138,931 research outputs found
Acceleration Rates and Injection Efficiencies in Oblique Shocks
The rate at which particles are accelerated by the first-order Fermi
mechanism in shocks depends on the angle, \teq{\Tbone}, that the upstream
magnetic field makes with the shock normal. The greater the obliquity the
greater the rate, and in quasi-perpendicular shocks rates can be hundreds of
times higher than those seen in parallel shocks. In many circumstances
pertaining to evolving shocks (\eg, supernova blast waves and interplanetary
traveling shocks), high acceleration rates imply high maximum particle energies
and obliquity effects may have important astrophysical consequences. However,
as is demonstrated here, the efficiency for injecting thermal particles into
the acceleration mechanism also depends strongly on obliquity and, in general,
varies inversely with \teq{\Tbone}. The degree of turbulence and the resulting
cross-field diffusion strongly influences both injection efficiency and
acceleration rates. The test particle \mc simulation of shock acceleration used
here assumes large-angle scattering, computes particle orbits exactly in
shocked, laminar, non-relativistic flows, and calculates the injection
efficiency as a function of obliquity, Mach number, and degree of turbulence.
We find that turbulence must be quite strong for high Mach number, highly
oblique shocks to inject significant numbers of thermal particles and that only
modest gains in acceleration rates can be expected for strong oblique shocks
over parallel ones if the only source of seed particles is the thermal
background.Comment: 24 pages including 6 encapsulated figures, as a compressed,
uuencoded, Postscript file. Accepted for publication in the Astrophysical
Journa
Inverse Bremsstrahlung in Shocked Astrophysical Plasmas
There has recently been interest in the role of inverse bremsstrahlung, the
emission of photons by fast suprathermal ions in collisions with ambient
electrons possessing relatively low velocities, in tenuous plasmas in various
astrophysical contexts. This follows a long hiatus in the application of
suprathermal ion bremsstrahlung to astrophysical models since the early 1970s.
The potential importance of inverse bremsstrahlung relative to normal
bremsstrahlung, i.e. where ions are at rest, hinges upon the underlying
velocity distributions of the interacting species. In this paper, we identify
the conditions under which the inverse bremsstrahlung emissivity is significant
relative to that for normal bremsstrahlung in shocked astrophysical plasmas. We
determine that, since both observational and theoretical evidence favors
electron temperatures almost comparable to, and certainly not very deficient
relative to proton temperatures in shocked plasmas, these environments
generally render inverse bremsstrahlung at best a minor contributor to the
overall emission. Hence inverse bremsstrahlung can be safely neglected in most
models invoking shock acceleration in discrete sources such as supernova
remnants. However, on scales > 100pc distant from these sources, Coulomb
collisional losses can deplete the cosmic ray electrons, rendering inverse
bremsstrahlung, and perhaps bremsstrahlung from knock-on electrons, possibly
detectable.Comment: 13 pages, including 2 figures, using apjgalley format; to appear in
the January 10, 2000 issue, of the Astrophysical Journa
Table-lookup algorithm for pattern recognition: ELLTAB (Elliptical Table)
Remotely sensed unit is assigned to category by merely looking up its channel readings in four-dimensional table. Approach makes it possible to process multispectral scanner data using a minicomputer
A Study for a Tracking Trigger at First Level for CMS at SLHC
It is expected that the LHC accelerator and experiments will undergo a
luminosity upgrade which will commence after several years of running. This
part of the LHC operations is referred to as Super-LHC (SLHC) and is expected
to provide beams of an order of magnitude larger luminosity (1035cm-2sec-1)
than the current design. Preliminary results are presented from a feasibility
study for a First Level Tracking Trigger for CMS at the SLHC using the data of
the inner tracking detector. As a model for these studies the current CMS pixel
detector with the same pixel size and radial distances from the beam has been
used. Monte Carlo studies have been performed using the full CMS simulation
package (OSCAR) and the occupancy of such a detector at SLHC beam conditions
has been calculated. The design of an electron trigger which uses both the
calorimeter energy depositions and the pixel data to identify isolated
electrons and photons has been investigated. Results on the tracker occupancy
and the electron trigger performance are presentedComment: Presented at LECC, Heidelberg 200
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FT-IR microanalysis of mineral separates from primitive meteorites: techniques, problems and solutions
From the Introduction: We compared several methods of infrared micro spectroscopy using an FT-IR microscope and workbench. This is part of a project to assemble a database of infrared and optical spectra from mineral separates from meteorites, for comparison with astronomical data. Since we usually have to work with small amounts of material (original grain sizes often <50 m), special sample preparation and analytical procedures
have to be applied
Biodetection grinder
Work on a biodetection grinder is summarized. It includes development of the prototype grinder, second generation grinder, and the production version of the grinder. Tests showed the particle size distribution was satisfactory and biological evaluation confirmed the tests
Single stage experimental evaluation of slotted rotor and stator blading. Part I - Analysis and design
Analysis and design of slotted rotor and stator blading for application to compressors in advanced airbreathing propulsion system
Procedure for analysis of nickel-cadmium cell materials
Quality control procedures include analyses on electrolyte, active materials, and separators for nickel cadmium cell materials. Tests range from the visual/mechanical inspection of cells to gas sampling, electrolyte extract, electrochemical tests, and physical measurements
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