34,452 research outputs found
Microscopic resolution broadband dielectric spectroscopy
Results are presented for a non-contact measurement system capable of micron level spatial resolution. It utilises the novel electric potential sensor (EPS) technology, invented at Sussex, to image the electric field above a simple composite dielectric material. EP sensors may be regarded as analogous to a magnetometer and require no adjustments or offsets during either setup or use. The sample consists of a standard glass/epoxy FR4 circuit board, with linear defects machined into the surface by a PCB milling machine. The sample is excited with an a.c. signal over a range of frequencies from 10 kHz to 10 MHz, from the reverse side, by placing it on a conducting sheet connected to the source. The single sensor is raster scanned over the surface at a constant working distance, consistent with the spatial resolution, in order to build up an image of the electric field, with respect to the reference potential. The results demonstrate that both the surface defects and the internal dielectric variations within the composite may be imaged in this way, with good contrast being observed between the glass mat and the epoxy resin
Mission analysis of solar powered aircraft
The effect of a real mission scenario on a solar powered airplane configuration which had been developed in previous work were assessed. The mission used was surveillance of crop conditions over a route from Phoenix to Tucson to Tombstone, Arizona. Appendices are attached which address the applicability of existing platforms and payloads to do this mission
Three-dimensional theory of stimulated Raman scattering
We present a three-dimensional theory of stimulated Raman scattering
(SRS) or superradiance. In particular we address how the spatial and temporal
properties of the generated SRS beam, or Stokes beam, of radiation depends on
the spatial properties of the gain medium. Maxwell equations for the Stokes
field operators and of the atomic operators are solved analytically and a
correlation function for the Stokes field is derived. In the analysis we
identify a superradiating part of the Stokes radiation that exhibit beam
characteristics. We show how the intensity in this beam builds up in time and
at some point largely dominates the total Stokes radiation of the gain medium.
We show how the SRS depends on geometric factors such as the Fresnel number and
the optical depth, and that in fact these two factors are the only factors
describing the coherent radiation.Comment: 21 pages 14 figure
Meson spectroscopy and properties using Dyson-Schwinger equations
We study pseudoscalar and scalar mesons using a practical and symmetry
preserving truncation of QCD's Dyson-Schwinger equations. We investigate and
compare properties of ground and radially excited meson states. In addition to
exact results for radial meson excitations we also present results for meson
masses and decay constants from the chiral limit up to the charm-quark mass, e.
g., the mass of the chi_{c0} (2P) meson.Comment: 8 pages, 3 figures, To appear in the proceedings of 9th International
Workshop on Meson Production, Properties and Interaction (Meson2006), Krakow,
Poland, 9 - 13 June 200
String windings in the early universe
We study string dynamics in the early universe. Our motivation is the
proposal of Brandenberger and Vafa, that string winding modes may play a key
role in decompactifying three spatial dimensions. We model the universe as a
homogeneous but anisotropic 9-torus filled with a gas of excited strings. We
adopt initial conditions which fix the dilaton and the volume of the torus, but
otherwise assume all states are equally likely. We study the evolution of the
system both analytically and numerically to determine the late-time behavior.
We find that, although dynamical evolution can indeed lead to three large
spatial dimensions, such an outcome is not statistically favored.Comment: 26 pages, LaTeX, 4 eps figure
ULTRACAM photometry of the eclipsing cataclysmic variable OU Vir
We present high-speed, three-colour photometry of the faint eclipsing
cataclysmic variable OU Vir. For the first time in OU Vir, separate eclipses of
the white dwarf and bright spot have been observed. We use timings of these
eclipses to derive a purely photometric model of the system, obtaining a mass
ratio of q = 0.175 +/- 0.025, an inclination of i = 79.2 +/- 0.7 degrees and a
disc radius of Rd/a = 0.2315 +/- 0.0150. We separate the white dwarf eclipse
from the lightcurve and, by fitting a blackbody spectrum to its flux in each
passband, obtain a white dwarf temperature of T = 21700 +/- 1200 K and a
distance of D = 650 +/- 210 pc. Assuming that the primary obeys the Nauenberg
(1972) mass-radius relation for white dwarfs and allowing for temperature
effects, we also find a primary mass Mw/Msun = 0.90 +/- 0.19, primary radius
Rw/Rsun = 0.0097 +/- 0.0031 and orbital separation a/Rsun = 0.75 +/- 0.05.Comment: 8 pages LaTeX, 6 figures. Accepted by MNRAS; erratum added at end.
Mon.Not.Roy.Astron.Soc. 347 (2004) 1173, erratum in pres
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