinary quantum mechanics. In particular we discuss
conditions for these eigenstates to be {\em biorthogonal distributions}, and we
discuss few of their properties. We illustrate our results with two examples,
one in which the similarity map between the self-adjoint and the non
self-adjoint is bounded, with bounded inverse, and the other in which this is
not true. We also briefly propose an alternative strategy to deal with
and , based on the so-called {\em quasi *-algebras}.Comment: Accepted in Proceedings of the Royal Society
Tailored laser pulse chirp to maintain optimum radiation pressure acceleration of ions
Ion beams generated with ultra-intense lasers-plasma accelerators hold
promises to provide compact and affordable beams of relativistic ions. One of
the most efficient acceleration setups was demonstrated to be direct
acceleration by the laser's radiation pressure. Due to plasma instabilities
developing in the ultra-thin foils required for radiation pressure
acceleration, however, it is challenging to maintain stable acceleration over
long distances. Recent studies demonstrated, on the other hand, that specially
tailored laser pulses can shorten the required acceleration distance
suppressing the onset of plasma instabilities. Here we extend the concept of
specific laser pulse shapes to the experimentally accessible parameter of a
frequency chirp. We present a novel analysis of how a laser pulse chirp may be
used to drive a foil target constantly maintaining optimal radiation pressure
acceleration conditions for in dependence on the target's areal density and the
laser's local field strength. Our results indicate that an appropriately
frequency chirped laser pulse yields a significantly enhanced acceleration to
higher energies and over longer distances suppressing the onset of plasma
instabilities.Comment: 7 pages, 4 figure
Recent studies on flame stabilization of premixed turbulent gases
FLAME stabilization is of importance in the practical design
of ramjets and afterburners. It has been studied
extensively in recent years, particularly with reference to
bluff-body flame-holders. In the present survey we describe
the investigations relating to flame holding by bluff bodies as well as new techniques (e.g.,. flame holding by the use of reverse jets) which may prove to be of practical importance in new engine configurations. In Section II we consider the flow field downstream of a bluff-body flame-holder which includes the recirculation zone behind the body and a region of flame spreading farther downstream.
Explicit reference is made to crucial experiments which illustrate the nature and magnitude of the velocity field, the physical extent, the temperature, and the gas composition of the recirculation zone. Experimental studies and theoretical predictions of the angle of flame spreading, as well as some observations on unstable flow and the onset of blowoff, will be reviewed.
The variation of blowoff velocity with flame-holder design,
pressure, and mixture composition is considered briefly in
Section III both for single and for adjacent bluff bodies. Also included is a summary of results for blowoff velocities obtained with a reverse-jet flame-holder and with wall recesses. Theoretical studies on the mechanism of flame stabilization form the subject of Section IV. We shall indicate the points on which various proposed models agree and disagree with experiment and attempt to formulate a composite description which is consistent with most of the currently available experimental data both for bluff-body and for reverse-jet flameholders
Charm and longitudinal structure functions with the Kharzeev-Levin-Nardi model
We use the Kharzeev-Levin-Nardi model of the low gluon distributions to
fit recent HERA data on charm and longitudinal structure functions. Having
checked that this model gives a good description of the data, we use it to
predict and to be measured in a future electron-ion collider. The
results interpolate between those obtained with the de Florian-Sassot and
Eskola-Paukkunen-Salgado nuclear gluon distributions. The conclusion of this
exercise is that the KLN model, simple as it is, may still be used as an
auxiliary tool to make estimates both for heavy ion and electron-ion
collisions.Comment: 6 pages, 7 figure
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