700 research outputs found
A Review of Micro-Contact Physics for Microelectromechanical Systems (MEMS) Metal Contact Switches
Innovations in relevant micro-contact areas are highlighted, these include, design, contact resistance modeling, contact materials, performance and reliability. For each area the basic theory and relevant innovations are explored. A brief comparison of actuation methods is provided to show why electrostatic actuation is most commonly used by radio frequency microelectromechanical systems designers. An examination of the important characteristics of the contact interface such as modeling and material choice is discussed. Micro-contact resistance models based on plastic, elastic-plastic and elastic deformations are reviewed. Much of the modeling for metal contact micro-switches centers around contact area and surface roughness. Surface roughness and its effect on contact area is stressed when considering micro-contact resistance modeling. Finite element models and various approaches for describing surface roughness are compared. Different contact materials to include gold, gold alloys, carbon nanotubes, composite gold-carbon nanotubes, ruthenium, ruthenium oxide, as well as tungsten have been shown to enhance contact performance and reliability with distinct trade offs for each. Finally, a review of physical and electrical failure modes witnessed by researchers are detailed and examined
Modeling Micro-Porous Surfaces for Secondary Electron Emission Control to Suppress Multipactor
This work seeks to understand how the topography of a surface can be engineered to control secondary electron emission (SEE) for multipactor suppression. Two unique, semi-empirical models for the secondary electron yield (SEY) of a micro-porous surface are derived and compared. The first model is based on a two-dimensional (2D) pore geometry. The second model is based on a three-dimensional (3D) pore geometry. The SEY of both models is shown to depend on two categories of surface parameters: chemistry and topography. An important parameter in these models is the probability of electron emissions to escape the surface pores. This probability is shown by both models to depend exclusively on the aspect ratio of the pore (the ratio of the pore height to the pore diameter). The increased accuracy of the 3D model (compared to the 2D model) results in lower electron escape probabilities with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes. The use of an additive metal-deposition process (instead of the more commonly used subtractive metal-etch process) provided geometrically ideal pores which were necessary to accurately assess the 2D and 3D models. Comparison of the experimentally measured SEY data with model predictions from both the 2D and 3D models illustrates the improved accuracy of the 3D model. For a micro-porous gold surface consisting of pores with aspect ratios of two and a 50% pore density, the 3D model predicts that the maximum total SEY will be one. This provides optimal engineered surface design objectives to pursue for multipactor suppression using gold surfaces
New Measurement of the Cosmic-Ray Positron Fraction from 5 to 15 GeV
We present a new measurement of the cosmic-ray positron fraction at energies
between 5 and 15 GeV with the balloon-borne HEAT-pbar instrument in the spring
of 2000. The data presented here are compatible with our previous measurements,
obtained with a different instrument. The combined data from the three HEAT
flights indicate a small positron flux of non-standard origin above 5 GeV. We
compare the new measurement with earlier data obtained with the HEAT-e+-
instrument, during the opposite epoch of the solar cycle, and conclude that our
measurements do not support predictions of charge sign dependent solar
modulation of the positron abundance at 5 GeV.Comment: accepted for publication in PR
Non-Thermal Production of WIMPs, Cosmic Excesses and -rays from the Galactic Center
In this paper we propose a dark matter model and study aspects of its
phenomenology. Our model is based on a new dark matter sector with a U(1)'
gauge symmetry plus a discrete symmetry added to the Standard Model of particle
physics. The new fields of the dark matter sector have no hadronic charges and
couple only to leptons. Our model can not only give rise to the observed
neutrino mass hierarchy, but can also generate the baryon number asymmetry via
non-thermal leptogenesis. The breaking of the new U(1)' symmetry produces
cosmic strings. The dark matter particles are produced non-thermally from
cosmic string loop decay which allows one to obtain sufficiently large
annihilation cross sections to explain the observed cosmic ray positron and
electron fluxes recently measured by the PAMELA, ATIC, PPB-BETS, Fermi-LAT, and
HESS experiments while maintaining the required overall dark matter energy
density. The high velocity of the dark matter particles from cosmic string loop
decay leads to a low phase space density and thus to a dark matter profile with
a constant density core in contrast to what happens in a scenario with
thermally produced cold dark matter where the density keeps rising towards the
center. As a result, the flux of gamma rays radiated from the final leptonic
states of dark matter annihilation from the Galactic center is suppressed and
satisfies the constraints from the HESS gamma-ray observations.Comment: 23 pages, 2 figure
Energy Spectra, Altitude Profiles and Charge Ratios of Atmospheric Muons
We present a new measurement of air shower muons made during atmospheric
ascent of the High Energy Antimatter Telescope balloon experiment. The muon
charge ratio mu+ / mu- is presented as a function of atmospheric depth in the
momentum interval 0.3-0.9 GeV/c. The differential mu- momentum spectra are
presented between 0.3 and about 50 GeV/c at atmospheric depths between 13 and
960 g/cm^2. We compare our measurements with other recent data and with Monte
Carlo calculations of the same type as those used in predicting atmospheric
neutrino fluxes. We find that our measured mu- fluxes are smaller than the
predictions by as much as 70% at shallow atmospheric depths, by about 20% at
the depth of shower maximum, and are in good agreement with the predictions at
greater depths. We explore the consequences of this on the question of
atmospheric neutrino production.Comment: 11 pages, 8 figures, to appear in Phys. Rev. D (2000
Proton and Helium Spectra from the CREAM-III Flight
Primary cosmic-ray elemental spectra have been measured with the
balloon-borne Cosmic Ray Energetics And Mass (CREAM) experiment since 2004. The
third CREAM payload (CREAM-III) flew for 29 days during the 2007-2008 Antarctic
season. Energies of incident particles above 1 TeV are measured with a
calorimeter. Individual elements are clearly separated with a charge resolution
of ~0.12 e (in charge units) and ~0.14 e for protons and helium nuclei,
respectively, using two layers of silicon charge detectors. The measured proton
and helium energy spectra at the top of the atmosphere are harder than other
existing measurements at a few tens of GeV. The relative abundance of protons
to helium nuclei is 9.53+-0.03 for the range of 1 TeV/n to 63 TeV/n. The ratio
is considerably smaller than other measurements at a few tens of GeV/n. The
spectra become softer above ~20 TeV. However, our statistical uncertainties are
large at these energies and more data are needed
Positron Propagation and Fluxes from Neutralino Annihilation in the Halo
Supersymmetric neutralinos are one of the most promising candidates for the
dark matter in the Universe. If they exist, they should make up some fraction
of the Milky Way halo. We investigate the fluxes of positrons expected at the
Earth from neutralino annihilation in the halo. Positron propagation is treated
in a diffusion model including energy loss. The positron source function
includes contributions from both continuum and monochromatic positrons. We find
that, for a "canonical" halo model and propagation parameters, the fluxes are
generally too low to be visible. Given the large uncertainties in both
propagation and halo structure, it is however possible to obtain observable
fluxes. We also investigate the shapes of the positron spectra, including fits
to a feature indicated by the results of the HEAT experiment.Comment: 16 pages, 19 figures, uses revte
The Energy Spectra and Relative Abundances of Electrons and Positrons in the Galactic Cosmic Radiation
Observations of cosmic-ray electrons and positrons have been made with a new
balloon-borne detector, HEAT (the "High-Energy Antimatter Telescope"), first
flown in 1994 May from Fort Sumner, NM. We describe the instrumental approach
and the data analysis procedures, and we present results from this flight. The
measurement has provided a new determination of the individual energy spectra
of electrons and positrons from 5 GeV to about 50 GeV, and of the combined
"all-electron" intensity (e+ + e-) up to about 100 GeV. The single power-law
spectral indices for electrons and positrons are alpha = 3.09 +/- 0.08 and 3.3
+/- 0.2, respectively. We find that a contribution from primary sources to the
positron intensity in this energy region, if it exists, must be quite small.Comment: latex2e file, 30 pages, 15 figures, aas2pp4.sty and epsf.tex needed.
To appear in May 10, 1998 issue of Ap.
- …