2,374 research outputs found
On the Role of Metastable States in Low Pressure Oxygen Discharges
We use the one-dimensional object-oriented particle-in-cell Monte Carlo
collision code {\tt oopd1} to explore the spatio-temporal evolution of the
electron heating mechanism in a capacitively coupled oxygen discharge in the
pressure range 10 -- 200 mTorr. The electron heating is most significant in the
sheath vicinity during the sheath expansion phase. We explore how including and
excluding detachment by the singlet metastable states O(a) and O(b) influences the heating mechanism, the
effective electron temperature and electronegativity, in the oxygen discharge.
We demonstrate that the detachment processes have a significant influence on
the discharge properties, in particular for the higher pressures. At 10 mTorr
the time averaged electron heating shows mainly ohmic heating in the plasma
bulk (the electronegative core) and at higher pressures there is no ohmic
heating in the plasma bulk, that is electron heating in the sheath regions
dominates.Comment: submitted to AIP Conference Proceeding
Analytical analysis of small-amplitude perturbations in the shallow ice stream approximation
International audienceNew analytical solutions describing the effects of small-amplitude perturbations in boundary data on flow in the shallow ice stream approximation are presented. These solutions are valid for a non-linear Weertman-type sliding law and for Newtonian ice rheology. Comparison is made with corresponding solutions of the shallow ice sheet approximation, and with solutions of the full Stokes equations. The shallow ice stream approximation is commonly used to describe large-scale ice stream flow over a weak bed, while the shallow ice sheet approximation forms the basis of most current large-scale ice sheet models. It is found that the shallow ice stream approximation overestimates the effects of bedrock perturbations on surface topography for wavelengths less than about 5 to 10 ice thicknesses, the exact number depending on values of surface slope and slip ratio. For high slip ratios, the shallow ice stream approximation gives a very simple description of the relationship between bed and surface topography, with the corresponding transfer amplitudes being close to unity for any given wavelength. The shallow ice stream estimates for the timescales that govern the transient response of ice streams to external perturbations are considerably more accurate than those based on the shallow ice sheet approximation. In contrast to the shallow ice sheet approximation, the shallow ice stream approximation correctly reproduces the short-wavelength limit of the kinematic phase speed. In accordance with the full system solutions, the shallow ice sheet approximation predicts surface fields to react weakly to spatial variations in basal slipperiness with wavelengths less than about 10 to 20 ice thicknesses
Bringing the norm to the ‘Burgs: Gender and design at two Virginia Normal Schools 1908-1928
The purpose of this presentation is to compare James Madison University and the University of Mary Washington from their start with the passing of a 1908 General Assembly bill creating state normal schools for women in Harrisonburg and Fredericksburg, Virginia. The focus is on the two schools from their creation, with an emphasis on how gender contributed to the architectural styles of both campuses and how Southern cultural ideals and Progressive Era ideals of the early 20th century shaped the experiences of the women during the first two decades of the two institutions\u27 histories
Transient magnetotransport through a quantum wire
We consider an ideal parabolic quantum wire in a perpendicular magnetic
field. A simple Gaussian shaped scattering potential well or hill is flashed
softly on and off with its maximum at , mimicking a temporary broadening
or narrowing of the wire. By an extension of the Lippmann-Schwinger formalism
to time-dependent scattering potentials we investigate the effects on the
continuous current that is driven through the quantum wire with a vanishingly
small forward bias. The Lippmann-Schwinger approach to the scattering process
enables us to investigate the interplay between geometrical effects and effects
caused by the magnetic field.Comment: RevTeX (pdf-LaTeX), 11 pages with 15 included jpg figure
Afterglow Light Curves and Broken Power Laws: A Statistical Study
In gamma-ray burst research it is quite common to fit the afterglow light
curves with a broken power law to interpret the data. We apply this method to a
computer simulated population of afterglows and find systematic differences
between the known model parameters of the population and the ones derived from
the power law fits. In general, the slope of the electron energy distribution
is overestimated from the pre-break light curve slope while being
underestimated from the post-break slope. We also find that the jet opening
angle derived from the fits is overestimated in narrow jets and underestimated
in wider ones. Results from fitting afterglow light curves with broken power
laws must therefore be interpreted with caution since the uncertainties in the
derived parameters might be larger than estimated from the fit. This may have
implications for Hubble diagrams constructed using gamma-ray burst data.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
Hartree-Fock dynamics in highly excited quantum dots
Time-dependent Hartree-Fock theory is used to describe density oscillations
of symmetry-unrestricted two-dimensional nanostructures. In the small amplitude
limit the results reproduce those obtained within a perturbative approach such
as the linearized time-dependent Hartree-Fock one. The nonlinear regime is
explored by studying large amplitude oscillations in a non-parabolic potential,
which are shown to introduce a strong coupling with internal degrees of
freedom. This excitation of internal modes, mainly of monopole and quadrupole
character, results in sizeable modifications of the dipole absorption.Comment: 4 pages, 4 embedded figure
Energy Injection Episodes in Gamma Ray Bursts: The Light Curves and Polarization Properties of GRB 021004
Several GRB afterglow light curves deviate strongly from the power law decay
observed in most bursts. We show that these variations can be accounted for by
including refreshed shocks in the standard fireball model previously used to
interpret the overall afterglow behavior. As an example we consider GRB 021004
that exhibited strong light curve variations and has a reasonably well
time-resolved polarimetry. We show that the light curves in the R-band, X-rays
and in the radio can be accounted for by four energy injection episodes in
addition to the initial event. The polarization variations are shown to be a
consequence of the injections.Comment: 4 pages, 2 figures. To appear in ApJ
Halley Research Station, Antarctica: calving risks and monitoring strategies
The British Antarctic Survey's Halley Research Station is located on the Brunt Ice Shelf, Antarctica, where it is potentially vulnerable to calving events. Existing historical records show that the Brunt Ice Shelf is currently extended further into the Weddell Sea than it was before its last large calving event, so a new calving event may be overdue. We describe three different possible future scenarios for a large-scale calving event on Brunt Ice Shelf, and conclude that the currently most threatening scenario for the Halley Research Station is a calving event on the neighbouring Stancomb–Wills Glacier Tongue, with subsequent detrimental consequences for the stability of the Brunt Ice Shelf. Based on available data, we suggest an increasing likelihood of this scenario occurring after 2020. We furthermore describe ongoing monitoring efforts aimed at giving advanced warning of an imminent calving event
Magnetization in short-period mesoscopic electron systems
We calculate the magnetization of the two-dimensional electron gas in a
short-period lateral superlattice, with the Coulomb interaction included in
Hartree and Hartree-Fock approximations. We compare the results for a finite,
mesoscopic system modulated by a periodic potential, with the results for the
infinite periodic system. In addition to the expected strong exchange effects,
the size of the system, the type and the strength of the lateral modulation
leave their fingerprints on the magnetization.Comment: RevTeX4, 10 pages with 14 included postscript figures To be published
in PRB. Replaced to repair figure
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