17,049 research outputs found
Development of a 200 W CW High Efficiency Traveling Wave Tube at 12 GHz
The design, development, and test results are reported for an experimental PPM focused, traveling-wave tube that produces 235 watts of CW RF power over 85 MHz centered at 12.080 GHz. The tube uses a coupled cavity RF circuit with a velocity taper for greater than 30 percent basic efficiency. Overall efficiency of 51 percent is achieved by means of a nine stage depressed collector designed at NASA Lewis Research Center. This collector is cooled by direct radiation to deep space
Power filtration of CMB observational data
We propose a power filter Gp for linear reconstruction of the CMB signal from
observational maps. This Gp filter preserves the power spectrum of the CMB
signal in contrast to the Wiener filter which diminishes the power spectrum of
the reconstructed CMB signal. We demonstrate how peak statistics and a cluster
analysis can be used to estimate the probability of the presence of a CMB
signal in observational records. The efficiency of the Gp filter is
demonstrated on a toy model of an observational record consisting of a CMB
signal and noise in the form of foreground point sources.Comment: 17 pages; 4 figures; submitted to International Journal of Modern
Physic
Numerical indications on the semiclassical limit of the flipped vertex
We introduce a technique for testing the semiclassical limit of a quantum
gravity vertex amplitude. The technique is based on the propagation of a
semiclassical wave packet. We apply this technique to the newly introduced
"flipped" vertex in loop quantum gravity, in order to test the intertwiner
dependence of the vertex. Under some drastic simplifications, we find very
preliminary, but surprisingly good numerical evidence for the correct classical
limit.Comment: 4 pages, 8 figure
Transitions in non-conserving models of Self-Organized Criticality
We investigate a random--neighbours version of the two dimensional
non-conserving earthquake model of Olami, Feder and Christensen [Phys. Rev.
Lett. {\bf 68}, 1244 (1992)]. We show both analytically and numerically that
criticality can be expected even in the presence of dissipation. As the
critical level of conservation, , is approached, the cut--off of the
avalanche size distribution scales as . The
transition from non-SOC to SOC behaviour is controlled by the average branching
ratio of an avalanche, which can thus be regarded as an order
parameter of the system. The relevance of the results are discussed in
connection to the nearest-neighbours OFC model (in particular we analyse the
relevance of synchronization in the latter).Comment: 8 pages in latex format; 5 figures available upon reques
Peaks and Troughs in Helioseismology: The Power Spectrum of Solar Oscillations
I present a matched-wave asymptotic analysis of the driving of solar
oscillations by a general localised source. The analysis provides a simple
mathematical description of the asymmetric peaks in the power spectrum in terms
of the relative locations of eigenmodes and troughs in the spectral response.
It is suggested that the difference in measured phase function between the
modes and the troughs in the spectrum will provide a key diagnostic of the
source of the oscillations. I also suggest a form for the asymmetric line
profiles to be used in the fitting of solar power spectra.
Finally I present a comparison between the numerical and asymptotic
descriptions of the oscillations. The numerical results bear out the
qualitative features suggested by the asymptotic analysis but suggest that
numerical calculations of the locations of the troughs will be necessary for a
quantitative comparison with the observations.Comment: 18 pages + 8 separate figures. To appear in Ap
Local models of stellar convection II: Rotation dependence of the mixing length relations
We study the mixing length concept in comparison to three-dimensional
numerical calculations of convection with rotation. In a limited range, the
velocity and temperature fluctuations are linearly proportional to the
superadiabaticity, as predicted by the mixing length concept and in accordance
with published results. The effects of rotation are investigated by varying the
Coriolis number, Co = 2 Omega tau, from zero to roughly ten, and by calculating
models at different latitudes. We find that \alpha decreases monotonically as a
function of the Coriolis number. This can be explained by the decreased spatial
scale of convection and the diminished efficiency of the convective energy
transport, the latter of which leads to a large increase of the
superadibaticity, \delta = \nabla - \nabla_ad as function of Co. Applying a
decreased mixing length parameter in a solar model yields very small
differences in comparison to the standard model within the convection zone. The
main difference is the reduction of the overshooting depth, and thus the depth
of the convection zone, when a non-local version of the mixing length concept
is used. Reduction of \alpha by a factor of roughly 2.5 is sufficient to
reconcile the difference between the model and helioseismic results. The
numerical results indicate reduction of \alpha by this order of magnitude.Comment: Final published version, 8 pages, 9 figure
The CoRoT Evolution and Seismic Tools Activity: Goals and Tasks
The forthcoming data expected from space missions such as CoRoT require the
capacity of the available tools to provide accurate models whose numerical
precision is well above the expected observational errors. In order to secure
that these tools meet the specifications, a team has been established to test
and, when necessary, to improve the codes available in the community. The CoRoT
evolution and seismic tool activity (ESTA) has been set up with this mission.
Several groups have been involved. The present paper describes the motivation
and the organisation of this activity, providing the context and the basis for
the presentation of the results that have been achieved so far. This is not a
finished task as future even better data will continue to demand more precise
and complete tools for asteroseismology.Comment: 11 pages, 3 figures, accepted for publication in Astrophysics and
Space Science, 'CoRoT ESTA' special volum
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