6,396 research outputs found
Spatial Structure of Ion Beams in an Expanding Plasma
We report spatially resolved perpendicular and parallel, to the magnetic field, ion velocity distribution function (IVDF) measurements in an expanding argon helicon plasma. The parallel IVDFs, obtained through laser induced fluorescence (LIF), show an ion beam with v ≈ 8000 m/s flowing downstream and confined to the center of the discharge. The ion beam is measurable for tens of centimeters along the expansion axis before the LIF signal fades, likely a result of metastable quenching of the beam ions. The parallel ion beam velocity slows in agreement with expectations for the measured parallel electric field. The perpendicular IVDFs show an ion population with a radially outward flow that increases with distance from the plasma axis. Structures aligned to the expanding magnetic field appear in the DC electric field, the electron temperature, and the plasma density in the plasma plume. These measurements demonstrate that at least two-dimensional and perhaps fully three-dimensional models are needed to accurately describe the spontaneous acceleration of ion beams in expanding plasmas
Field-theoretical approach to particle oscillations in absorbing matter
The oscillations in absorbing matter are considered. The standard model
based on optical potential does not describe the total transition
probability as well as the channel corresponding to absorption of the
-particle. We calculate directly the off-diagonal matrix element in the
framework of field-theoretical approach. Contrary to one-particle model, the
final state absorption does not tend to suppress the channels mentioned above
or, similarly, calculation with hermitian Hamiltonian leads to increase the
corresponding values. The model reproduces all the results on the particle
oscillations, however it is oriented to the description of the above-mentioned
channels. Also we touch on the problem of infrared singularities. The approach
under study is infrared-free.Comment: 27 pages, 8 figure
The Digital Sky Project: Prototyping Virtual Observatory Technologies
Astronomy is entering a new era as multiple, large area, digital sky surveys
are in production. The resulting datasets are truly remarkable in their own
right; however, a revolutionary step arises in the aggregation of complimentary
multi-wavelength surveys (i.e., the cross-identification of a billion sources).
The federation of these large datasets is already underway, and is producing a
major paradigm shift as Astronomy has suddenly become an immensely data-rich
field. This new paradigm will enable quantitatively and qualitatively new
science, from statistical studies of our Galaxy and the large-scale structure
in the universe, to discoveries of rare, unusual, or even completely new types
of astronomical objects and phenomena. Federating and then exploring these
large datasets, however, is an extremely challenging task. The Digital Sky
project was initiated with this task in mind and is working to develop the
techniques and technologies necessary to solve the problems inherent in
federating these large databases, as well as the mining of the resultant
aggregate data.Comment: 8 pages, 3 Figures, uses newpasp.sty (included). To be published in
the proceedings of the conference "Virtual Observatories of the Future,"
editors R.J. Brunner, S.G. Djorgovski, and Alex S. Szala
The Next Generation of the Montage Image Mosaic Toolkit
The scientific computing landscape has evolved dramatically in the past few years, with new schemes for organizing and storing data that reflect the growth in size and complexity of astronomical data sets. In response to this changing landscape, we are, over the next two years, deploying the next generation of the Montage toolkit ([ascl:1010.036]). The first release (October 2015) supports multi-dimensional data sets ("data cubes"), and insertion of XMP/AVM tags that allows images to "drop-in" to the WWT. The same release offers a beta-version of web-based interactive visualization of images; this includes wrappers for visualization in Python. Subsequent releases will support HEALPix (now standard in cosmic background experiments); incorporation of Montage into package managers (which enable automated management of software builds), and support for a library that will enable Montage to be called directly from Python. This next generation toolkit will inherit the architectural benefits of the current engine - component based tools, ANSI-C portability across Unix platforms and scalability for distributed processing. With the expanded functionality under development, Montage can be viewed not simply as a mosaic engine, but as a scalable, portable toolkit for managing, organizing and processing images
Design of the Spitzer Space Telescope Heritage Archive
It is predicted that Spitzer Space Telescope’s cryogen will run out in April 2009, and the final reprocessing for the cryogenic mission is scheduled to end in April 2011, at which time the Spitzer archive will be transferred to the NASA/IPAC Infrared Science Archive (IRSA) for long-term curation. The Spitzer Science Center (SSC) and IRSA are collaborating to design and deploy the Spitzer Heritage Archive (SHA), which will supersede the current Spitzer archive. It will initially contain the raw and final reprocessed cryogenic science products, and will eventually incorporate the final products from the Warm mission. The SHA will be accompanied by tools deemed necessary to extract the full science content of the archive and by comprehensive documentation
A Theoretical Analysis of Two-Stage Recommendation for Cold-Start Collaborative Filtering
In this paper, we present a theoretical framework for tackling the cold-start
collaborative filtering problem, where unknown targets (items or users) keep
coming to the system, and there is a limited number of resources (users or
items) that can be allocated and related to them. The solution requires a
trade-off between exploitation and exploration as with the limited
recommendation opportunities, we need to, on one hand, allocate the most
relevant resources right away, but, on the other hand, it is also necessary to
allocate resources that are useful for learning the target's properties in
order to recommend more relevant ones in the future. In this paper, we study a
simple two-stage recommendation combining a sequential and a batch solution
together. We first model the problem with the partially observable Markov
decision process (POMDP) and provide an exact solution. Then, through an
in-depth analysis over the POMDP value iteration solution, we identify that an
exact solution can be abstracted as selecting resources that are not only
highly relevant to the target according to the initial-stage information, but
also highly correlated, either positively or negatively, with other potential
resources for the next stage. With this finding, we propose an approximate
solution to ease the intractability of the exact solution. Our initial results
on synthetic data and the Movie Lens 100K dataset confirm the performance gains
of our theoretical development and analysis
Network information and connected correlations
Entropy and information provide natural measures of correlation among
elements in a network. We construct here the information theoretic analog of
connected correlation functions: irreducible --point correlation is measured
by a decrease in entropy for the joint distribution of variables relative
to the maximum entropy allowed by all the observed variable
distributions. We calculate the ``connected information'' terms for several
examples, and show that it also enables the decomposition of the information
that is carried by a population of elements about an outside source.Comment: 4 pages, 3 figure
Synthetic RNA Silencing of Actinorhodin Biosynthesis in Streptomyces coelicolor A3(2)
We demonstrate the first application of synthetic RNA gene silencers in Streptomyces coelicolor A3(2). Peptide nucleic acid and expressed antisense RNA silencers successfully inhibited actinorhodin production. Synthetic RNA silencing was target-specific and is a new tool for gene regulation and metabolic engineering studies in Streptomyces.Peer reviewe
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