120,168 research outputs found
Gallium Arsenide preparation and QE Lifetime Studies using the ALICE Photocathode Preparation Facility
In recent years, Gallium Arsenide (GaAs) type photocathodes have become
widely used as electron sources in modern Energy Recovery Linac based light
sources such as the Accelerators and Lasers in Combined Experiments (ALICE) at
Daresbury Laboratory and as polarised electron source for the proposed
International Linear Collider (ILC). Once activated to a Low Electron Affinity
(LEA) state and illuminated by a laser, these materials can be used as a
high-brightness source of both polarised and un-polarised electrons. This paper
presents an effective multi-stage preparation procedure including heat
cleaning, atomic hydrogen cleaning and the activation process for a GaAs
photocathode. The stability of quantum efficiency (QE) and lifetime of
activated to LEA state GaAs photocathode have been studied in the ALICE
load-lock photocathode preparation facility which has a base pressure in the
order of 10^-11 mbar. These studies are supported by further experimental
evidence from surface science techniques such as X-ray Photoelectron
Spectroscopy (XPS) to demonstrate the processes at the atomic level.Comment: Presented at First International Particle Accelerator Conference,
IPAC'10, Kyoto, Japan, from 23 to 28 May 201
Effect of hyperon bulk viscosity on neutron-star r-modes
Neutron stars are expected to contain a significant number of hyperons in
addition to protons and neutrons in the highest density portions of their
cores. Following the work of Jones, we calculate the coefficient of bulk
viscosity due to nonleptonic weak interactions involving hyperons in
neutron-star cores, including new relativistic and superfluid effects. We
evaluate the influence of this new bulk viscosity on the gravitational
radiation driven instability in the r-modes. We find that the instability is
completely suppressed in stars with cores cooler than a few times 10^9 K, but
that stars rotating more rapidly than 10-30% of maximum are unstable for
temperatures around 10^10 K. Since neutron-star cores are expected to cool to a
few times 10^9 K within seconds (much shorter than the r-mode instability
growth time) due to direct Urca processes, we conclude that the gravitational
radiation instability will be suppressed in young neutron stars before it can
significantly change the angular momentum of the star.Comment: final PRD version, minor typos etc correcte
Space shuttle entry and landing navigation analysis
A navigation system for the entry phase of a Space Shuttle mission which is an aided-inertial system which uses a Kalman filter to mix IMU data with data derived from external navigation aids is evaluated. A drag pseudo-measurement used during radio blackout is treated as an additional external aid. A comprehensive truth model with 101 states is formulated and used to generate detailed error budgets at several significant time points -- end-of-blackout, start of final approach, over runway threshold, and touchdown. Sensitivity curves illustrating the effect of variations in the size of individual error sources on navigation accuracy are presented. The sensitivity of the navigation system performance to filter modifications is analyzed. The projected overall performance is shown in the form of time histories of position and velocity error components. The detailed results are summarized and interpreted, and suggestions are made concerning possible software improvements
Single stage experimental evaluation of variable geometry guide vanes and stators. Part 1 - Analysis and design
Variable geometry concepts applied to guide vanes and stators in single stage compressor
Molecular Evolution in Time Dependent Environments
The quasispecies theory is studied for dynamic replication landscapes. A
meaningful asymptotic quasispecies is defined for periodic time dependencies.
The quasispecies' composition is constantly changing over the oscillation
period. The error threshold moves towards the position of the time averaged
landscape for high oscillation frequencies and follows the landscape closely
for low oscillation frequencies.Comment: 5 pages, 3 figures, Latex, uses Springer documentclass llncs.cl
The evolution of the cluster X-ray scaling relations in the WARPS sample at 0.6<z<1.0
The X-ray properties of a sample of 11 high-redshift (0.6<z<1.0) clusters
observed with Chandra and/or XMM are used to investigate the evolution of the
cluster scaling relations. The observed evolution of the L-T and M-L relations
is consistent with simple self-similar predictions, in which the properties of
clusters reflect the properties of the universe at their redshift of
observation. When the systematic effect of assuming isothermality on the
derived masses of the high-redshift clusters is taken into account, the
high-redshift M-T and Mgas-T relations are also consistent with self-similar
evolution. Under the assumption that the model of self-similar evolution is
correct and that the local systems formed via a single spherical collapse, the
high-redshift L-T relation is consistent with the high-z clusters having formed
at a significantly higher redshift than the local systems. The data are also
consistent with the more realistic scenario of clusters forming via the
continuous accretion of material. The slope of the L-T relation at
high-redshift (B=3.29+/-0.38) is consistent with the local relation, and
significantly steeper then the self-similar prediction of B=2. This suggests
that the non-gravitational processes causing the steepening occurred at z>1 or
in the early stages of the clusters' formation, prior to their observation. The
properties of the intra-cluster medium at high-redshift are found to be similar
to those in the local universe. The mean surface-brightness profile slope for
the sample is 0.66+/-0.05, the mean gas mass fractions within R2500 and R200
are 0.073+/-0.010 and 0.12+/-0.02 respectively, and the mean metallicity of the
sample is 0.28+/-0.16 solar.Comment: 23 pages, 17 figures. Accepted for publication in MNRAS. Revised to
match accepted version: reanalysed data with latest calibrations, several
minor changes. Conclusions unchange
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