442 research outputs found
Vortex soliton tori with multiple nested phase singularities in dissipative media
We show the existence of stable two- and three-dimensional vortex solitons
carrying multiple, spatially separated, single-charge topological dislocations
nested around a vortex-ring core. Such new nonlinear states are supported by
elliptical gain landscapes in focusing nonlinear media with two-photon
absorption. The separation between the phase dislocations is dictated mostly by
the geometry of gain landscape and it only slightly changes upon variation of
the gain or absorption strength.Comment: 17 pages, 5 figures, to appear in Physical Review
Low Mach-number collisionless electrostatic shocks and associated ion acceleration
The existence and properties of low Mach-number () electrostatic
collisionless shocks are investigated with a semi-analytical solution for the
shock structure. We show that the properties of the shock obtained in the
semi-analytical model can be well reproduced in fully kinetic Eulerian
Vlasov-Poisson simulations, where the shock is generated by the decay of an
initial density discontinuity. Using this semi-analytical model, we study the
effect of electron-to-ion temperature ratio and presence of impurities on both
the maximum shock potential and Mach number. We find that even a small amount
of impurities can influence the shock properties significantly, including the
reflected light ion fraction, which can change several orders of magnitude.
Electrostatic shocks in heavy ion plasmas reflect most of the hydrogen impurity
ions.Comment: In Plasma Physics and Controlled Fusio
Boundary effect of a partition in a quantum well
The paper wishes to demonstrate that, in quantum systems with boundaries,
different boundary conditions can lead to remarkably different physical
behaviour. Our seemingly innocent setting is a one dimensional potential well
that is divided into two halves by a thin separating wall. The two half wells
are populated by the same type and number of particles and are kept at the same
temperature. The only difference is in the boundary condition imposed at the
two sides of the separating wall, which is the Dirichlet condition from the
left and the Neumann condition from the right. The resulting different energy
spectra cause a difference in the quantum statistically emerging pressure on
the two sides. The net force acting on the separating wall proves to be nonzero
at any temperature and, after a weak decrease in the low temperature domain, to
increase and diverge with a square-root-of-temperature asymptotics for high
temperatures. These observations hold for both bosonic and fermionic type
particles, but with quantitative differences. We work out several analytic
approximations to explain these differences and the various aspects of the
found unexpectedly complex picture.Comment: LaTeX (with iopart.cls, iopart10.clo and iopart12.clo), 28 pages, 17
figure
Quantum Force Induced on a Partition Wall in a Harmonic Potential
Boundary effects in quantum mechanics are examined by considering a partition
wall inserted at the centre of a harmonic oscillator system. We put an equal
number of particles on both sides of the impenetrable wall keeping the system
under finite temperatures. When the wall admits distinct boundary conditions on
the two sides, then a net force is induced on the wall. We study the
temperature behaviour of the induced force both analytically and numerically
under the combination of the Dirichlet and the Neumann conditions, and
determine its scaling property for two statistical cases of the particles:
fermions and bosons. We find that the force has a nonvanishing limit at zero
temperature T = 0 and exhibits scalings characteristic to the statistics of the
particles. We also see that for higher temperatures the force decreases
according to 1/sqrt{T}, in sharp contrast to the case of the infinite potential
well where it diverges according to sqrt{T}. The results suggest that, if such
a nontrivial partition wall can be realized, it may be used as a probe to
examine the profile of the potentials and the statistics of the particles
involved.Comment: 22 pages, 7 figures, typos corrected, references adde
One dimensional drift-diffusion between two absorbing boundaries: application to granular segregation
Motivated by a novel method for granular segregation, we analyze the one
dimensional drift-diffusion between two absorbing boundaries. The time
evolution of the probability distribution and the rate of absorption are given
by explicit formulae, the splitting probability and the mean first passage time
are also calculated. Applying the results we find optimal parameters for
segregating binary granular mixtures.Comment: RevTeX, 5 pages, 6 figure
Analysis of climate change scenarios based on modelling of the seasonal dynamics of a Danubian copepod species
Climate change is one of the most crucial ecological problems of our age with great influence.
Seasonal dynamics of aquatic communities are â among others â regulated by the climate, especially
by temperature. In this case study we attempted the simulation modelling of the seasonal dynamics of a
copepod species, Cyclops vicinus, which ranks among the zooplankton community, based on a
quantitative database containing ten years of data from the Danubeâs Göd area. We set up a simulation
model predicting the abundance of Cyclops vicinus by considering only temperature as it affects the
abundance of population. The model was adapted to eight years of daily temperature data observed
between 1981 and 1994 and was tested successfully with the additional data of two further years. The
model was run with the data series of climate change scenarios specified for the period around 2070-
2100. On the other hand we looked for the geographically analogous areas with the Göd region which are
mostly similar to the future climate of the Göd area. By means of the above-mentioned points we can get
a view how the climate of the region will change by the end of the 21st century, and the way the seasonal
dynamics of a chosen planktonic crustacean species may follow this change. According to our results the
area of Göd will be similar to the northern region of Greece. The maximum abundance of the examined
species occurs a month to one and a half months earlier, moreover larger variances are expected between
years in respect of the abundance. The deviations are expected in the direction of smaller or significantly
larger abundance not observed earlier
Topological light bullets supported by spatio-temporal gain
We reveal that the competition between diffraction, cubic nonlinearity,
two-photon absorption, and gain localized in both space and time results in
arrest of collapse, suppression of azimuthal modula-tion instabilities for
spatiotemporal wavepackets, and formation of stable three-dimensional light
bul-lets. We show that Gaussian spatiotemporal gain landscapes support bright,
fundamental light bullets, while gain landscapes featuring a ring-like spatial
and a Gaussian temporal shapes may support stable vortex bullets carrying
topological phase dislocations.Comment: 19 pages, 5 figures, to appear in Physical Review
Experimental determination of the He(,)Be reaction cross section above the Be proton separation threshold
The He(,)Be reaction plays a major role both in the
BBN producing the majority of the primordial Li, and in the pp-chain, where
it is the branching point. As a few-nucleon system, this reaction is often used
to validate ab-initio theoretical calculations and/or test R-matrix theory and
code implementations. For the latter, experimental data in an extended energy
range is of crucial importance to test the fit and extrapolation capabilities
of the different codes. The He(,)Be reaction cross
section has been measured by several groups up to the first resonance
( MeV) in the reaction. However, only one dataset exists
above the Be proton separation threshold measured in a narrow energy range
( MeV). In this work we extend the available experimental
capture cross section database to the energy range of known Be levels. The
activation method was used. The experiment was performed using a thin-window
gas cell with two high-purity Al foils as entrance and exit windows. The
activity of the Be nuclei implanted in the exit/catcher foil was measured
by detecting the yield of the emitted ~rays using shielded high-purity
germanium detectors. New experimental He(,)Be reaction
cross section data were obtained for the first time in the
MeV energy region. The new dataset with about 0.2 MeV step covers the energy
range of known levels and particle separation thresholds. No prominent
structures are observer around the Be levels. The reaction cross section is
slowly increasing with increasing energy. Above the Li threshold, a
decrease starts in the cross section trend. The overall structure of the cross
section suggest a broad resonance peaking around MeV Be
excitation energy, with a width of 8 MeV.Comment: Accepted for publication in PR
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