18,525 research outputs found
The effect of parallel static and microwave electric fields on excited hydrogen atoms
Motivated by recent experiments we analyse the classical dynamics of a
hydrogen atom in parallel static and microwave electric fields. Using an
appropriate representation and averaging approximations we show that resonant
ionisation is controlled by a separatrix, and provide necessary conditions for
a dynamical resonance to affect the ionisation probability.
The position of the dynamical resonance is computed using a high-order
perturbation series, and estimate its radius of convergence. We show that the
position of the dynamical resonance does not coincide precisely with the
ionisation maxima, and that the field switch-on time can dramatically affect
the ionisation signal which, for long switch times, reflects the shape of an
incipient homoclinic. Similarly, the resonance ionisation time can reflect the
time-scale of the separatrix motion, which is therefore longer than
conventional static field Stark ionisation. We explain why these effects should
be observed in the quantum dynamics.
PACs: 32.80.Rm, 33.40.+f, 34.10.+x, 05.45.Ac, 05.45.MtComment: 47 pages, 20 figure
Orbital and stochastic far-UV variability in the nova-like system V3885 Sgr
Highly time-resolved time-tagged FUSE satellite spectroscopic data are
analysed to establish the far-ultraviolet (FUV) absorption line characteristics
of the nova-like cataclysmic variable binary, V3885 Sgr. We determine the
temporal behaviour of low (Ly_beta, CIII, NIII) and high (SIV, PV, OVI) ion
species, and highlight corresponding orbital phase modulated changes in these
lines. On average the absorption troughs are blueshifted due to a low velocity
disc wind outflow. Very rapid (~ 5 min) fluctuations in the absorption lines
are isolated, which are indicative of stochastic density changes. Doppler
tomograms of the FUV lines are calculated which provide evidence for structures
where a gas stream interacts with the accretion disc. We conclude that the line
depth and velocity changes as a function of orbital phase are consistent with
an asymmetry that has its origin in a line-emitting, localised disc-stream
interaction region.Comment: Accepted for publication in MNRA
Meson-meson interactions -- from static to dynamic valence quarks
A method for the extraction of an effective meson-meson potential from Green
functions, which can be obtained from a lattice simulation, is presented.
Simulations are carried out for compact QED and QCD in four dimensions using
the quenched approximation and the hopping parameter expansion. In a further
study, a heavy-light meson is considered employing a conjugate gradient
algorithm for the light propagators. Due to the Pauli exclusion principle, the
results for QED indicate the existence of a hard core, but for QCD there is
strong attraction at small meson distances.Comment: 4 pages, uuencoded gziped postscript file, contribution to
LATTICE'95, Melbourne, Australia (list of authors completed
AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field
We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations
of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds.
Above the 210 microJy/beam (5sigma) detection level, the EVN clearly detects
two radio sources in a field that encompasses the HDF and part of the Hubble
Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013,
low-luminosity FR-I radio source located within the HDF itself) and VLA
J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system).
A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI
detections of all three sources suggest that most of the radio emission of
these particular sources (including the dusty starburst) is generated by an
embedded AGN.Comment: 4 pages, 1 figure; Accepted by Astron. & Astrophys Letters ... See
http://www.nfra.nl/~mag/hdf_evn.htm
Simplified Methodology to Estimate the Maximum Liquid Helium (LHe) Cryostat Pressure from a Vacuum Jacket Failure
The aircraft-based Stratospheric Observatory for Infrared Astronomy (SOFIA) is a platform for multiple infrared astronomical observation experiments. These experiments carry sensors cooled to liquid helium temperatures. The liquid helium supply is contained in large (i.e., 10 liters or more) vacuum-insulated dewars. Should the dewar vacuum insulation fail, the inrushing air will condense and freeze on the dewar wall, resulting in a large heat flux on the dewar's contents. The heat flux results in a rise in pressure and the actuation of the dewar pressure relief system. A previous NASA Engineering and Safety Center (NESC) assessment provided recommendations for the wall heat flux that would be expected from a loss of vacuum and detailed an appropriate method to use in calculating the maximum pressure that would occur in a loss of vacuum event. This method involved building a detailed supercritical helium compressible flow thermal/fluid model of the vent stack and exercising the model over the appropriate range of parameters. The experimenters designing science instruments for SOFIA are not experts in compressible supercritical flows and do not generally have access to the thermal/fluid modeling packages that are required to build detailed models of the vent stacks. Therefore, the SOFIA Program engaged the NESC to develop a simplified methodology to estimate the maximum pressure in a liquid helium dewar after the loss of vacuum insulation. The method would allow the university-based science instrument development teams to conservatively determine the cryostat's vent neck sizing during preliminary design of new SOFIA Science Instruments. This report details the development of the simplified method, the method itself, and the limits of its applicability. The simplified methodology provides an estimate of the dewar pressure after a loss of vacuum insulation that can be used for the initial design of the liquid helium dewar vent stacks. However, since it is not an exact tool, final verification of the dewar pressure vessel design requires a complete, detailed real fluid compressible flow model of the vent stack.
The wall heat flux resulting from a loss of vacuum insulation increases the dewar pressure,
which actuates the pressure relief mechanism and results in high-speed flow through the dewar
vent stack. At high pressures, the flow can be choked at the vent stack inlet, at the exit, or at an
intermediate transition or restriction.
During previous SOFIA analyses, it was observed that there was generally a readily identifiable
section of the vent stack that would limit the flow e.g., a small diameter entrance or an orifice.
It was also found that when the supercritical helium was approximated as an ideal gas at the
dewar condition, the calculated mass flow rate based on choking at the limiting entrance or
transition was less than the mass flow rate calculated using the detailed real fluid model2. Using
this lower mass flow rate would yield a conservative prediction of the dewars wall heat flux
capability. The simplified method of the current work was developed by building on this
observation
Calculation of the nucleon axial charge in lattice QCD
Protons and neutrons have a rich structure in terms of their constituents,
the quarks and gluons. Understanding this structure requires solving Quantum
Chromodynamics (QCD). However QCD is extremely complicated, so we must
numerically solve the equations of QCD using a method known as lattice QCD.
Here we describe a typical lattice QCD calculation by examining our recent
computation of the nucleon axial charge.Comment: Prepared for Scientific Discovery through Advanced Computing (SciDAC
2006), Denver, Colorado, June 25-29 200
Kinetic Ising model in an oscillating field: Finite-size scaling at the dynamic phase transition
We study hysteresis for a two-dimensional, spin-1/2, nearest-neighbor,
kinetic Ising ferromagnet in an oscillating field, using Monte Carlo
simulations. The period-averaged magnetization is the order parameter for a
proposed dynamic phase transition (DPT). To quantify the nature of this
transition, we present the first finite-size scaling study of the DPT for this
model. Evidence of a diverging correlation length is given, and we provide
estimates of the transition frequency and the critical indices ,
and .Comment: Accepted by Physical Review Letters. 9 page
Octet baryon electromagnetic form factors in nuclear medium
We study the octet baryon electromagnetic form factors in nuclear matter
using the covariant spectator quark model extended to the nuclear matter
regime. The parameters of the model in vacuum are fixed by the study of the
octet baryon electromagnetic form factors. In nuclear matter the changes in
hadron properties are calculated by including the relevant hadron masses and
the modification of the pion-baryon coupling constants calculated in the
quark-meson coupling model. In nuclear matter the magnetic form factors of the
octet baryons are enhanced in the low region, while the electric form
factors show a more rapid variation with . The results are compared with
the modification of the bound proton electromagnetic form factors observed at
Jefferson Lab. In addition, the corresponding changes for the bound neutron are
predicted.Comment: Version accepted for publication in J.Phys. G. Few changes. 40 pages,
14 figures and 8 table
Nucleon structure in the chiral regime with domain wall fermions on an improved staggered sea
Moments of unpolarized, helicity, and transversity distributions,
electromagnetic form factors, and generalized form factors of the nucleon are
presented from a preliminary analysis of lattice results using pion masses down
to 359 MeV. The twist two matrix elements are calculated using a mixed action
of domain wall valence quarks and asqtad staggered sea quarks and are
renormalized perturbatively. Several observables are extrapolated to the
physical limit using chiral perturbation theory. Results are compared with
experimental moments of quark distributions and electromagnetic form factors
and phenomenologically determined generalized form factors, and the
implications on the transverse structure and spin content of the nucleon are
discussed.Comment: Talks of J.W. Negele and D.B. Renner at Lattice 200
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