18,270 research outputs found
Control of long-range correlations in turbulence
The character of turbulence depends on where it develops. Turbulence near
boundaries, for instance, is different than in a free stream. To elucidate the
differences between flows, it is instructive to vary the structure of
turbulence systematically, but there are few ways of stirring turbulence that
make this possible. In other words, an experiment typically examines either a
boundary layer or a free stream, say, and the structure of the turbulence is
fixed by the geometry of the experiment. We introduce a new active grid with
many more degrees of freedom than previous active grids. The additional degrees
of freedom make it possible to control various properties of the turbulence. We
show how long-range correlations in the turbulent velocity fluctuations can be
shaped by changing the way the active grid moves. Specifically, we show how not
only the correlation length but also the detailed shape of the correlation
function depends on the correlations imposed in the motions of the grid. Until
now, large-scale structure had not been adjustable in experiments. This new
capability makes possible new systematic investigations into turbulence
dissipation and dispersion, for example, and perhaps in flows that mimic
features of boundary layers, free streams, and flows of intermediate character.Comment: This paper has been accepted to Experiments in Fluids. 25 pages, 10
figure
Landau damping of Bogoliubov excitations in optical lattices at finite temperature
We study the damping of Bogoliubov excitations in an optical lattice at
finite temperatures. For simplicity, we consider a Bose-Hubbard tight-binding
model and limit our analysis to the lowest excitation band. We use the Popov
approximation to calculate the temperature dependence of the number of
condensate atoms in each lattice well. We calculate the Landau
damping of a Bogoliubov excitation in an optical lattice due to coupling to a
thermal cloud of excitations. While most of the paper concentrates on 1D
optical lattices, we also briefly present results for 2D and 3D lattices. For
energy conservation to be satisfied, we find that the excitations in the
collision process must exhibit anomalous dispersion ({\it i.e.} the excitation
energy must bend upward at low momentum), as also exhibited by phonons in
superfluid . This leads to the sudden disappearance of all damping
processes in -dimensional simple cubic optical lattice when , where is the on-site interaction, and is the hopping matrix
element. Beliaev damping in a 1D optical lattice is briefly discussed.Comment: 28 pages, 9 figure
Numerical calculations of a high brilliance synchrotron source and on issues with characterizing strong radiation damping effects in non-linear Thomson/Compton backscattering experiments
A number of theoretical calculations have studied the effect of radiation
reaction forces on radiation distributions in strong field counter-propagating
electron beam-laser interactions, but could these effects - including quantum
corrections - be observed in interactions with realistic bunches and focusing
fields, as is hoped in a number of soon to be proposed experiments? We present
numerical calculations of the angularly resolved radiation spectrum from an
electron bunch with parameters similar to those produced in laser wakefield
acceleration experiments, interacting with an intense, ultrashort laser pulse.
For our parameters, the effects of radiation damping on the angular
distribution and energy distribution of \emph{photons} is not easily
discernible for a "realistic" moderate emittance electron beam. However,
experiments using such a counter-propagating beam-laser geometry should be able
to measure such effects using current laser systems through measurement of the
\emph{electron beam} properties. In addition, the brilliance of this source is
very high, with peak spectral brilliance exceeding
photonssmmmrad% bandwidth with
approximately 2% efficiency and with a peak energy of 10 MeV.Comment: 11 figures, 11 page
Collective Excitations of a Two-Component Bose Condensate at Finite Temperature
We compare the collective modes for Bose-condensed systems with two
degenerate components with and without spontaneous intercomponent coherence at
finite temperature using the time-dependent Hartree-Fock approximation. We show
that the interaction between the condensate and non-condensate in these two
cases results in qualitatively different collective excitation spectra. We show
that at zero temperature the single-particle excitations of the incoherent Bose
condensate can be probed by intercomponent excitations.Comment: 4 pages, 3 figures, Submitted to PR
Demonstration of an inductively coupled ring trap for cold atoms
We report the first demonstration of an inductively coupled magnetic ring trap for cold atoms. A uniform, ac magnetic field is used to induce current in a copper ring, which creates an opposing magnetic field that is time-averaged to produce a smooth cylindrically symmetric ring trap of radius 5 mm. We use a laser-cooled atomic sample to characterize the loading efficiency and adiabaticity of the magnetic potential, achieving a vacuum-limited lifetime in the trap. This technique is suitable for creating scalable toroidal waveguides for applications in matter-wave interferometry, offering long interaction times and large enclosed areas
Superfluid density and condensate fraction in the BCS-BEC crossover regime at finite temperatures
The superfluid density is a fundamental quantity describing the response to a
rotation as well as in two-fluid collisional hydrodynamics. We present
extensive calculations of the superfluid density \rho_s in the BCS-BEC
crossover regime of a uniform superfluid Fermi gas at finite temperatures. We
include strong-coupling or fluctuation effects on these quantities within a
Gaussian approximation. We also incorporate the same fluctuation effects into
the BCS single-particle excitations described by the superfluid order parameter
\Delta and Fermi chemical potential \mu, using the Nozi\`eres and Schmitt-Rink
(NSR) approximation. This treatment is shown to be necessary for consistent
treatment of \rho_s over the entire BCS-BEC crossover. We also calculate the
condensate fraction N_c as a function of the temperature, a quantity which is
quite different from the superfluid density \rho_s. We show that the mean-field
expression for the condensate fraction N_c is a good approximation even in the
strong-coupling BEC regime. Our numerical results show how \rho_s and N_c
depend on temperature, from the weak-coupling BCS region to the BEC region of
tightly-bound Cooper pair molecules. In a companion paper by the authors
(cond-mat/0609187), we derive an equivalent expression for \rho_s from the
thermodynamic potential, which exhibits the role of the pairing fluctuations in
a more explicit manner.Comment: 32 pages, 12 figure
Volcanic Gas Emissions Mapping Using a Mass Spectrometer System
The visualization of hazardous gaseous emissions at volcanoes using in-situ mass spectrometry (MS) is a key step towards a better comprehension of the geophysical phenomena surrounding eruptive activity. In-Situ gas data consisting of helium, carbon dioxide, sulfur dioxide, and other gas species, were acquired with an MS system. MS and global position system (GPS) data were plotted on ground imagery, topography, and remote sensing data collected by a host of instruments during the second Costa Rica Airborne Research and Technology Applications (CARTA) mission This combination of gas and imaging data allowed 3-dimensional (3-D) visualization of the volcanic plume end the mapping of gas concentration at several volcanic structures and urban areas This combined set of data has demonstrated a better tool to assess hazardous conditions by visualizing and modeling of possible scenarios of volcanic activity. The MS system is used for in-situ measurement of three-dimensional gas concentrations at different volcanic locations with three different transportation platforms, aircraft, auto, and hand carried. The demonstration for urban contamination mapping is also presented as another possible use for the MS system
- …