15 research outputs found
Coherent Tunneling of Atoms from Bose-condensed Gases at Finite Temperatures
Tunneling of atoms between two trapped Bose-condensed gases at finite
temperatures is explored using a many-body linear response tunneling formalism
similar to that used in superconductors. To lowest order, the tunneling
currents can be expressed quite generally in terms of the single-particle
Green's functions of the isolated Bose gases. A coherent first-order tunneling
Josephson current between two atomic Bose-condensates is found, in addition to
coherent and dissipative contributions from second-order
condensate-noncondensate and noncondensate-noncondensate tunneling. Our work is
a generalization of Meier and Zwerger, who recently treated tunneling between
uniform atomic Bose gases. We apply our formalism to the analysis of an
out-coupling experiment induced by light wave fields, using a simple
Bogoliubov-Popov quasiparticle approximation for the trapped Bose gas. For
tunneling into the vacuum, we recover the results of Japha, Choi, Burnett and
Band, who recently pointed out the usefulness of studying the spectrum of
out-coupled atoms. In particular, we show that the small tunneling current of
noncondensate atoms from a trapped Bose gas has a broad spectrum of energies,
with a characteristic structure associated with the Bogoliubov quasiparticle
u^2 and v^2 amplitudes.Comment: 26 pages, 5 figures, minor changes, to appear in PR
Dynamic correlation functions in one-dimensional quasi-condensates
We calculate the static and dynamic single-particle correlation functions in
one-dimensional (1D) trapped Bose gases and discuss experimental measurements
that can directly probe such correlation functions. Using a quantized
hydrodynamic theory for the low energy excitations, we calculate both the
static and dynamic single-particle correlation functions for a 1D Bose gas that
is a phase-fluctuating quasi-condensate. For the static (equal-time)
correlation function, our approximations and results are equivalent to those of
Petrov, Shlyapnikov and Walraven. The Fourier transform of the static
single-particle correlation function gives the momentum distribution, which can
be measured using Doppler-sensitive Bragg scattering experiments on a highly
elongated Bose gas. We show how a two-photon Raman out-coupling experiment can
measure the characteristic features of the dynamic or time-dependent
single-particle correlation function of a 1D Bose quasi-condensate.Comment: 19 pages, 4 figures; submitted to Phys. Rev.
Finite-temperature correlations in the one-dimensional trapped and untrapped Bose gases
We calculate the dynamic single-particle and many-particle correlation
functions at non-zero temperature in one-dimensional trapped repulsive Bose
gases. The decay for increasing distance between the points of these
correlation functions is governed by a scaling exponent that has a universal
expression in terms of observed quantities. This expression is valid in the
weak-interaction Gross-Pitaevskii as well as in the strong-interaction
Girardeau-Tonks limit, but the observed quantities involved depend on the
interaction strength. The confining trap introduces a weak center-of-mass
dependence in the scaling exponent. We also conjecture results for the
density-density correlation function.Comment: 18 pages, Latex, Revtex
Bosonizing one-dimensional cold atomic gases
We present results for the long-distance asymptotics of correlation functions
of mesoscopic one-dimensional systems with periodic and open (Dirichlet)
boundary conditions, as well as at finite temperature in the thermodynamic
limit. The results are obtained using Haldane's harmonic-fluid approach (also
known as ``bosonization''), and are valid for both bosons and fermions, in
weakly and strongly interacting regimes. The harmonic-fluid approach and the
method to compute the correlation functions using conformal transformations are
explained in great detail. As an application relevant to one-dimensional
systems of cold atomic gases, we consider the model of bosons interacting with
a zero-range potential. The Luttinger-liquid parameters are obtained from the
exact solution by solving the Bethe-ansatz equations in finite-size systems.
The range of applicability of the approach is discussed, and the prefactor of
the one-body density matrix of bosons is fixed by finding an appropriate
parametrization of the weak-coupling result. The formula thus obtained is shown
to be accurate, when compared with recent diffusion Montecarlo calculations,
within less than 10%. The experimental implications of these results for Bragg
scattering experiments at low and high momenta are also discussed.Comment: 39 pages + 14 EPS figures; typos corrected, references update
Current status of Melcor 2.2 for fusion safety analyses
MELCOR is an integral code developed by Sandia National Laboratories (SNL) for the US Nuclear Regulatory Commission (USNRC) to perform severe accident analyses of Light Water Reactors (LWR). More recently, MELCOR capabilities are being extended also to analyze non-LWR fission technologies. Within the European MELCOR User Group (EMUG), organized in the framework of the USNRC Cooperative Severe Accident Research Program (CSARP), an activity on the evaluation of the applicability of MELCOR 2.2 for fusion safety analyses has been launched and it has been coordinated by ENEA. The aim of the activity was to identify the physical models to be possibly implemented in MELCOR 2.2 necessary for fusion safety analyses, and to check if those models are already available in MELCOR 1.8.6 fusion version, developed by Idaho National Laboratory (INL). From this activity, a list of modeling needs that emerged from the safety analyses of fusion-related installations has been
identified and described. Then, the importance of the various needs, intended as the priority for model implementation in the MELCOR 2.2 code, has been evaluated according to the technical expert judgment of the authors. In the present paper, the identified modeling needs are discussed. The ultimate goal would be to propose to have a single integrated MELCOR 2.2 code release capable to cover both fission and fusion applications
Momentum distribution and correlation function of quasicondensates in elongated traps
We calculate the spatial correlation function and momentum distribution of a
phase-fluctuating, elongated three-dimensional condensate, in a trap and in
free expansion. We take the inhomogeneous density profile into account
{\it{via}} a local density approximation. We find an almost Lorentzian momentum
distribution, in stark contrast with a Heisenberg-limited Thomas-Fermi
condensate.Comment: 5 pages, 2 figures; final version, references update
ATHLET-CD/COCOSYS Analyses of Severe Accidents in Fukushima Daiichi Units 2 and 3: German Contribution to the OECD/NEA BSAF Project, Phase 1
The Dilution Dependency of Multigroup Uncertainties
The propagation of nuclear data uncertainties through reactor physics calculation has received attention through the Organization for Economic Cooperation and DevelopmentâNuclear Energy Agencyâs Uncertainty Analysis in Modelling (UAM) benchmark. A common strategy for performing lattice physics uncertainty analysis involves starting with nuclear data and covariance matrix which is typically available at infinite dilution. To describe the uncertainty of all multigroup physics parametersâincluding those at finite dilutionâadditional calculations must be performed that relate uncertainties in an infinite dilution cross-section to those at the problem dilution. Two potential methods for propagating dilution-related uncertainties were studied in this work. The first assumed a correlation between continuous-energy and multigroup cross-sectional data and uncertainties, which is convenient for direct implementation in lattice physics codes. The second is based on a more rigorous approach involving the Monte Carlo sampling of resonance parameters in evaluated nuclear data using the TALYS software. When applied to a light water fuel cell, the two approaches show significant differences, indicating that the assumption of the first method did not capture the complexity of physics parameter data uncertainties. It was found that the covariance of problem-dilution multigroup parameters for selected neutron cross-sections can vary significantly from their infinite-dilution counterparts
Safety insights from forensics evaluations at Daiichi
Although it is clear that the accident signatures from each affected unit at the Fukushima Daiichi Nuclear Power Station [Daiichi] differ, much is not known about the end-state of core materials within these units. Some of this uncertainty can be attributed to a lack of information related to cooling system operation and cooling water injection. There is also uncertainty in our understanding of phenomena affecting: a) in-vessel core damage progression during severe accidents in boiling water reactors (BWRs), and b) accident progression after vessel failure (ex-vessel progression) for BWRs and Pressurized Water Reactors (PWRs). These uncertainties arise due to limited full scale prototypic data. Similar to what occurred after the accident at Three Mile Island Unit 2, these Daiichi units offer the international community a means to reduce such uncertainties by obtaining prototypic data from multiple full-scale BWR severe accidents.
Information obtained from Daiichi is required to inform Decontamination and Decommissioning activities, improving the ability of the Tokyo Electric Power Company (TEPCO) to characterize potential hazards and to ensure the safety of workers involved with cleanup activities. This paper reports initial results from the US Forensics Effort to utilize examination information obtained by TEPCO to enhance the safety of existing and future nuclear power plant designs. In this paper, three examples are presented in which examination information, such as visual images, dose surveys, sample evaluations, and muon tomography examinations, along with data from plant instrumentation, are used to obtain significant safety insights in the areas of component performance, fission product release and transport, debris end-state location, and combustible gas generation and transport. In addition to reducing uncertainties related to severe accident modeling progression, these insights confirm actions, such as the importance of water addition and containment venting, that are emphasized in updated guidance for severe accident prevention, mitigation, and emergency planning