3,108 research outputs found
Analysis of the effects of baffles on combustion instability
An analytical model has been developed for predicting the effects of baffles on combustion instability. This model has been developed by coupling an acoustic analysis of the wave motion within baffled chambers with a model for the oscillatory combustion response of a propellant droplet developed by Heidmann. A computer program was developed for numerical solution of the resultant coupled equations. Diagnostic calculations were made to determine the reasons for the improper prediction. These calculations showed that the chosen method of representing the combustion response was a very poor approximation. At the end of the program, attempts were made to minimize this effect but the model still improperly predicts the stability trends. Therefore, it is recommended that additional analysis be done with an improved approximation
A Reconnaissance of the Deeper Jamaican Coral Reef Fish Communities
A submersible was used to make repetitive dives in Jamaica to depths of 25 m, 50 m, and 100 m. With increasing depth there was a decline in both species and the number of individuals. The territorial damselfish and mixed-species groups of herbivorous fishes were conspicuously absent at 100 m. Few unique species appeared with increasing depth and thus the deep community resembled depauperate versions of the shallower communities. Twelve species were shared between the 3 depths but there was no significant correlation in ranked relative abundance. Thus increasing depth also influenced community organization
Novel insights into transfer processes in the reaction 16O+208Pb at sub-barrier energies
The collision of the doubly-magic nuclei O+Pb is a benchmark
in nuclear reaction studies. Our new measurements of back-scattered
projectile-like fragments at sub-barrier energies show show that transfer of 2
protons () is much more probable than -particle transfer.
transfer probabilities are strongly enhanced compared to expectations for the
sequential transfer of two uncorrelated protons; at energies around the fusion
barrier absolute probabilities for two proton transfer are similar to those for
one proton transfer. This strong enhancement indicates strong pairing
correlations in O, and suggests evidence for the occurrence of a nuclear
supercurrent of two-proton Cooper pairs in this reaction, already at energies
well below the fusion barrier.Comment: 5 pages, 3 figure
Effects of Nuclear Structure on Quasi-fission
The quasi-fission mechanism hinders fusion of heavy systems because of a mass
flow between the reactants, leading to a re-separation of more symmetric
fragments in the exit channel. A good understanding of the competition between
fusion and quasi-fission mechanisms is expected to be of great help to optimize
the formation and study of heavy and superheavy nuclei. Quantum microscopic
models, such as the time-dependent Hartree-Fock approach, allow for a treatment
of all degrees of freedom associated to the dynamics of each nucleon. This
provides a description of the complex reaction mechanisms, such as
quasi-fission, with no parameter adjusted on reaction mechanisms. In
particular, the role of the deformation and orientation of a heavy target, as
well as the entrance channel magicity and isospin are investigated with
theoretical and experimental approaches.Comment: Invited talk to NSRT12. To be published in Eur. Phys. J. Web of Con
A Problematic Set of Two-Loop Self-Energy Corrections
We investigate a specific set of two-loop self-energy corrections involving
squared decay rates and point out that their interpretation is highly
problematic. The corrections cannot be interpreted as radiative energy shifts
in the usual sense. Some of the problematic corrections find a natural
interpretation as radiative nonresonant corrections to the natural line shape.
They cannot uniquely be associated with one and only one atomic level. While
the problematic corrections are rather tiny when expressed in units of
frequency (a few Hertz for hydrogenic P levels) and do not affect the
reliability of quantum electrodynamics at the current level of experimental
accuracy, they may be of importance for future experiments. The problems are
connected with the limitations of the so-called asymptotic-state approximation
which means that atomic in- and out-states in the S-matrix are assumed to have
an infinite lifetime.Comment: 12 pages, 3 figures (New J. Phys., in press, submitted 28th May
Microcavities coupled to multilevel atoms
A three-level atom in the -configuration coupled to a microcavity is
studied. The two transitions of the atom are assumed couple to different
counterpropagating mode pairs in the cavity. We analyze the dynamics both, in
the strong-coupling and the bad cavity limit. We find that compared to a
two-level setup, the third atomic state and the additional control field modes
crucially modify the system dynamics and enable more advanced control schemes.
All results are explained using appropriate dressed state and eigenmode
representations. As potential applications, we discuss optical switching and
turnstile operations and detection of particles close to the resonator surface.Comment: 14 pages, 9 figure
Lamb Shift of Laser-Dressed Atomic States
We discuss radiative corrections to an atomic two-level system subject to an
intense driving laser field. It is shown that the Lamb shift of the
laser-dressed states, which are the natural state basis of the combined
atom-laser system, cannot be explained in terms of the Lamb shift received by
the atomic bare states which is usually observed in spectroscopic experiments.
In the final part, we propose an experimental scheme to measure these
corrections based on the incoherent resonance fluorescence spectrum of the
driven atom.Comment: 4 pages, 1 figure, submitted for publicatio
Colloids in light fields: particle dynamics in random and periodic energy landscapes
The dynamics of colloidal particles in potential energy landscapes have
mainly been investigated theoretically. In contrast, here we discuss the
experimental realization of potential energy landscapes with the help of light
fields and the observation of the particle dynamics by video microscopy. The
experimentally observed dynamics in periodic and random potentials are compared
to simulation and theoretical results in terms of, e.g. the mean-squared
displacement, the time-dependent diffusion coefficient or the non-Gaussian
parameter. The dynamics are initially diffusive followed by intermediate
subdiffusive behaviour which again becomes diffusive at long times. How
pronounced and extended the different regimes are, depends on the specific
conditions, in particular the shape of the potential as well as its roughness
or amplitude but also the particle concentration. Here we focus on dilute
systems, but the dynamics of interacting systems in external potentials, and
thus the interplay between particle-particle and particle-potential
interactions, is also mentioned briefly. Furthermore, the observed dynamics of
dilute systems resemble the dynamics of concentrated systems close to their
glass transition, with which it is compared. The effect of certain potential
energy landscapes on the dynamics of individual particles appears similar to
the effect of interparticle interactions in the absence of an external
potential
Coherent control in a decoherence-free subspace of a collective multi-level system
Decoherence-free subspaces (DFS) in systems of dipole-dipole interacting
multi-level atoms are investigated theoretically. It is shown that the
collective state space of two dipole-dipole interacting four-level atoms
contains a four-dimensional DFS. We describe a method that allows to populate
the antisymmetric states of the DFS by means of a laser field, without the need
of a field gradient between the two atoms. We identify these antisymmetric
states as long-lived entangled states. Further, we show that any single-qubit
operation between two states of the DFS can be induced by means of a microwave
field. Typical operation times of these qubit rotations can be significantly
shorter than for a nuclear spin system.Comment: 15 pages, 11 figure
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