8,843 research outputs found
Casting copper to tungsten for high-power arc lamp cathodes
Voids forming at interface when copper is cast onto tungsten can be eliminated by adding wetting agent during casting process. Small amount of copper and nickel are cast onto thoriated tungsten insert, insert is recast with more copper to form electrode. Good thermal conductance results in long-lived cathode
Computer program for pulsed thermocouples with corrections for radiation effects
A pulsed thermocouple was used for measuring gas temperatures above the melting point of common thermocouples. This was done by allowing the thermocouple to heat until it approaches its melting point and then turning on the protective cooling gas. This method required a computer to extrapolate the thermocouple data to the higher gas temperatures. A method that includes the effect of radiation in the extrapolation is described. Computations of gas temperature are provided, along with the estimate of the final thermocouple wire temperature. Results from tests on high temperature combustor research rigs are presented
Generic features of Einstein-Aether black holes
We reconsider spherically symmetric black hole solutions in Einstein-Aether
theory with the condition that this theory has identical PPN parameters as
those for general relativity, which is the main difference from the previous
research. In contrast with previous study, we allow superluminal propagation of
a spin-0 Aether-gravity wave mode. As a result, we obtain black holes having a
spin-0 "horizon" inside an event horizon. We allow a singularity at a spin-0
"horizon" since it is concealed by the event horizon. If we allow such a
configuration, the kinetic term of the Aether field can be large enough for
black holes to be significantly different from Schwarzschild black holes with
respect to ADM mass, innermost stable circular orbit, Hawking temperature, and
so on. We also discuss whether or not the above features can be seen in more
generic vector-tensor theories.Comment: 9 pages, 9 figures, basic equations and their analytic arguments are
adde
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The impact of market and supply configurations on the costs of tendering in the construction industry
The cost of tendering in the construction industry is widely suspected to be excessive, but there is little robust empirical evidence to demonstrate this. It also seems that innovative working practices may reduce the costs of undertaking construction projects and the consequent improvement in relationships should increase overall value for money. The aim of this proposed research project is to develop mechanisms for measuring the true costs of tendering based upon extensive in-house data collection undertaken in a range of different construction firms. The output from this research will enable all participants in the construction process to make better decisions about how to select members of the team and identify the price and scope of their obligations
Epitaxial growth of 6H silicon carbide in the temperature range 1320 C to 1390 C
High-quality epitaxial layers of 6H SiC have been grown on 6H SiC substrates with the grown direction perpendicular to the crystal c-axis. The growth was by chemical vapor deposition from methyltrichlorosilane (CH3SiCl3) in hydrogen at temperatures in the range of 1320 to 1390 C. Epitaxial layers up to 80 microns thick were grown at rates of 0.4 microns/min. Attempts at growth on the (0001) plane of 6H SiC substrates under similar conditions resulted in polycrystalline cubic SiC layers. Optical and X-ray diffraction techniques were used to characterize the grown layers
Scalar radiation from Chameleon-shielded regions
I study the profile of the Chameleon field around a radially pulsating mass.
Focusing on the case in which the background (static) Chameleon profile
exhibits a thin-shell, I add small perturbations to the source in the form of
time-dependent radial pulsations. It is found that the Chameleon field inherits
a time-dependence, there is a resultant scalar radiation from the region of the
source and the metric outside the spherically symmetric mass is not static.
This has several interesting and potentially testable consequences.Comment: 4 pages, 4 figures, slightly edited version matching the journal
versio
Solar system tests of scalar field models with an exponential potential
We consider a scenario where a scalar field has dynamics ruled by an
exponential potential, such as those arising from some quintessence type
models, and aim at obtaining phenomenological manifestations of this entity
within our Solar System. To do so, we assume a perturbative regime, derive the
perturbed Schwarzschild metric, and extract the relevant post-Newtonian
parameters.Comment: 5 page
Strong field effects on binary systems in Einstein-aether theory
"Einstein-aether" theory is a generally covariant theory of gravity
containing a dynamical preferred frame. This article continues an examination
of effects on the motion of binary pulsar systems in this theory, by
incorporating effects due to strong fields in the vicinity of neutron star
pulsars. These effects are included through an effective approach, by treating
the compact bodies as point particles with nonstandard, velocity dependent
interactions parametrized by dimensionless "sensitivities". Effective
post-Newtonian equations of motion for the bodies and the radiation damping
rate are determined. More work is needed to calculate values of the
sensitivities for a given fluid source, so precise constraints on the theory's
coupling constants cannot yet be stated. It is shown, however, that strong
field effects will be negligible given current observational uncertainties if
the dimensionless couplings are less than roughly 0.01 and two conditions that
match the PPN parameters to those of pure general relativity are imposed. In
this case, weak field results suffice and imply one further condition on the
couplings. Thus, there exists a one-parameter family of Einstein-aether
theories with "small-enough" couplings that passes all current observational
tests. No conclusion can yet be reached for large couplings.Comment: 23 pages, 1 figure; v2: fixed error in Eqn. (70) and resulting bounds
on c'
Estimating spinning binary parameters and testing alternative theories of gravity with LISA
We investigate the effect of spin-orbit and spin-spin couplings on the
estimation of parameters for inspiralling compact binaries of massive black
holes, and for neutron stars inspiralling into intermediate-mass black holes,
using hypothetical data from the proposed Laser Interferometer Space Antenna
(LISA). We work both in Einstein's theory and in alternative theories of
gravity of the scalar-tensor and massive-graviton types. We restrict the
analysis to non-precessing spinning binaries, i.e. to cases where the spins are
aligned normal to the orbital plane. We find that the accuracy with which
intrinsic binary parameters such as chirp mass and reduced mass can be
estimated within general relativity is degraded by between one and two orders
of magnitude. We find that the bound on the coupling parameter omega_BD of
scalar-tensor gravity is significantly reduced by the presence of spin
couplings, while the reduction in the graviton-mass bound is milder. Using fast
Monte-Carlo simulations of 10^4 binaries, we show that inclusion of spin terms
in massive black-hole binaries has little effect on the angular resolution or
on distance determination accuracy. For stellar mass inspirals into
intermediate-mass black holes, the angular resolution and the distance are
determined only poorly, in all cases considered. We also show that, if LISA's
low-frequency noise sensitivity can be extrapolated from 10^-4 Hz to as low as
10^-5 Hz, the accuracy of determining both extrinsic parameters (distance, sky
location) and intrinsic parameters (chirp mass, reduced mass) of massive
binaries may be greatly improved.Comment: 29 pages, 9 figures. Matches version accepted in Physical Review D.
More stringent checks in the inversion of the Fisher matri
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