131 research outputs found
Ripples in Tapped or Blown Powder
We observe ripples forming on the surface of a granular powder in a container
submitted from below to a series of brief and distinct shocks. After a few
taps, the pattern turns out to be stable against any further shock of the same
amplitude. We find experimentally that the characteristic wavelength of the
pattern is proportional to the amplitude of the shocks. Starting from
consideration involving Darcy's law for air flow through the porous granulate
and avalanche properties, we build up a semi-quantitative model which fits
satisfactorily the set of experimental observations as well as a couple of
additional experiments.Comment: 7 pages, four postscript figures, submitted PRL 11/19/9
Evolution on a smooth landscape
We study in detail a recently proposed simple discrete model for evolution on
smooth landscapes. An asymptotic solution of this model for long times is
constructed. We find that the dynamics of the population are governed by
correlation functions that although being formally down by powers of (the
population size) nonetheless control the evolution process after a very short
transient. The long-time behavior can be found analytically since only one of
these higher-order correlators (the two-point function) is relevant. We compare
and contrast the exact findings derived herein with a previously proposed
phenomenological treatment employing mean field theory supplemented with a
cutoff at small population density. Finally, we relate our results to the
recently studied case of mutation on a totally flat landscape.Comment: Revtex, 15 pages, + 4 embedded PS figure
Sub-wavelength waveguide loaded by a complementary electric metamaterial for vacuum electron devices
We report the electromagnetic properties of a waveguide loaded by complementary electric split ring resonators (CeSRRs) and the application of the waveguide in vacuum electronics. The S-parameters of the CeSRRs in free space are calculated using the HFSS code and are used to retrieve the effective permittivity and permeability in an effective medium theory. The dispersion relation of a waveguide loaded with the CeSRRs is calculated by two approaches: by direct calculation with HFSS and by calculation with the effective medium theory; the results are in good agreement. An improved agreement is obtained using a fitting procedure for the permittivity tensor in the effective medium theory. The gain of a backward wave mode of the CeSRR-loaded waveguide interacting with an electron beam is calculated by two methods: by using the HFSS model and traveling wave tube theory; and by using a dispersion relation derived in the effective medium model. Results of the two methods are in very good agreement. The proposed all-metal structure may be useful in miniaturized vacuum electron devices.United States. Department of Energy (Grant DE-SC0010075
Controlled Dynamics of Interfaces in a Vibrated Granular Layer
We present experimental study of a topological excitation, {\it interface},
in a vertically vibrated layer of granular material. We show that these
interfaces, separating regions of granular material oscillation with opposite
phases, can be shifted and controlled by a very small amount of an additional
subharmonic signal, mixed with the harmonic driving signal. The speed and the
direction of interface motion depends sensitively on the phase and the
amplitude of the subharmonic driving.Comment: 4 pages, 6 figures, RevTe
Dynamics of grain ejection by sphere impact on a granular bed
The dynamics of grain ejection consecutive to a sphere impacting a granular
material is investigated experimentally and the variations of the
characteristics of grain ejection with the control parameters are
quantitatively studied. The time evolution of the corona formed by the ejected
grains is reported, mainly in terms of its diameter and height, and favourably
compared with a simple ballistic model. A key characteristic of the granular
corona is that the angle formed by its edge with the horizontal granular
surface remains constant during the ejection process, which again can be
reproduced by the ballistic model. The number and the kinetic energy of the
ejected grains is evaluated and allows for the calculation of an effective
restitution coefficient characterizing the complex collision process between
the impacting sphere and the fine granular target. The effective restitution
coefficient is found to be constant when varying the control parameters.Comment: 9 page
Analysis and optimization of a free-electron laser with an irregular waveguide
Using a time-dependent approach the analysis and optimization of a planar
FEL-amplifier with an axial magnetic field and an irregular waveguide is
performed. By applying methods of nonlinear dynamics three-dimensional
equations of motion and the excitation equation are partly integrated in an
analytical way. As a result, a self-consistent reduced model of the FEL is
built in special phase space. The reduced model is the generalization of the
Colson-Bonifacio model and takes into account the intricate dynamics of
electrons in the pump magnetic field and the intramode scattering in the
irregular waveguide. The reduced model and concepts of evolutionary computation
are used to find optimal waveguide profiles. The numerical simulation of the
original non-simplified model is performed to check the effectiveness of found
optimal profiles. The FEL parameters are chosen to be close to the parameters
of the experiment (S. Cheng et al. IEEE Trans. Plasma Sci. 1996, vol. 24, p.
750), in which a sheet electron beam with the moderate thickness interacts with
the TE01 mode of a rectangular waveguide. The results strongly indicate that
one can improve the efficiency by a factor of five or six if the FEL operates
in the magnetoresonance regime and if the irregular waveguide with the
optimized profile is used
Projectile interactions in granular impact cratering
We present evidence for the interactions between a ball and the container
boundaries, as well as between two balls, that are mediated by the granular
medium during impact cratering. The presence of the bottom boundary affects the
final penetration depth only for low drop heights with shallow filling, in
which case, surprisingly, the penetration becomes deeper. By contrast the
presence of the side wall causes less penetration and also an effective
repulsion. Repulsion is also found for two balls dropped side-by-side
Sub-wavelength waveguide loaded by a complementary electric metamaterial for vacuum electron devices
Active Negative Index Metamaterial Powered by an Electron Beam
A novel active negative index metamaterial that derives its gain from an
electron beam is intro- duced. The metamaterial consists of a stack of
equidistant parallel metal plates perforated by a periodic array of holes
shaped as complementary split-ring resonators. It is shown that this structure
supports a negative-index transverse magnetic electromagnetic mode that can
resonantly interact with a relativistic electron beam. Such metamaterial can be
used as a coherent radiation source or a particle accelerator.Comment: 5 pages, 4 figure
Mutator Dynamics on a Smooth Evolutionary Landscape
We investigate a model of evolutionary dynamics on a smooth landscape which
features a ``mutator'' allele whose effect is to increase the mutation rate. We
show that the expected proportion of mutators far from equilibrium, when the
fitness is steadily increasing in time, is governed solely by the transition
rates into and out of the mutator state. This results is a much faster rate of
fitness increase than would be the case without the mutator allele. Near the
fitness equilibrium, however, the mutators are severely suppressed, due to the
detrimental effects of a large mutation rate near the fitness maximum. We
discuss the results of a recent experiment on natural selection of E. coli in
the light of our model.Comment: 4 pages, 3 figure
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