5,965 research outputs found
Electron surface layer at the interface of a plasma and a dielectric wall
We study the potential and the charge distribution across the interface of a
plasma and a dielectric wall. For this purpose, the charge bound to the wall is
modelled as a quasi-stationary electron surface layer which satisfies Poisson's
equation and minimizes the grand canonical potential of the wall-thermalized
excess electrons constituting the wall charge. Based on an effective model for
a graded interface taking into account the image potential and the offset of
the conduction band to the potential just outside the dielectric, we
specifically calculate the potential and the electron distribution for
magnesium oxide, silicon dioxide and sapphire surfaces in contact with a helium
discharge. Depending on the electron affinity of the surface, we find two
vastly different behaviors. For negative electron affinity, electrons do not
penetrate into the wall and an external surface charge is formed in the image
potential, while for positive electron affinity, electrons penetrate into the
wall and a space charge layer develops in the interior of the dielectric. We
also investigate how the electron surface layer merges with the bulk of the
dielectric.Comment: 15 pages, 9 figures, accepted versio
Electron heating mechanisms in dual frequency capacitive discharges
We discuss electron heating mechanisms in the sheath regions of dual-frequency capacitive discharges, with the twin aims of identifying the dominant mechanisms and supplying closed-form expressions from which the heating power can be estimated. We show that the heating effect produced by either Ohmic or collisionless heating is much larger when the discharge is excited by a superposition of currents at two frequencies than if either current had acted alone. This coupling effect occurs because the lower frequency current, while not directly heating the electrons to any great extent, strongly affects the spatial structure of the discharge in the sheath regions
Auger de-excitation of metastable molecules at metallic surfaces
We study secondary electron emission from metallic surfaces due to Auger
de-excitation of diatomic metastable molecules. Our approach is based on an
effective model for the two active electrons involved in the process -- a
molecular electron described by a linear combination of atomic orbitals when it
is bound and a two-center Coulomb wave when it is not and a metal electron
described by the eigenfunctions of a step potential -- and employs Keldysh
Green's functions. Solving the Dyson equation for the retarded Green's function
by exponential resummation we are able to treat time-nonlocal self-energies and
to avoid the wide-band approximation.Results are presented for the
de-excitation of \NitrogenDominantMetastableState\ on aluminum and tungsten and
discussed in view of previous experimental and theoretical investigations. We
find quantitative agreement with experimental data for tungsten indicating that
the effective model captures the physics of the process quite well. For
aluminum we predict secondary electron emission due to Auger de-excitation to
be one to two orders of magnitude smaller than the one found for resonant
charge-transfer and subsequent auto-detachment.Comment: 15 pages, 9 figures, revised version using an improved
single-electron basi
Oseledets' Splitting of Standard-like Maps
For the class of differentiable maps of the plane and, in particular, for
standard-like maps (McMillan form), a simple relation is shown between the
directions of the local invariant manifolds of a generic point and its
contribution to the finite-time Lyapunov exponents (FTLE) of the associated
orbit. By computing also the point-wise curvature of the manifolds, we produce
a comparative study between local Lyapunov exponent, manifold's curvature and
splitting angle between stable/unstable manifolds. Interestingly, the analysis
of the Chirikov-Taylor standard map suggests that the positive contributions to
the FTLE average mostly come from points of the orbit where the structure of
the manifolds is locally hyperbolic: where the manifolds are flat and
transversal, the one-step exponent is predominantly positive and large; this
behaviour is intended in a purely statistical sense, since it exhibits large
deviations. Such phenomenon can be understood by analytic arguments which, as a
by-product, also suggest an explicit way to point-wise approximate the
splitting.Comment: 17 pages, 11 figure
Electron heating by sheaths in radio frequency discharges
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Magpie: towards a semantic web browser
Web browsing involves two tasks: finding the right web page and then making sense of its content. So far, research has focused on supporting the task of finding web resources through ‘standard’ information retrieval mechanisms, or semantics-enhanced search. Much less attention has been paid to the second problem. In this paper we describe Magpie, a tool which supports the
interpretation of web pages. Magpie offers complementary knowledge sources, which a reader can call upon to quickly gain access to any background knowledge relevant to a web resource. Magpie automatically associates an ontologybased
semantic layer to web resources, allowing relevant services to be invoked within a standard web browser. Hence, Magpie may be seen as a step towards a semantic web browser. The functionality of Magpie is illustrated using examples of how it has been integrated with our lab’s web resources
Three months of informational trends in COVID-19 across New York City
© The Author(s) 2020. Published by Oxford University Press on behalf of Faculty of Public Health. All rights reserved. For permissions, please e-mail: [email protected]. In the midst of widespread community transmission of coronavirus disease 2019 (COVID-19) in New York, residents have sought information about COVID-19. We analyzed trends in New York State (NYS) and New York City (NYC) data to quantify the extent of COVID-19-related queries. Data on the number of 311 calls in NYC, Google Trend data on the search term \u27Coronavirus\u27 and information about trends in COVID-19 cases in NYS and the USA were compiled from multiple sources. There were 1228 994 total calls to 311 between 22 January 2020 and 22 April 2020, with 50 845 calls specific to COVID-19 in the study period. The proportion of 311 calls related to COVID-19 increased over time, while the \u27interest over time\u27 of the search term \u27Coronavirus\u27 has exponentially increased since the end of February 2020. It is vital that public health officials provide clear and up-to-date information about protective measures and crucial communications to respond to information-seeking behavior across NYC
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