1,503 research outputs found
Collective excitations of atomic Bose-Einstein condensates
We apply linear-response analysis of the Gross-Pitaevskii equation to obtain
the excitation frequencies of a Bose-Einstein condensate confined in a
time-averaged orbiting potential trap. Our calculated values are in excellent
agreement with those observed in a recent experiment.Comment: 11 pages, 2 Postscript figures, uses psbox.tex for automatic figure
inclusion. More info at http://amo.phy.gasou.edu/bec.htm
Transparent boundary conditions - the pole condition approach
A new approach to derive transparent boundary conditions
(TBCs) for wave, Schr¨odinger and drift-diffusion
equations is presented. It relies on the pole condition approach and distinguishes physical reasonable and unreasonable solutions by the location of the singularities of the spatial Laplace transform U of the exterior solution. By the condition that U is analytic in some region TBCs are established. To realize the pole condition numerically, a Möbius transform is used to map the region of analyticity to the unit disc. There the Laplace transform is expanded in a power series. The equations coupling the coefficients of the power series with the interior provide the TBC. Numerical result for the damped wave equation show that the error introduced by truncating the power series decays exponentially in the number of coefficients
Transparent Boundary Conditions Based on the Pole Condition
Transparent boundary conditions for polygonal two-dimensional domains based on the pole condition approach are presented. The discretization of the exterior is done by innite trapezoids, which allows to dene a generalized distance variable. Taking the Laplace transform of the solution w.r.t the distance variable, incoming and outgoing solutions can be distinguished by the location of the singularities.
Using special ansatz and test functions, the condition
on the location of the singularities yields a new
algorithmic realization of transparent boundary conditions
Moisture - Gravity Wave Interactions in a Multiscale Environment
Starting from the conservation laws for mass, momentum and energy together with
a three species, bulk microphysic model, a model for the interaction of internal gravity waves and
deep convective hot towers is derived by using multiscale asymptotic techniques.
From the resulting leading order equations, a closed model is obtained by applying weighted
averages to the smallscale hot towers without requiring further closure approximations. The resulting
model is an extension of the linear, anelastic equations, into which moisture enters as the area fraction
of saturated regions on the microscale with two way coupling between the large and small scale.
Moisture reduces the effective stability in the model and defines a potential temperature sourceterm
related to the net effect of latent heat release or consumption by microscale up- and downdrafts.
The dispersion relation and group velocity of the system is analyzed and moisture is found to have
several effects: It reduces energy transport by waves, increases the vertical wavenumber but decreases
the slope at which wave packets travel and it introduces a lower horizontal cutoff wavenumber, below
which modes turn into evanescent. Further, moisture can cause critical layers.
Numerical examples for steady-state and time-dependent mountain waves are shown and the effects
of moisture on these waves are investigated
Single Particle Tracking for Simultaneous Long and Short Electron Bunches in the BESSY II Storage Ring
A scheme where 1.5 ps and 15 ps long bunches rms can be stored simultaneously in the BESSY II storage ring has recently been proposed BESSYVSR . This paper presents simulations of single particle beam dynamics influenced by superconducting cavities used for the strong longitudinal beam focusing. The effect of RF jitter on very short bunches is investigated and results are discussed. Furthermore, possible effects on beam dynamics during ramp up and ramp down of the high gradient fields in the cavities are studied. The primary goal is to reveal preliminary design specifications for RF jitter on the basis of single particle dynamic
Probing dipolar effects with condensate shape oscillation
We discuss the low energy shape oscillations of a magnetic trapped atomic
condensate including the spin dipole interaction. When the nominal isotropic
s-wave interaction strength becomes tunable through a Feshbach resonance (e.g.
as for Rb atoms), anisotropic dipolar effects are shown to be detectable
under current experimental conditions [E. A. Donley {\it et al.}, Nature {\bf
412}, 295 (2001)].Comment: revised version, submitte
Stationary states of the Gross-Pitaevskii equation with linear counterpart
We study the stationary solutions of the Gross-Pitaevskii equation that
reduce, in the limit of vanishing non-linearity, to the eigenfunctions of the
associated Schr\"odinger equation. By providing analytical and numerical
support, we conjecture an existence condition for these solutions in terms of
the ratio between their proper frequency (chemical potential) and the
corresponding linear eigenvalue. We also give approximate expressions for the
stationary solutions which become exact in the opposite limit of strong
non-linearity. For one-dimensional systems these solutions have the form of a
chain of dark or bright solitons depending on the sign of the non-linearity. We
demonstrate that in the case of negative non-linearity (attractive interaction)
the norm of the solutions is always bounded for dimensions greater than one.Comment: 15 pages, 6 figures, LaTe
Enhancement of the Deuteron-Fusion Reactions in Metals and its Experimental Implications
Recent measurements of the reaction d(d,p)t in metallic environments at very
low energies performed by different experimental groups point to an enhanced
electron screening effect. However, the resulting screening energies differ
strongly for divers host metals and different experiments. Here, we present new
experimental results and investigations of interfering processes in the
irradiated targets. These measurements inside metals set special challenges and
pitfalls which make them and the data analysis particularly error-prone. There
are multi-parameter collateral effects which are crucial for the correct
interpretation of the observed experimental yields. They mainly originate from
target surface contaminations due to residual gases in the vacuum as well as
from inhomogeneities and instabilities in the deuteron density distribution in
the targets. In order to address these problems an improved differential
analysis method beyond the standard procedures has been implemented. Profound
scrutiny of the other experiments demonstrates that the observed unusual
changes in the reaction yields are mainly due to deuteron density dynamics
simulating the alleged screening energy values. The experimental results are
compared with different theoretical models of the electron screening in metals.
The Debye-H\"{u}ckel model that has been previously proposed to explain the
influence of the electron screening on both nuclear reactions and radioactive
decays could be clearly excluded.Comment: 22 pages, 12 figures, REVTeX4, 2-column format. Submitted to Phys.
Rev. C; accepte
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