18,181 research outputs found
Invariant manifolds and orbit control in the solar sail three-body problem
In this paper we consider issues regarding the control and orbit transfer of solar sails in the circular restricted Earth-Sun system. Fixed points for solar sails in this system have the linear dynamical properties of saddles crossed with centers; thus the fixed points are dynamically unstable and control is required. A natural mechanism of control presents itself: variations in the sail's orientation. We describe an optimal controller to control the sail onto fixed points and periodic orbits about fixed points. We find this controller to be very robust, and define sets of initial data using spherical coordinates to get a sense of the domain of controllability; we also perform a series of tests for control onto periodic orbits. We then present some mission strategies involving transfer form the Earth to fixed points and onto periodic orbits, and controlled heteroclinic transfers between fixed points on opposite sides of the Earth. Finally we present some novel methods to finding periodic orbits in circumstances where traditional methods break down, based on considerations of the Center Manifold theorem
Interpreting the time variable RM observed in the core region of the TeV blazar Mrk 421
In this work we interpret and discuss the time variable rotation measure (RM)
found, for the first time over a 1-yr period, in the core region of a blazar.
These results are based on a one-year, multi-frequency (15, 24, and 43 GHz)
Very Long Baseline Array (VLBA) monitoring of the TeV blazar Markarian 421 (Mrk
421). We investigate the Faraday screen properties and its location with
respect to the jet emitting region. Given that the 43 GHz radio core flux
density and the RM time evolution suggest a similar trend, we explore the
possible connection between the RM and the accretion rate. Among the various
scenarios that we explore, the jet sheath is the most promising candidate for
being the main source of Faraday rotation. During the one-year observing period
the RM trend shows two sign reversals, which may be qualitatively interpreted
within the context of the magnetic tower models. We invoke the presence of two
nested helical magnetic fields in the relativistic jet with opposite
helicities, whose relative contribution produce the observed RM values. The
inner helical field has the poloidal component () oriented in the
observer's direction and produces a positive RM, while the outer helical field,
with in the opposite direction, produces a negative RM. We assume
that the external helical field dominates the contribution to the observed RM,
while the internal helical field dominates when a jet perturbation arises
during the second observing epoch. Being the intrinsic polarization angle
parallel to the jet axis, a pitch angle of the helical magnetic field
is required. Additional scenarios are also considered to
explain the observed RM sign reversals.Comment: 6 pages, 2 figures. Published on MNRA
Gravitational waveforms with controlled accuracy
A partially first-order form of the characteristic formulation is introduced
to control the accuracy in the computation of gravitational waveforms produced
by highly distorted single black hole spacetimes. Our approach is to reduce the
system of equations to first-order differential form on the angular
derivatives, while retaining the proven radial and time integration schemes of
the standard characteristic formulation. This results in significantly improved
accuracy over the standard mixed-order approach in the extremely nonlinear
post-merger regime of binary black hole collisions.Comment: Revised version, published in Phys. Rev. D, RevTeX, 16 pages, 4
figure
Comparative analysis of neutronics/thermal-hydraulics multi-scale coupling for LWR analysis
The aim of the research described in this paper is to perform consistent comparative analyses of two different approaches for coupling of two-scale, two-physics phenomena in reactor core calculations. The physical phenomena of interest are the neutronics and the thermal-hydraulics core behaviors and their interactions, while the spatial scales are the “global” (assembly/channel-wise) and the “local” (pin/sub-channel-wise). The objective is three-fold: qualification of coupled code systems by consistent step-by-step cross-comparison (in order to understand the prediction deviations in both neutronics and thermal-hydraulics parameters); assessment of fine scale (local/subchannel-wise) thermal-hydraulic effects; and evaluation of the impact of on-line modeling of interactions of the two spatial scales. The reported work is within the cooperation between the Universidad Politécnica de Madrid (UPM), Spain and the Pennsylvania State University (PSU), USA. The paper first presents the two multi-scale coupled code systems followed by cross-comparisons for steady state calculations. Selected results are discussed to highlight some of the issues involved in comparative analysis of coupled multi-scale simulations. The transient comparisons are subject of future work and publications
Benchmark model to assess community structure in evolving networks
Detecting the time evolution of the community structure of networks is
crucial to identify major changes in the internal organization of many complex
systems, which may undergo important endogenous or exogenous events. This
analysis can be done in two ways: considering each snapshot as an independent
community detection problem or taking into account the whole evolution of the
network. In the first case, one can apply static methods on the temporal
snapshots, which correspond to configurations of the system in short time
windows, and match afterwards the communities across layers. Alternatively, one
can develop dedicated dynamic procedures, so that multiple snapshots are
simultaneously taken into account while detecting communities, which allows us
to keep memory of the flow. To check how well a method of any kind could
capture the evolution of communities, suitable benchmarks are needed. Here we
propose a model for generating simple dynamic benchmark graphs, based on
stochastic block models. In them, the time evolution consists of a periodic
oscillation of the system's structure between configurations with built-in
community structure. We also propose the extension of quality comparison
indices to the dynamic scenario.Comment: 11 pages, 7 figures, 3 table
Spin Information from Vector-Meson Decay in Photoproduction
For the photoproduction of vector mesons, all single and double spin
observables involving vector meson two-body decays are defined consistently in
the center of mass. These definitions yield a procedure for
extracting physically meaningful single and double spin observables that are
subject to known rules concerning their angle and energy evolution. As part of
this analysis, we show that measuring the two-meson decay of a photoproduced
or does not determine the vector meson's vector polarization, but
only its tensor polarization. The vector meson decay into lepton pairs is also
insensitive to the vector meson's vector polarization, unless one measures the
spin of one of the leptons. Similar results are found for all double spin
observables which involve observation of vector meson decay. To access the
vector meson's vector polarization, one therefore needs to either measure the
spin of the decay leptons, make an analysis of the background interference
effects or relate the vector meson's vector polarization to other accessible
spin observables.Comment: 22 pages, 3 figure
Characteristics of the chiral phase transition in nonlocal quark models
The characteristics of the chiral phase transition are analyzed within the
framework of chiral quark models with nonlocal interactions in the mean field
approximation. In the chiral limit, we develop a semi-analytic framework which
allows us to explicitly determine the phase transition curve, the position of
the critical points, some relevant critical exponents, etc. For the case of
finite current quark masses, we show the behavior of various thermodynamical
and chiral response functions across the phase transition.Comment: 19 pages, 5 figures. Figures 1 and 2 modified, references added,
minor changes in the presentation and in the discussion of results. Accepted
for publication in Phys. Rev.
Improved graphene blisters by ultrahigh pressure sealing
Graphene is a very attractive material for nanomechanical devices and
membrane applications. Graphene blisters based on silicon oxide micro-cavities
are a simple but relevant example of nanoactuators. A drawback of this
experimental set up is that gas leakage through the graphene-SiO2 interface
contributes significantly to the total leak rate. Here we study the diffusion
of air from pressurized graphene drumheads on SiO2 micro-cavities and propose a
straightforward method to improve the already strong adhesion between graphene
and the underlying SiO2 substrate, resulting in reduced leak rates. This is
carried out by applying controlled and localized ultrahigh pressure (> 10 GPa)
with an Atomic Force Microscopy diamond tip. With this procedure, we are able
to significantly approach the graphene layer to the SiO2 surface around the
drumheads, thus enhancing the interaction between them allowing us to better
seal the graphene-SiO2 interface, which is reflected in up to ~ 4 times lower
leakage rates. Our work opens an easy way to improve the performance of
graphene as a gas membrane on a technological relevant substrate such as SiO2.Comment: pages 19, 4 figures + supplementary informatio
Coherent coupling between surface plasmons and excitons in semiconductor nanocrystals
We present an experimental demonstration of strong coupling between a surface
plasmon propagating on a planar silver substrate, and the lowest excited state
of CdSe nanocrystals. Variable-angle spectroscopic ellipsometry measurements
demonstrated the formation of plasmon-exciton mixed states, characterized by a
Rabi splitting of 82 meV at room temperature. Such a coherent
interaction has the potential for the development of plasmonic non-linear
devices, and furthermore, this system is akin to those studied in cavity
quantum electrodynamics, thus offering the possibility to study the regime of
strong light-matter coupling in semiconductor nanocrystals at easily accessible
experimental conditions.Comment: 12 pages, 4 figure
The STROBE extensions: protocol for a qualitative assessment of content and a survey of endorsement
Introduction The STrengthening the Reporting of OBservational studies in Epidemiology (STROBE) Statement was developed in response to inadequate reporting of observational studies. In recent years, several extensions to STROBE have been created to provide more nuanced field-specific guidance for authors. The content and the prevalence of extension endorsement have not yet been assessed. Accordingly, there are two aims: (1) to classify changes made in the extensions to identify strengths and weaknesses of the original STROBE checklist and (2) to determine the prevalence and typology of endorsement by journals in fields related to extensions. Methods and analysis Two independent researchers will assess additions in each extension. Additions will be coded as â field specific' (FS) or â not field specific' (NFS). FS is defined as particularly relevant information for a single field and guidance provided generally cannot be extrapolated beyond that field. NFS is defined as information that reflects epidemiological or methodological tenets and can be generalised to most, if not all, types of observational research studies. Intraclass correlation will be calculated to measure reviewers' concordance. On disagreement, consensus will be sought. Individual additions will be grouped by STROBE checklist items to identify the frequency and distribution of changes. Journals in fields related to extensions will be identified through National Library of Medicine PubMed Broad Subject Terms, screened for eligibility and further distilled via Ovid MEDLINE® search strategies for observational studies. Text describing endorsement will be extracted from each journal's website. A classification scheme will be created for endorsement types and the prevalence of endorsement will be estimated. Analyses will use NVivo V.11 and SAS University Edition. Ethics and dissemination This study does not require ethical approval as it does not involve human participants. This study has been preregistered on Open Science Framework.Peer ReviewedPostprint (author's final draft
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