91,651 research outputs found
Kassiopeia: A Modern, Extensible C++ Particle Tracking Package
The Kassiopeia particle tracking framework is an object-oriented software
package using modern C++ techniques, written originally to meet the needs of
the KATRIN collaboration. Kassiopeia features a new algorithmic paradigm for
particle tracking simulations which targets experiments containing complex
geometries and electromagnetic fields, with high priority put on calculation
efficiency, customizability, extensibility, and ease of use for novice
programmers. To solve Kassiopeia's target physics problem the software is
capable of simulating particle trajectories governed by arbitrarily complex
differential equations of motion, continuous physics processes that may in part
be modeled as terms perturbing that equation of motion, stochastic processes
that occur in flight such as bulk scattering and decay, and stochastic surface
processes occuring at interfaces, including transmission and reflection
effects. This entire set of computations takes place against the backdrop of a
rich geometry package which serves a variety of roles, including initialization
of electromagnetic field simulations and the support of state-dependent
algorithm-swapping and behavioral changes as a particle's state evolves. Thanks
to the very general approach taken by Kassiopeia it can be used by other
experiments facing similar challenges when calculating particle trajectories in
electromagnetic fields. It is publicly available at
https://github.com/KATRIN-Experiment/Kassiopei
Using an Ellipsoid Model to Track and Predict the Evolution and Propagation of Coronal Mass Ejections
We present a method for tracking and predicting the propagation and evolution
of coronal mass ejections (CMEs) using the imagers on the STEREO and SOHO
satellites. By empirically modeling the material between the inner core and
leading edge of a CME as an expanding, outward propagating ellipsoid, we track
its evolution in three-dimensional space. Though more complex empirical CME
models have been developed, we examine the accuracy of this relatively simple
geometric model, which incorporates relatively few physical assumptions,
including i) a constant propagation angle and ii) an azimuthally symmetric
structure. Testing our ellipsoid model developed herein on three separate CMEs,
we find that it is an effective tool for predicting the arrival of density
enhancements and the duration of each event near 1 AU. For each CME studied,
the trends in the trajectory, as well as the radial and transverse expansion
are studied from 0 to ~.3 AU to create predictions at 1 AU with an average
accuracy of 2.9 hours.Comment: 18 pages, 11 figure
Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario
We develop an improved method for tracking the nuclear flame during the
deflagration phase of a Type Ia supernova, and apply it to study the variation
in outcomes expected from the gravitationally confined detonation (GCD)
paradigm. A simplified 3-stage burning model and a non-static ash state are
integrated with an artificially thickened advection-diffusion-reaction (ADR)
flame front in order to provide an accurate but highly efficient representation
of the energy release and electron capture in and after the unresolvable flame.
We demonstrate that both our ADR and energy release methods do not generate
significant acoustic noise, as has been a problem with previous ADR-based
schemes. We proceed to model aspects of the deflagration, particularly the role
of buoyancy of the hot ash, and find that our methods are reasonably
well-behaved with respect to numerical resolution. We show that if a detonation
occurs in material swept up by the material ejected by the first rising bubble
but gravitationally confined to the white dwarf (WD) surface (the GCD
paradigm), the density structure of the WD at detonation is systematically
correlated with the distance of the deflagration ignition point from the center
of the star. Coupled to a suitably stochastic ignition process, this
correlation may provide a plausible explanation for the variety of nickel
masses seen in Type Ia Supernovae.Comment: 14 pages, 10 figures, accepted to the Astrophysical Journa
On-chip spectropolarimetry by fingerprinting with random surface arrays of nanoparticles
Optical metasurfaces revolutionized the approach to moulding the propagation
of light by enabling simultaneous control of the light phase, momentum,
amplitude and polarization. Thus, instantaneous spectropolarimetry became
possible by conducting parallel intensity measurements of differently
diffracted optical beams. Various implementations of this very important
functionality have one feature in common - the determination of wavelength
utilizes dispersion of the diffraction angle, requiring tracking the diffracted
beams in space. Realization of on-chip spectropolarimetry calls thereby for
conceptually different approaches. In this work, we demonstrate that random
nanoparticle arrays on metal surfaces, enabling strong multiple scattering of
surface plasmon polaritons (SPPs), produce upon illumination complicated SPP
scattered patterns, whose angular spectra are uniquely determined by the
polarization and wavelength of light, representing thereby spectropolarimetric
fingerprints. Using um-sized circular arrays of randomly distributed
{\mu}m-sized gold nanoparticles (density ~ 75 {\mu}m}) fabricated on
gold films, we measure angular distributions of scattered SPP waves using the
leakage radiation microscopy and find that the angular SPP spectra obtained for
normally incident light beams different in wavelength and/or polarization are
distinctly different. Our approach allows one to realize on-chip
spectropolarimetry by fingerprinting using surface nanostructures fabricated
with simple one-step electron-beam lithography.Comment: 22 pages, 5 figure
Structure and Evolution of Giant Cells in Global Models of Solar Convection
The global scales of solar convection are studied through three-dimensional
simulations of compressible convection carried out in spherical shells of
rotating fluid which extend from the base of the convection zone to within 15
Mm of the photosphere. Such modelling at the highest spatial resolution to date
allows study of distinctly turbulent convection, revealing that coherent
downflow structures associated with giant cells continue to play a significant
role in maintaining the strong differential rotation that is achieved. These
giant cells at lower latitudes exhibit prograde propagation relative to the
mean zonal flow, or differential rotation, that they establish, and retrograde
propagation of more isotropic structures with vortical character at mid and
high latitudes. The interstices of the downflow networks often possess strong
and compact cyclonic flows. The evolving giant-cell downflow systems can be
partly masked by the intense smaller scales of convection driven closer to the
surface, yet they are likely to be detectable with the helioseismic probing
that is now becoming available. Indeed, the meandering streams and varying
cellular subsurface flows revealed by helioseismology must be sampling
contributions from the giant cells, yet it is difficult to separate out these
signals from those attributed to the faster horizontal flows of
supergranulation. To aid in such detection, we use our simulations to describe
how the properties of giant cells may be expected to vary with depth, how their
patterns evolve in time, and analyze the statistical features of correlations
within these complex flow fields.Comment: 22 pages, 16 figures (color figures are low res), uses emulateapj.cls
Latex class file, Results shown during a Press release at the AAS meeting in
June 2007. Submitted to Ap
A new model for deflagration fronts in reactive fluids
We present a new way of modeling deflagration fronts in reactive fluids, the
main emphasis being on turbulent thermonuclear deflagration fronts in white
dwarfs undergoing a Type Ia supernova explosion. Our approach is based on a
level set method which treats the front as a mathematical discontinuity and
allows full coupling between the front geometry and the flow field. With only
minor modifications, this method can also be applied to describe contact
discontinuities. Two different implementations are described and their
physically correct behaviour for simple testcases is shown. First results of
the method applied to the concrete problems of Type Ia supernovae and chemical
hydrogen combustion are briefly discussed; a more extensive analysis of our
astrophysical simulations is given in (Reinecke et al. 1998, MPA Green Report
1122b).Comment: 11 pages, 13 figures, accepted by A&A, corrected and extended
according to referee's comment
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