903 research outputs found
Reliability Testing of AlGaN/GaN HEMTs Under Multiple Stressors
We performed an experiment on AlGaN/GaN HEMTs with high voltage and high power as stressors. We found that devices tested under high power generally degraded more than those tested under high voltage. In particular, the high-voltage-tested devices did not degrade significantly as suggested by some papers in the literature. The same papers in the literature also suggest that high voltages cause cracks and pits. However, the high-voltage-tested devices in this study do not exhibit cracks or pits in TEM images, while the high-power-tested devices exhibit pits
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. © 2017 Author(s)
Supersymmetry and the positron excess in cosmic rays
Recently the HEAT balloon experiment has confirmed an excess of high-energy
positrons in cosmic rays. They could come from annihilation of dark matter in
the galactic halo. We discuss expectations for the positron signal in cosmic
rays from the lightest superpartner. The simplest interpretations are
incompatible with the size and shape of the excess if the relic LSPs evolved
from thermal equilbrium. Non-thermal histories can describe a sufficient
positron rate. Reproducing the energy spectrum is more challenging, but perhaps
possible. The resulting light superpartner spectrum is compatible with collider
physics, the muon anomalous magnetic moment, Z-pole electroweak data, and other
dark matter searches.Comment: 4 pages, 2 figures, references added, minor wording change
Innovator resilience potential: A process perspective of individual resilience as influenced by innovation project termination
Innovation projects fail at an astonishing rate. Yet, the negative effects of innovation project failures on the team members of these projects have been largely neglected in research streams that deal with innovation project failures. After such setbacks, it is vital to maintain or even strengthen project members’ innovative capabilities for subsequent innovation projects. For this, the concept of resilience, i.e. project members’ potential to positively adjust (or even grow) after a setback such as an innovation project failure, is fundamental. We develop the second-order construct of innovator resilience potential, which consists of six components – self-efficacy, outcome expectancy, optimism, hope, self-esteem, and risk propensity – that are important for project members’ potential of innovative functioning in innovation projects subsequent to a failure. We illustrate our theoretical findings by means of a qualitative study of a terminated large-scale innovation project, and derive implications for research and management
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
Accelerator Testing of the General Antiparticle Spectrometer, a Novel Approach to Indirect Dark Matter Detection
We report on recent accelerator testing of a prototype general antiparticle
spectrometer (GAPS). GAPS is a novel approach for indirect dark matter searches
that exploits the antideuterons produced in neutralino-neutralino
annihilations. GAPS captures these antideuterons into a target with the
subsequent formation of exotic atoms. These exotic atoms decay with the
emission of X-rays of precisely defined energy and a correlated pion signature
from nuclear annihilation. This signature uniquely characterizes the
antideuterons. Preliminary analysis of data from a prototype GAPS in an
antiproton beam at the KEK accelerator in Japan has confirmed the
multi-X-ray/pion star topology and indicated X-ray yields consistent with prior
expectations. Moreover our success in utilizing solid rather than gas targets
represents a significant simplification over our original approach and offers
potential gains in sensitivity through reduced dead mass in the target area.Comment: 18 pages, 9 figures, submitted to JCA
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
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