1,777 research outputs found
The architecture of DDMl: a recursively structured data driven machine
Journal ArticleAn architecture for a highly modular, recursively structured class of machines is presented. DDMl is an instance of such a machine structure, and is capable of executing machine language programs which are data driven (data flow) nets. These nets may represent arbitrary amounts of concurrency as well as arbitrary amounts of pipelining. DDMl is a fully distributed multi-processing system composed of completely asynchronous modules. The architecture allows for limitless physical extensibility without necessitating special programming or special hardware to support individual machines of widely varying sizes. DDMl is capable of automatically and dynamically allocating concurrent tasks to the available physical resources. The essential characteristics of the highly parallel, pipelined machine language are also described along with its method for execution on DDMl
Dataflow computers: a tutorial and survey
Journal ArticleThe demand for very high performance computer has encouraged some researchers in the computer science field to consider alternatives to the conventional notions of program and computer organization. The dataflow computer is one attempt to form a new collection of consistent systems ideas to improve both computer performance and to alleviate the software design problems induced by the construction of highly concurrent programs
Small-amplitude normal modes of a vortex in a trapped Bose-Einstein condensate
We consider a cylindrically symmetric trap containing a small Bose-Einstein
condensate with a singly quantized vortex on the axis of symmetry. A
time-dependent variational Lagrangian analysis yields the small-amplitude
dynamics of the vortex and the condensate, directly determining the equations
of motion of the coupled normal modes. As found previously from the Bogoliubov
equations, there are two rigid dipole modes and one anomalous mode with a
negative frequency when seen in the laboratory frame.Comment: 4 pages, no figures, Revte
Conserving and Gapless Approximations for an Inhomogeneous Bose Gas at Finite Temperatures
We derive and discuss the equations of motion for the condensate and its
fluctuations for a dilute, weakly interacting Bose gas in an external potential
within the self--consistent Hartree--Fock--Bogoliubov (HFB) approximation.
Account is taken of the depletion of the condensate and the anomalous Bose
correlations, which are important at finite temperatures. We give a critical
analysis of the self-consistent HFB approximation in terms of the
Hohenberg--Martin classification of approximations (conserving vs gapless) and
point out that the Popov approximation to the full HFB gives a gapless
single-particle spectrum at all temperatures. The Beliaev second-order
approximation is discussed as the spectrum generated by functional
differentiation of the HFB single--particle Green's function. We emphasize that
the problem of determining the excitation spectrum of a Bose-condensed gas
(homogeneous or inhomogeneous) is difficult because of the need to satisfy
several different constraints.Comment: plain tex, 19 page
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Thorium-Uranium-233 Oxide (Kilorod) Facility--Rod Fabrication Process and Equipment
A fabrication facility with related process equipment has been constructed to fabricate fuel rods containing /sup 233/U and thorium oxide by the bulk oxide-vibratory compaction route and is now in operation at the Oak Ridge National Laboratory. While the initial motivation for the facility was to fabricate /sup 233/U-bearing 1/2-in.-dia x 46-in.-long Zircaloy-2 clad fuel rods for criticality experiments at the Brookhaven National Laboratory, sufficient flexibility has been incorporated into the facility to accommodate a variety of development work on a pilot-plant scale. The facility consists of a number of permanent alpha-tight cubicles which are shielded with 41/4-in. steel. The fabrication process is carried out remotely in these cubicles with the exception of several gloved-hand operations which occur where the dexterity required for manipulation exceeds that of the remote castle-type tongs. The fabrication equipment that has been installed in the facility performs the operations associated with oxide powder comminution and classification, vibratory compaction of oxide in fuel tubes, tube closure, fuel rod decontamination, and fuel rod inspection. Both the facility and the process equipment are described in detail, and detailed assembly drawings of the process equipment are included. (auth
Stability of Bose condensed atomic Li-7
We study the stability of a Bose condensate of atomic Li in a (harmonic
oscillator) magnetic trap at non-zero temperatures. In analogy to the stability
criterion for a neutron star, we conjecture that the gas becomes unstable if
the free energy as a function of the central density of the cloud has a local
extremum which conserves the number of particles. Moreover, we show that the
number of condensate particles at the point of instability decreases with
increasing temperature, and that for the temperature interval considered, the
normal part of the gas is stable against density fluctuations at this point.Comment: Submitted for publication in Physical Review
Condensate Heating by Atomic Losses
Atomic Bose-Einstein condensate is heated by atomic losses. Predicted
depletion ranges from 1% for a uniform 3D condensate to around 10% for a
quasi-1D condensate in a harmonic trap.Comment: 4 pages in RevTex, 1 eps figur
Theory of coherent Bragg spectroscopy of a trapped Bose-Einstein condensate
We present a detailed theoretical analysis of Bragg spectroscopy from a
Bose-Einstein condensate at T=0K. We demonstrate that within the linear
response regime, both a quantum field theory treatment and a meanfield
Gross-Pitaevskii treatment lead to the same value for the mean evolution of the
quasiparticle operators. The observable for Bragg spectroscopy experiments,
which is the spectral response function of the momentum transferred to the
condensate, can therefore be calculated in a meanfield formalism. We analyse
the behaviour of this observable by carrying out numerical simulations in
axially symmetric three-dimensional cases and in two dimensions. An approximate
analytic expression for the observable is obtained and provides a means for
identifying the relative importance of three broadening and shift mechanisms
(meanfield, Doppler, and finite pulse duration) in different regimes. We show
that the suppression of scattering at small values of q observed by
Stamper-Kurn et al. [Phys. Rev. Lett. 83, 2876 (1999)] is accounted for by the
meanfield treatment, and can be interpreted in terms of the interference of the
u and v quasiparticle amplitudes. We also show that, contrary to the
assumptions of previous analyses, there is no regime for trapped condensates
for which the spectral response function and the dynamic structure factor are
equivalent. Our numerical calculations can also be performed outside the linear
response regime, and show that at large laser intensities a significant
decrease in the shift of the spectral response function can occur due to
depletion of the initial condensate.Comment: RevTeX4 format, 16 pages plus 7 eps figures; Update to published
version: minors changes and an additional figure. (To appear in Phys. Rev. A
Free expansion of Bose-Einstein condensates with quantized vortices
The expansion of Bose-Einstein condensates with quantized vortices is studied
by solving numerically the time-dependent Gross-Pitaevskii equation at zero
temperature. For a condensate initially trapped in a spherical harmonic
potential, we confirm previous results obtained by means of variational methods
showing that, after releasing the trap, the vortex core expands faster than the
radius of the atomic cloud. This could make the detection of vortices feasible,
by observing the depletion of the density along the axis of rotation. We find
that this effect is significantly enhanced in the case of anisotropic
disc-shaped traps. The results obtained as a function of the anisotropy of the
initial configuration are compared with the analytic solution for a
noninteracting gas in 3D as well as with the scaling law predicted for an
interacting gas in 2D.Comment: 5 pages, 6 postscript figure
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