112,171 research outputs found
Human comfort in relation to sinusoidal vibration
An investigation was made to assess the overall subjective comfort levels to sinusoidal excitations over the range 1 to 19 Hz using a two axis electrohydraulic vibration simulator. Exposure durations of 16 minutes, 25 minutes, 1 hour, and 2.5 hours have been considered. Subjects were not exposed over such durations, but were instructed to estimate the overall comfort levels preferred had they been constantly subjected to vibration over such durations
Analytical expressions for fringe fields in multipole magnets
Fringe fields in multipole magnets can have a variety of effects on the
linear and nonlinear dynamics of particles moving along an accelerator
beamline. An accurate model of an accelerator must include realistic models of
the magnet fringe fields. Fringe fields for dipoles are well understood and can
be modelled at an early stage of accelerator design in such codes as MAD8, MADX
or ELEGANT. However, usually it is not until the final stages of a design
project that it is possible to model fringe fields for quadrupoles or higher
order multipoles. Even then, existing techniques rely on the use of a numerical
field map, which will usually not be available until the magnet design is well
developed. Substitutes for the full field map exist but these are typically
based on expansions about the origin and rely heavily on the assumption that
the beam travels more or less on axis throughout the beam line. In some types
of machine (for example, a non-scaling FFAG such as EMMA) this is not a good
assumption.
In this paper, a method for calculating fringe fields based on analytical
expressions is presented, which allows fringe field effects to be included at
the start of an accelerator design project. The magnetostatic Maxwell equations
are solved analytically and a solution that fits all orders of multipoles
derived. Quadrupole fringe fields are considered in detail as these are the
ones that give the strongest effects. Two examples of quadrupole fringe fields
are presented. The first example is a magnet in the LHC inner triplet, which
consists of a set of four quadrupoles providing the final focus to the beam,
just before the interaction point. Quadrupoles in EMMA provide the second
example. In both examples, the analytical expressions derived in this paper for
quadrupole fringe fields provide a good approximation to the field maps
obtained from a numerical magnet modelling code.Comment: 27 pages, 39 figures. The figures are new with respect to the
previous version, Several mistakes also correcte
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
A Cosmic Microwave Background Radiation Polarimeter Using Superconducting Bearings
Measurements of the polarization of the cosmic microwave background (CMB)
radiation are expected to significantly increase our understanding of the early
universe. We present a design for a CMB polarimeter in which a cryogenically
cooled half wave plate rotates by means of a high-temperature superconducting
(HTS) bearing. The design is optimized for implementation in MAXIPOL, a
balloon-borne CMB polarimeter. A prototype bearing, consisting of commercially
available ring-shaped permanent magnet and an array of YBCO bulk HTS material,
has been constructed. We measured the coefficient of friction as a function of
several parameters including temperature between 15 and 80 K, rotation
frequency between 0.3 and 3.5 Hz, levitation distance between 6 and 10 mm, and
ambient pressure between 10^{-7} and 1 torr. The low rotational drag of the HTS
bearing allows rotations for long periods of time with minimal input power and
negligible wear and tear thus making this technology suitable for a future
satellite mission.Comment: 6 pages, IEEE-Transactions of Applied Superconductivity, 2003, Vol.
13, in pres
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
Nature of fault planes in solid neutron star matter
The properties of tectonic earthquake sources are compared with those deduced
here for fault planes in solid neutron-star matter. The conclusion that
neutron-star matter cannot exhibit brittle fracture at any temperature or
magnetic field is significant for current theories of pulsar glitches, and of
the anomalous X-ray pulsars and soft-gamma repeaters.Comment: 5 AAS LaTeX pages 1 eps figur
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