49,058 research outputs found
Transistorized Marx bank pulse circuit provides voltage multiplication with nanosecond rise-time
Base-triggered avalanche transistor circuit used in a Marx bank pulser configuration provides voltage multiplication with nanosecond rise-time. The avalanche-mode transistors replace conventional spark gaps in the Marx bank. The delay time from an input signal to the output signal to the output is typically 6 nanoseconds
Precision capacitor has improved temperature and operational stability
Vacuum dielectric capacitor is fabricated from materials with very low temperature coefficients of expansion. This precision capacitor in the 1000-2000 picofarad range has a near-zero temperature coefficient of capacitance, eliminates ion chamber action caused by air ionization in the dielectric, and minimizes electromagnetic field charging effects
Bayesian Restricted Likelihood Methods: Conditioning on Insufficient Statistics in Bayesian Regression
Bayesian methods have proven themselves to be successful across a wide range
of scientific problems and have many well-documented advantages over competing
methods. However, these methods run into difficulties for two major and
prevalent classes of problems: handling data sets with outliers and dealing
with model misspecification. We outline the drawbacks of previous solutions to
both of these problems and propose a new method as an alternative. When working
with the new method, the data is summarized through a set of insufficient
statistics, targeting inferential quantities of interest, and the prior
distribution is updated with the summary statistics rather than the complete
data. By careful choice of conditioning statistics, we retain the main benefits
of Bayesian methods while reducing the sensitivity of the analysis to features
of the data not captured by the conditioning statistics. For reducing
sensitivity to outliers, classical robust estimators (e.g., M-estimators) are
natural choices for conditioning statistics. A major contribution of this work
is the development of a data augmented Markov chain Monte Carlo (MCMC)
algorithm for the linear model and a large class of summary statistics. We
demonstrate the method on simulated and real data sets containing outliers and
subject to model misspecification. Success is manifested in better predictive
performance for data points of interest as compared to competing methods
Continued development of doped-germanium photoconductors for astronomical observations at wavelengths from 30 to 120 micrometers
The development of doped-germanium detectors which have optimized performance in the 30- to 120-mu m wavelength range and are capable of achieving the objectives of the infrared astronomical satellite (IRAS) space mission is discussed. Topics covered include the growth and evaluation of Ge:Ga and Ge:Be crystals, procedures for the fabrication and testing of detectors, irradiance calculations, detector responsivity, and resistance measurements through MOSFET. Test data are presented in graphs and charts
Wear and Fatigue of Railway Track Caused by Contamination, Sanding and Surface Damage
The wheel rail contact operates in an arduous
environment. Damage to the surface of either
component is possible during manufacture, installation,
or operation. The question arises as to how tolerant is
the railway wheel or section of track to surface
indentation or damage.
In this work a twin disc simulation has been used
to relate the level of surface damage (as well as the way
it is generated) to the fatigue life of the surfaces. A
related problem is the presence of solid contamination
on the track. Sand (applied for improved adhesion) or
track ballast material can cause damage to the rail and
wheel surfaces. These mechanisms have been explored
to assess the effect on contact fatigue life and wear. The
disc specimens have been either artificially damaged
(with dents and scratches) or run with particles of sand
or ballast material. The discs were then loaded and
rotated at realistic conditions of contact pressure and
controlled slip.
For normal operation of the contact, either dry or
with water lubrication, surface dents and scratches have
little effect on fatigue life. The normal plastic flow in
the rail surface layer acts to close up dents. The failure
of the disc is then by fatigue cracking across the whole
surface with no particular preference to the dent
location. Alternatively, if the contact is lubricated with
oil then this plastic flow is greatly reduced and the dents
act as stress raisers and fatigue cracks initiate from their
trailing edge.
Sand or ballast particles are crushed as they enter
the wheel/rail contact. The fragments indent the surfaces
and rapidly roughen the contact faces. The surface
indentation is relatively minor, but the presence of
particles increases the level of traction (over the wet
case) and promotes further surface plastic flow. This
can reduce the residual fatigue life of the contact.
Further, high concentrations of sand were shown to
promote a low cycle fatigue process that caused very
high wear by the spallation of material.
The twin disc simulations have shown that, under
conditions similar to that of wheel/rail operation,
surface damage is not a primary cause of fatigue failure.
However, wear is greatly accelerated by the presence of
solid contaminants and some evidence of a low cycle
fatigue process was observed for sanded contacts
A numerical model of twin disc test arrangement for the evaluation of railway wheel wear prediction methods
Twin disc tests are commonly used to study wear in railway materials. In this work the implementation of a numerical model of the twin disc arrangement is given, which reproduces the distribution of tangential forces over the contact patch between the two discs. Wear is subsequently calculated by relating the forces and creepage between the two discs using three different wear functions found in the literature. The resulting wear rates are compared with experimental data for discs made of common railway wheel and rail steels. This allows a comparison and assessment of the validity of the different wear algorithms considered
On the accretion origin of a vast extended stellar disk around the Andromeda galaxy
We present the discovery of an inhomogenous, low-surface brightness, extended
disk-like structure around the Andromeda galaxy (M31) based on a large
kinematic survey of more than 2800 stars with the Keck/DEIMOS spectrograph. The
stellar structure spans radii from 15 kpc out to ~40 kpc, with detections out
to R ~ 70 kpc. The constituent stars lag the expected velocity of circular
orbits in the plane of the M31 disk by ~40 kms and have a velocity dispersion
of ~30 kms. The color range on the upper RGB shows a large spread indicative of
a population with a significant range of metallicity. The mean metallicity of
the population, measured from Ca II equivalent widths, is [Fe/H] = -0.9 +/-
0.2. The morphology of the structure is irregular at large radii, and shows a
wealth of substructures which must be transitory in nature, and are almost
certainly tidal debris. The presence of these substructures indicates that the
global entity was formed by accretion. This extended disk follows smoothly on
from the central parts of M31 disk with an exponential density law of
scale-length of 5.1 +/- 0.1 kpc, similar to that of the bright inner disk. The
population possesses similar kinematic and abundance properties over the entire
region where it is detected in the survey. We estimate that the structure
accounts for approximately 10% of the total luminosity of the M31 disk, and
given the huge scale, contains ~30% of the total disk angular momentum. This
finding indicates that at least some galactic stellar disks are vastly larger
than previously thought and are formed, at least in their outer regions,
primarily by accretion. [abridged]Comment: 20 pages, 30 figures, ApJ submitte
Elastic precursor of the transformation from glycolipid-nanotube to -vesicle
By the combination of optical tweezer manipulation and digital video
microscopy, the flexural rigidity of single glycolipid "nano" tubes has been
measured below the transition temperature at which the lipid tubules are
transformed into vesicles. Consequently, we have found a clear reduction of the
rigidity obviously before the transition as temperature increasing. Further
experiments of infrared spectroscopy (FT-IR) and differential scanning
calorimetry (DSC) have suggested a microscopic change of the tube walls,
synchronizing with the precursory softening of the nanotubes.Comment: 9 pages, 6 figure
A near zero velocity dispersion stellar component in the Canes Venatici dwarf spheroidal galaxy
We present a spectroscopic survey of the newly-discovered Canes Venatici
dwarf galaxy using the Keck/DEIMOS spectrograph. Two stellar populations of
distinct kinematics are found to be present in this galaxy: an extended,
metal-poor component, of half-light radius 7'.8(+2.4/-2.1), which has a
velocity dispersion of 13.9(+3.2/-2.5) km/s, and a more concentrated
(half-light radius 3'.6(+1.1/-0.8) metal-rich component of extremely low
velocity dispersion. At 99% confidence, the upper limit to the central velocity
dispersion of the metal-rich population is 1.9 km/s. This is the lowest
velocity dispersion ever measured in a galaxy. We perform a Jeans analysis on
the two components, and find that the dynamics of the structures can only be
consistent if we adopt extreme (and unlikely) values for the scale length and
velocity dispersion of the metal-poor population. With a larger radial velocity
sample and improved measurements of the density profile of the two populations,
we anticipate that it will be possible to place strong constraints on the
central distribution of the dark matter in this galaxy.Comment: 5 pages, 7 figures, accepted by MNRA
The synergistic effects of slip ring-brush design and materials
Synergistic effects of slip ring-brush design and fabrication for vacuum application determined by friction, wear, electrical noise, and dielectric strength dat
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