7,173 research outputs found
Transverse Instabilities of Coasting Beams with Space Charge
Transverse beam stability is strongly affected by the beam space charge.
Usually it is analyzed with the rigid-beam model. However this model is only
valid when a bare (not affected by the space charge) tune spread is small
compared to the space charge tune shift. This condition specifies a relatively
small area of parameters which, however, is the most interesting for practical
applications. The Landau damping rate and the beam Schottky spectra are
computed assuming that validity condition is satisfied. The results are applied
to a round Gaussian beam. The stability thresholds are described by simple fits
for the cases of chromatic and octupole tune spreads.Comment: 6 pages, 2 figures, accepted by Phys. Rev. ST - Accel. Beam
The effect of electron surface scattering on fine metal particle electromagnetic radiation absorption
The magnetic dipole absorption cross section of a spherically shaped metal
particle was calculated in terms of kinetic approach. The particle considered
was placed in the field of a plane electromagnetic wave. The model of boundary
conditions was investigated taking into account the dependence of the
reflectivity coefficient both on the surface roughness parameter and on the
electron incidence angle. The results obtained were compared with theoretical
computation results for a model of combined diffusion-specular boundary
conditions of Fuchs.Comment: 9 pages, 5 figure
Strong effect of weak diffusion on scalar turbulence at large scales
Passive scalar turbulence forced steadily is characterized by the velocity
correlation scale, , injection scale, , and diffusive scale, . The
scales are well separated if the diffusivity is small, , and one
normally says that effects of diffusion are confined to smaller scales, . However, if the velocity is single scale one finds that a weak dependence
of the scalar correlations on the molecular diffusivity persists to even larger
scales, e.g. \cite{95BCKL}. We consider the case of
and report a counter-intuitive result -- the emergence of a new range of large
scales, , where the diffusivity shows a strong effect on
scalar correlations.Comment: 4 pages, 1 figure, submitted to Physics of Fluid
Transient Cherenkov radiation from an inhomogeneous string excited by an ultrashort laser pulse at superluminal velocity
An optical response of one-dimensional string made of dipoles with a
periodically varying density excited by a spot of light moving along the string
at the superluminal (sub-luminal) velocity is theoretically studied. The
Cherenkov radiation in such system is rather unusual, possessing both transient
and resonant character. We show that under certain conditions, in addition to
the resonant Cherenkov peak another Doppler-like frequency appears in the
radiation spectrum. Both linear (small-signal) and nonlinear regimes as well as
different string topologies are considered.Comment: accepted to Phys. Rev.
Growing condensate in two-dimensional turbulence
We report a numerical study, supplemented by phenomenological explanations,
of ``energy condensation'' in forced 2D turbulence in a biperiodic box.
Condensation is a finite size effect which occurs after the standard inverse
cascade reaches the size of the system. It leads to emergence of a coherent
vortex dipole. We show that the time growth of the dipole is self-similar, and
it contains most of the injected energy, thus resulting in an energy spectrum
which is markedly steeper than the standard one. Once the coherent
component is subtracted, however, the remaining fluctuations have a spectrum
close to . The fluctuations decay slowly as the coherent part grows.Comment: 4 pages, 4 figures. This version includes some additional
phenomenological explanations of the results, additional references and
improved figure
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