5,453 research outputs found
Detailed opacity calculations for astrophysical applications
Nowadays, several opacity codes are able to provide data for stellar
structure models, but the computed opacities may show significant differences.
In this work, we present state-of-the-art precise spectral opacity
calculations, illustrated by stellar applications. The essential role of
laboratory experiments to check the quality of the computed data is underlined.
We review some X-ray and XUV laser and Z-pinch photo-absorption measurements as
well as X-ray emission spectroscopy experiments involving hot dense plasmas
produced by ultra-high-intensity laser irradiation. The measured spectra are
systematically compared with the fine-structure opacity code SCO-RCG. Focus is
put on iron, due to its crucial role in understanding asteroseismic
observations of Cephei-type and Slowly Pulsating B stars, as well as of
the Sun. For instance, in Cephei-type stars, the iron-group opacity
peak excites acoustic modes through the "kappa-mechanism". A particular
attention is paid to the higher-than-predicted iron opacity measured at the
Sandia Z-machine at solar interior conditions. We discuss some theoretical
aspects such as density effects, photo-ionization, autoionization or the
"filling-the-gap" effect of highly excited states.Comment: submitted to "Atoms
Ion stopping in dense plasma target for high energy density physics
The basic physics of nonrelativistic and electromagnetic ion stopping in hot and ionized plasma targets is thoroughly updated. Corresponding projectile-target interactions involve enhanced projectile ionization and coupling with target free electrons leading to significantly larger energy losses in hot targets when contrasted to their cold homologues. Standard stoppping formalism is framed around the most economical extrapolation of high velocity stopping in cold matter. Further elaborations pay attention to target electron coupling and nonlinearities due to enhanced projectile charge state, as well. Scaling rules are then used to optimize the enhanced stopping of MeV/amu ions in plasmas with electron linear densities nel ~ 10 18 -10 20 cm -2 . The synchronous firing of dense and strongly ionized plasmas with the time structure of bunched and energetic multicharged ion beam then allow to probe, for the first time, the long searched enhanced plasma stopping and projectile charge at target exit. Laser ablated plasmas (SPQR1) and dense linear plasma columns (SPQR2) show up as targets of choice in providing accurate and on line measurements of plasma parameters. Corresponding stopping results are of a central significance in asserting the validity of intense ion beam scenarios for driving thermonuclear pellets. Other applications of note feature thorium induced fission, novel ion sources and specific material processing through low energy ion beams. Last but not least, the given ion beam-plasma target interaction physics is likely to pave a way to the production and diagnostics of warm dense matter (WDM)
Imaging Jupiter's radiation belts down to 127 MHz with LOFAR
Context. Observing Jupiter's synchrotron emission from the Earth remains
today the sole method to scrutinize the distribution and dynamical behavior of
the ultra energetic electrons magnetically trapped around the planet (because
in-situ particle data are limited in the inner magnetosphere). Aims. We perform
the first resolved and low-frequency imaging of the synchrotron emission with
LOFAR at 127 MHz. The radiation comes from low energy electrons (~1-30 MeV)
which map a broad region of Jupiter's inner magnetosphere. Methods (see article
for complete abstract) Results. The first resolved images of Jupiter's
radiation belts at 127-172 MHz are obtained along with total integrated flux
densities. They are compared with previous observations at higher frequencies
and show a larger extent of the synchrotron emission source (>=4 ). The
asymmetry and the dynamic of east-west emission peaks are measured and the
presence of a hot spot at lambda_III=230 {\deg} 25 {\deg}. Spectral flux
density measurements are on the low side of previous (unresolved) ones,
suggesting a low-frequency turnover and/or time variations of the emission
spectrum. Conclusions. LOFAR is a powerful and flexible planetary imager. The
observations at 127 MHz depict an extended emission up to ~4-5 planetary radii.
The similarities with high frequency results reinforce the conclusion that: i)
the magnetic field morphology primarily shapes the brightness distribution of
the emission and ii) the radiating electrons are likely radially and
latitudinally distributed inside about 2 . Nonetheless, the larger extent
of the brightness combined with the overall lower flux density, yields new
information on Jupiter's electron distribution, that may shed light on the
origin and mode of transport of these particles.Comment: 10 pages, 12 figures, accepted for publication in A&A (27/11/2015) -
abstract edited because of limited character
Ergs: The Evolution of Shell Supernova Remnants
This paper reports on a workshop hosted by the University of Minnesota, March
23-26, 1997. It addressed fundamental dynamical issues associated with the
evolution of shell supernova remnants and the relationships between supernova
remnants and their environments. The workshop considered, in addition to
classical shell SNRs, dynamical issues involving X-ray filled composite
remnants and pulsar driven shells, such as that in the Crab Nebula.
Approximately 75 participants with wide ranging interests attended the
workshop. An even larger community helped through extensive on-line debates
prior to the meeting. Each of the several sessions, organized mostly around
chronological labels, also addressed some underlying, general physical themes:
How are SNR dynamics and structures modified by the character of the CSM and
the ISM and vice versa? How are magnetic fields generated in SNRs and how do
magnetic fields influence SNRs? Where and how are cosmic-rays (electrons and
ions) produced in SNRs and how does their presence influence or reveal SNR
dynamics? How does SNR blast energy partition into various components over time
and what controls conversion between components? In lieu of a proceedings
volume, we present here a synopsis of the workshop in the form of brief
summaries of the workshop sessions. The sharpest impressions from the workshop
were the crucial and under-appreciated roles that environments have on SNR
appearance and dynamics and the critical need for broad-based studies to
understand these beautiful, but enigmatic objects. \\Comment: 54 pages text, no figures, Latex (aasms4.sty). submitted to the PAS
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Parametrization of reflectometry fluctuation frequency spectra for systematic study of tokamak fusion plasma turbulence
We describe a way to parameterize power spectra extracted from fixed-frequency reflectometry data, with a view to systematic studies of turbulence properties in tokamak plasmas. Analysis of typical frequency spectra obtained from a new database suggests decomposition in a set of four key components: the direct current component, low-frequency fluctuations, broadband (BB) turbulence, and the noise level. For the decomposition in the identified components, different kinds of functions are tested and their fitting performance is analyzed to determine the optimal spectrum parametrization. In particular, for the BB turbulence, three models are compared qualitatively based on a number of representative spectrum test cases, notably the generalized Gaussian, the Voigt, and the Taylor model. In addition, quantitative performance testing is accomplished using the weighted residual sum of squares and the Bayesian information criterion in a large database including 350 000 spectra obtained in Tore Supra. Next, parametrization by the Taylor model is applied to Ohmically heated plasmas, and a BB energy basin is systematically observed in the core plasma region, which shrinks with decreasing radial position of the q = 1 surface. This basin might be explained by a drop of the density fluctuation level inside the q = 1 surface
Solar Terrestrial programs: A five year plan
Major projects to be initiated in the 1980-1985 period, designed to study the Sun, the heliosphere, Earth's magnetosphere, and the upper atmosphere involve the use of spacelab as well as free flying spacecraft. Current and recent investigations in these areas are reviewed and the guiding principles followed in planning future missions are examined. The implementation strategy, the planning process, and supporting research and technology are discussed
Modelling of microwave induced plasmas : the interplay between electromagnetism, plasma chemistry and transport
In this thesis we report on a theoretical/numerical study that is concerned with Microwave Induced Plasmas (MIPs) in general, and the application of a MIP to the Plasma-activated Chemical Vapour Deposition (PCVD) process that is used at Draka Comteq for the production of optical fibres in particular. This was performed in the framework of the STW project (ETF.6265) titled: Exploring the compositional freedom in space and chemistry of Microwave Induced Plasmas: An object oriented approach. The primary purpose of the project was to design an experimentally validated grand model for the microwave deposition plasma. Furthermore, in order to improve the deposition process, we studied how the different microwave components shape the electromagnetic field and thus the plasma. To deal with these objectives, two types of models have been used: 1. Models describing particular aspects. The Electromagnetic Model (chapter 3) is based on the finite difference frequency domain method and makes use of a two-dimensional orthocurvilinear grid. It is a full-vectorial approach for 3-field components that can be used as a design tool for symmetrical electromagnetic structures (chapter 6) and as a crucial part of Grand Models. The Electromagnetic Model was validated by means of a comparison of its results with those of known waveguide solutions. The Chemistry Model (chapter 5) is a zero space-dimensional version of a Grand Model giving a time-dependent description of the chemical plasma composition. The transport mechanisms, in reality originating from two or three dimensional structures, are described in the framework of the Chemistry model as effective source terms making use of convection and diffusion requencies. In this thesis we deal with a Chemistry Model for pulsed plasmas. The Deposition Model (chapter 11) has been developed as a tool for obtaining insight into the role of the plasmas used for optical fibre production. The model shows how plasma mechanisms such as diffusion and convection manifest themselves in the local deposition profile. The global deposition profile is the result of the convolution of various local profiles. This convolution is realized by the movement of the plasma-creating cavity along the tube in which the deposition takes place. Results of the global profile are compared with those of experimental measurements. 2. Models describing multiple aspects in a self-consistent manner using the insights of the electromagnetic, chemical and deposition aspects, a more complete plasma model is created. This so-called Grand Model (chapter 7) consists of a macroscopic description of plasmas based on balance equations (chapter 4) that compute the chemical composition in a self-consistent relation with the flow and the electromagnetic aspects. The boundary conditions are formed by the configuration of the plasma, the absorbed power, the overall pressure difference and the fill-chemistry. Two-dimensional Grand Models are created using the PLASIMO platform (described in chapter 2). A central role is played by the Electromagnetic Model described above, which is needed to describe the power absorbed by the plasma. The Grand Model has been validated by comparing it with experimental results of surface wave discharges (chapters 8 and 9) thus fulfilling the first objective of this study. Furthermore in chapter 10, it is used to investigate the confinement of the plasma inside the Draka reactor
The stellar atmosphere simulation code Bifrost
Context: Numerical simulations of stellar convection and photospheres have
been developed to the point where detailed shapes of observed spectral lines
can be explained. Stellar atmospheres are very complex, and very different
physical regimes are present in the convection zone, photosphere, chromosphere,
transition region and corona. To understand the details of the atmosphere it is
necessary to simulate the whole atmosphere since the different layers interact
strongly. These physical regimes are very diverse and it takes a highly
efficient massively parallel numerical code to solve the associated equations.
Aims: The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the convection
zone to the corona.
Methods: The code is subjected to a number of validation tests, among them
the Sod shock tube test, the Orzag-Tang colliding shock test, boundary
condition tests and tests of how the code treats magnetic field advection,
chromospheric radiation, radiative transfer in an isothermal scattering
atmosphere, hydrogen ionization and thermal conduction.
Results: Bifrost completes the tests with good results and shows near linear
efficiency scaling to thousands of computing cores
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