53,813 research outputs found
Dielectronic recombination rates, ionization equilibrium, and radiative emission rates for Mn ions in low-density high-temperature plasmas
The analysis of optically-thin far-ultraviolet and X-ray emission lines of multiply-charged ions is one of the basic methods for determining the temperatures and densities of laboratory and astrophysical plasmas. In addition, the energy balance in these plasmas can be significantly influenced by the emission of radiation from relatively low concentrations of multiple-charged atomic ions. Because the populations of the excited levels are expected to depart substantially from their local thermodynamic equilibrium values a detailed treatment of the elementary collisional and radiative processes must be employed in order to predict the emission line intensities. In this investigation the authors present the results of calculations based on a corona equilibrium model in which a detailed evaluation is made of the dielectronic recombination rate coefficients. The ionization and autoionization following inner-shell electron excitation from each ground state are balanced by direct radiative and dielectronic recombination. The spectral line intensities emitted by the low-lying excited states, which are assumed to undergo spontaneous radiative decay in times that are short compared with the collision time, are evaluated in terms of the corona ionization equilibrium distributions of the ground states and their electron-impact excitation states
Superfluid Phase Stability of He in Axially Anisotropic Aerogel
Measurements of superfluid He in 98% aerogel demonstrate the existence of
a metastable \emph{A}-like phase and a stable \emph{B}-like phase. It has been
suggested that the relative stability of these two phases is controlled by
anisotropic quasiparticle scattering in the aerogel. Anisotropic scattering
produced by axial compression of the aerogel has been predicted to stabilize
the axial state of superfluid He. To explore this possiblity, we used
transverse acoustic impedance to map out the phase diagram of superfluid He
in a % porous silica aerogel subjected to 17% axial compression. We
have previously shown that axial anisotropy in aerogel leads to optical
birefringence and that optical cross-polarization studies can be used to
characterize such anisotropy. Consequently, we have performed optical
cross-polarization experiments to verify the presence and uniformity of the
axial anisotropy in our aerogel sample. We find that uniform axial anisotropy
introduced by 17% compression does not stabilize the \emph{A}-like phase. We
also find an increase in the supercooling of the \emph{A}-like phase at lower
pressure, indicating a modification to \emph{B}-like phase nucleation in
\emph{globally} anisotropic aerogels.Comment: 4 pages, 4 figures, submitted to LT25 (25th International Conference
on Low Temperature Physics
Magnon-Phonon Quantum Correlation Thermometry
A large fraction of quantum science and technology requires low-temperature environments such as those afforded by dilution refrigerators. In these cryogenic environments, accurate thermometry can be difficult to implement, expensive, and often requires calibration to an external reference. Here, we theoretically propose a primary thermometer based on measurement of a hybrid system consisting of phonons coupled via a magnetostrictive interaction to magnons. Thermometry is based on a cross-correlation measurement in which the spectrum of back-action driven motion is used to scale the thermomechanical motion, providing a direct measurement of the phonon temperature independent of experimental parameters. Combined with a simple low-temperature compatible microwave cavity readout, this primary thermometer is expected to become a promising alternative for thermometry below 1 K
Simulation of large turbulent structures with the parabolic Navier-Stokes equations
The theoretical basis for well posed marching of a Parabolic Navier-Stokes (PNS) computational technique for supersonic flow is discussed and examples given to verify the analysis. It is demonstrated that stable computations can be made even with very small steps in the marching direction. The method is applied to cones at large angle of attack in high Reynolds number, supersonic flow. Streamline trajectories generated from the numerical solutions demonstrate the development of vortex structures on the lee side of the cone
Pairing mean-field theory for the dynamics of dissociation of molecular Bose-Einstein condensates
We develop a pairing mean-field theory to describe the quantum dynamics of
the dissociation of molecular Bose-Einstein condensates into their constituent
bosonic or fermionic atoms. We apply the theory to one, two, and
three-dimensional geometries and analyze the role of dimensionality on the atom
production rate as a function of the dissociation energy. As well as
determining the populations and coherences of the atoms, we calculate the
correlations that exist between atoms of opposite momenta, including the column
density correlations in 3D systems. We compare the results with those of the
undepleted molecular field approximation and argue that the latter is most
reliable in fermionic systems and in lower dimensions. In the bosonic case we
compare the pairing mean-field results with exact calculations using the
positive- stochastic method and estimate the range of validity of the
pairing mean-field theory. Comparisons with similar first-principle simulations
in the fermionic case are currently not available, however, we argue that the
range of validity of the present approach should be broader for fermions than
for bosons in the regime where Pauli blocking prevents complete depletion of
the molecular condensate.Comment: 16 pages, 10 figure
Classical Region of a Trapped Bose Gas
The classical region of a Bose gas consists of all single-particle modes that
have a high average occupation and are well-described by a classical field.
Highly-occupied modes only occur in massive Bose gases at ultra-cold
temperatures, in contrast to the photon case where there are highly-occupied
modes at all temperatures. For the Bose gas the number of these modes is
dependent on the temperature, the total number of particles and their
interaction strength. In this paper we characterize the classical region of a
harmonically trapped Bose gas over a wide parameter regime. We use a
Hartree-Fock approach to account for the effects of interactions, which we
observe to significantly change the classical region as compared to the
idealized case. We compare our results to full classical field calculations and
show that the Hartree-Fock approach provides a qualitatively accurate
description of classical region for the interacting gas.Comment: 6 pages, 5 figures; updated to include new results with interaction
Determining the Impact of Wind on System Costs via the Temporal Patterns of Load and Wind Generation
Wind Energy, System Costs, Alternative Energy, Electricity Generation, Environmental Economics and Policy, Resource /Energy Economics and Policy, Q4, Q42, Q54,
- …