125,200 research outputs found
The design and development of a constant-speed solar array drive
The design and development of a constant-speed solar array drive system for use in high power communications satellites is described. The relationship between continuity of motion in the solar array drive and spacecraft attitude disturbance is investigated. The selection of the system design based on the design requirements including spacecraft disturbance is discussed. The system comprises two main parts: the drive mechanism including small angle stepper motor and reduction gearing and the control electronics including ministepping drive circuits, such that a very small output step size is achieved. Factors contributing to discontinuities in motion are identified and discussed. Test methods for measurement of very small amplitudes of discontinuity at low rotational rates are described to assist in the testing of similar mechanisms
High Gradient Superconducting Cavity with Low Surface EM Fields and Well-Suppressed HOMS for the ILC
We present an optimized geometry for a 1.3 GHz superconducting cavity in
which the surface electromagnetic fields have been minimized and the bandwidth
of the fundamental mode has been maximized. We refer to this design as the New
Low Surface Field (NLSF) cavity. Earlier work focused on properties of the
fundamental mode. Here we additionally study higher order modes (HOMs), means
of damping them, and short range wake fields. A two-band circuit model is
employed in order to facilitate rapid characterization of cavity HOMs.Comment: Presented at First International Particle Accelerator Conference,
IPAC'10, Kyoto, Japan, from 23 to 28 May 201
Regression with strongly correlated data
This paper discusses linear regression of strongly correlated data that
arises, for example, in magnetohydrodynamic equilibrium reconstructions. We
have proved that, generically, the covariance matrix of the estimated
regression parameters for fixed sample size goes to zero as the correlations
become unity. That is, in this limit the estimated parameters are known with
perfect accuracy. Simple examples are shown to illustrate this effect and the
nature of the exceptional cases in which the estimate covariance does not go to
zero
Tachyon Defect Formation and Reheating in Brane-Antibrane Inflation
We study analytically the dynamical formation of lower dimensional branes at
the endpoint of brane-antibrane inflation through the condensation of
topological defects of the tachyon field which describes the instability of the
initial state. We then use this information to quantify the efficiency of the
reheating which is due to the coupling of time dependent tachyon background to
massless gauge fields which will be localized on the final state branes. We
improve upon previous estimates indicating that this can be an efficient
reheating mechanism for observers on the brane.Comment: 9 pages. Talk given at the 26th annual
Montreal-Rochester-Syracuse-Toronto Conference on High-Energy Physics: MRST
200
SRF cavity geometry optimization for the ILC with minimized surface e.m. fields and superior bandwidth
The main linacs of the ILC consist of nine-cell cavities based on the TESLA
design. In order to facilitate reaching higher gradients we have re-designed
the cavity shape. This leads to a reduction, comparable to several current
designs, in both the ratio of the surface electric field to the accelerating
field (Es/Ea) and the magnetic field to the accelerating field (Bs/Ea). The
bandwidth of the accelerating mode is also optimized. This new shape, which we
refer to as the New Low Surface Field (NLSF) design, bears comparison with the
Ichiro, Re-entrant and LSF designs.Comment: Presented at the 23rd Particle Accelerator Conference (PAC09)
Vancouver, Canad
Teaching Spoken English at Junior High School: A Comparison of TPR and PPP
This article reports on an experimental methods-comparison study, which was undertaken with beginner level junior high school students (aged 12 and 13) in Japan. The study aimed to investigate which type of teaching, Total Physical Response (TPR) or Present Practice Produce (PPP), was more effective in developing productive and receptive knowledge of a set of collocations. Results showed that both types of teaching had a significant impact upon the development of understanding and using the target language. However, there were no significant differences between the effectiveness of TPR and PPP, apart from a short-term benefit for PPP in terms of receptive knowledge. This shows that both types of teaching can have a positive impact upon learners of this age and level and that there is a need for further research to investigate the effectiveness of these communicative methodologies in this context
Mantle formation, coagulation and the origin of cloud/core-shine: II. Comparison with observations
Many dense interstellar clouds are observable in emission in the near-IR,
commonly referred to as "Cloudshine", and in the mid-IR, the so-called
"Coreshine". These C-shine observations have usually been explained with grain
growth but no model has yet been able to self-consistently explain the dust
spectral energy distribution from the near-IR to the submm. We want to
demonstrate the ability of our new core/mantle evolutionary dust model THEMIS
(The Heterogeneous dust Evolution Model at the IaS), which has been shown to be
valid in the far-IR and submm, to reproduce the C-shine observations. Our
starting point is a physically motivated core/mantle dust model. It consists of
3 dust populations: small aromatic-rich carbon grains; bigger core/mantle
grains with mantles of aromatic-rich carbon and cores either made of amorphous
aliphatic-rich carbon or amorphous silicate. We assume an evolutionary path
where these grains, when entering denser regions, may first form a second
aliphatic-rich carbon mantle (coagulation of small grains, accretion of carbon
from the gas phase), second coagulate together to form large aggregates, and
third accrete gas phase molecules coating them with an ice mantle. To compute
the corresponding dust emission and scattering, we use a 3D Monte-Carlo
radiative transfer code. We show that our global evolutionary dust modelling
approach THEMIS allows us to reproduce C-shine observations towards dense
starless clouds. Dust scattering and emission is most sensitive to the cloud
central density and to the steepness of the cloud density profile. Varying
these two parameters leads to changes, which are stronger in the near-IR, in
both the C-shine intensity and profile. With a combination of aliphatic-rich
mantle formation and low-level coagulation into aggregates, we can
self-consistently explain the observed C-shine and far-IR/submm emission
towards dense starless clouds.Comment: Paper accepted for publication in A&A with companion paper "Mantle
formation, coagulation and the origin of cloud/core-shine: I. Dust scattering
and absorption in the IR", A.P Jones, M. Koehler, N. Ysard, E. Dartois, M.
Godard, L. Gavila
Study of Beam Diagnostics with Trapped Modes in Third Harmonic Superconducting Cavities at FLASH
Off-axis beams passing through an accelerating cavity excite dipole modes
among other higher order modes (HOMs). These modes have linear dependence on
the transverse beam offset from the cavity axis. Therefore they can be used to
monitor the beam position within the cavity. The fifth dipole passband of the
third harmonic superconducting cavities at FLASH has modes trapped within each
cavity and do not propagate through the adjacent beam pipes, while most other
cavity modes do. This could enable the beam position measurement in individual
cavities. This paper investigates the possibility to use the fifth dipole band
for beam alignment in the third harmonic cavity module. Simulations and
measurements both with and without beam-excitations are presented. Various
analysis methods are used and compared. A good correlation of HOM signals to
the beam position is observed.Comment: 2nd International Particle Accelerator Conference (IPAC '11), San
Sebastian, Spain, Sep 4-9, 201
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