873 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
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
Ground-State Properties of a Rotating Bose-Einstein Condensate with Attractive Interaction
The ground state of a rotating Bose-Einstein condensate with attractive
interaction in a quasi-one-dimensional torus is studied in terms of the ratio
of the mean-field interaction energy per particle to the
single-particle energy-level spacing. The plateaus of quantized circulation are
found to appear if and only if with the lengths of the plateaus
reduced due to hybridization of the condensate over different angular-momentum
states.Comment: 4 pages, 2 figures, Accepted for publication in Physical Reveiw
Letter
RF Feedback and Detuning Studies for the BESSY Variable Pulse Length Storage Ring Higher Harmonic SC Cavities
For the feasibility of the BESSY VSR upgrade project of BESSY II two higher harmonic systems at a factor of 3 and 3.5 of the ring s RF fundamental of 500 MHz will be installed in the ring. Operating in continuous wave at high average accelerating field of 20 MV m and phased at zerocrossing, the superconducting cavities have to be detuned within tight margins to ensure stable operation and lowpower consumption at a loaded Q of 5 107. The field variation of the cavities is mainly driven by the repetitive transient beam loading of the envisaged complex bunch fill pattern in the ring. Within this work combined LLRF cavity and longitudinal beam dynamics simulation will demonstrate the limits for stable operation, especially the coupling between synchrotron oscillation and RF feedback settings. Further impact by beam current decay and top up injection shots are being simulate
NGC 6738: not a real open cluster
A photometric, astrometric and spectroscopic investigation of the poorly
studied open cluster NGC 6738 has been performed in order to ascertain its real
nature. NGC 6738 is definitely not a physical stellar ensemble: photometry does
not show a defined mean sequence, proper motions and radial velocities are
randomly distributed, spectro-photometric parallaxes range between 10 and 1600
pc, and the apparent luminosity function is identical to that of the
surrounding field. NGC 6738 therefore appears to be an apparent concentration
of a few bright stars projected on patchy background absorption.Comment: A&A, in press (compared with first submission to astro-ph, now Table
2 and Figure 4 are replaced with corrected versions
Influence of Transient Beam Loading on the Longitudinal Beam Dynamics at BESSY VSR
BESSY VSR, a scheme where 1.7 ps and 15 ps long bunches rms can be stored simultaneously in the BESSY II storage ring has recently been proposed [1]. The strong longitudinal bunch focusing is achieved by superconducting high gradient RF cavities. If the bunch fill pattern exhibits a significant inhomogeneity, e.g. due to gaps, transient beam loading causes a distortion of the longitudinal phase space which is different for each bunch. The result are variations along the fill pattern in synchronous phase, synchrotron fre quency and bunch shape. This paper presents investigations of transient beam loading and depicts the consequences on bunch length, phase stability and longitudinal multi bunch oscillations for the projected setup of BESSY VS
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
Beyond the Thomas-Fermi approximation for a trapped condensed Bose-Einstein gas
Corrections to the zero-temperature Thomas-Fermi description of a dilute
interacting condensed Bose-Einstein gas confined in an isotropic harmonic trap
arise due to the presence of a boundary layer near the condensate surface.
Within the Bogoliubov approximation, the various contributions to the
ground-state condensate energy all have terms of order R^{-4}ln R and R^{-4},
where R is the number-dependent dimensionless condensate radius in units of the
oscillator length. The zero-order hydrodynamic density-fluctuation amplitudes
are extended beyond the Thomas-Fermi radius through the boundary layer to
provide a uniform description throughout all space. The first-order correction
to the excitation frequencies is shown to be of order R^{-4}.Comment: 12 pages, 2 figures, revtex. Completely revised discussion of the
boundary-layer corrections to collective excitations, and two new figures
added. To appear in Phys. Rev. A (October, 1998
Structure and stability of bosonic clouds: alkali atoms with negative scattering length
We investigate the form and stability of a cloud of atoms confined in a
harmonic trap when the scattering length is negative. We find that, besides the
known low density metastable solution, a new branch of Bose condensate appears
at higher density when non locality effects in the attractive part are taken
into account. The transition between the two classes of solutions as a function
of the number of atoms can be either sharp or smooth according to the
strength and range of the attractive interaction. Use of tight traps is
favorable for investigating the evolution of the system as the strength of the
effective interaction increases with .Comment: 11 pages, Latex, 2 figures, to be published in Phys. Rev.
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