74 research outputs found
Reduced nonlinear description of Farley-Buneman instability
In the study on nonlinear wave-wave processes in an ionosphere and a
magnetosphere usually the main attention is paid to investigation of plasma
turbulence at well developed stage, when the wide spectrum of plasma wave is
present. On the other side, it is well known that even if the number of
cooperating waves remains small due to a competition of processes of their
instability and attenuation, the turbulence appears in the result of their
stochastic behavior. The regimes of nonlinear dynamics of low frequency waves
excited due to Farley-Buneman instability in weakly ionized and inhomogeneous
ionospheric plasma in the presence of electric current perpendicular to ambient
magnetic field are considered. The problem is essentially three dimensional and
difficult for full numerical simulation, but the strong collisional damping of
waves allow to assume that in this case a perturbed state of plasma can be
described as finite set of interacting waves, some of which are unstable and
other strongly damping. The proposed nonlinear model allow to make full study
of nonlinear stabilization, conditions of stochasticity and to consider the
different regimes and properties of few mode plasma turbulence.Comment: The extended version of work, published in AIP Conf. Proc. 993, 113
(2008
Modelling the light variability of the Ap star epsilon Ursae Majoris
We simulate the light variability of the Ap star epsUMa using the observed
surface distributions of Fe, Cr, Ca, Mn, Mg, Sr and Ti obtained with the help
of Doppler Imaging technique. Using all photometric data available we specified
light variations of epsUMa modulated by its rotation from far UV to IR. We
employed the LLmodels stellar model atmosphere code to predict the light
variability in different photometric systems. The rotational period of epsUMa
is refined to 5d088631(18). It is shown that the observed light variability can
be explained as a result of the redistribution of radiative flux from the UV
spectral region to the visual caused by the inhomogeneous surface distribution
of chemical elements. Among seven mapped elements, only Fe and Cr significantly
contribute to the amplitude of the observed light variability. In general, we
find a very good agreement between theory and observations. We confirm the
important role of Fe and Cr to the magnitude of the well-known depression
around 5200 \AA\ through the analysis of the peculiar -parameter. Finally,
we show that the abundance spots of considered elements cannot explain the
observed variability in near UV and index which are likely due to some
other causes. The inhomogeneous surface distribution of chemical elements can
explain most of the observed light variability of the A-type CP star epsUMa.Comment: Accepted in A&A, 10 pages, 9 figures, 3 table
The FERRUM project: laboratory-measured transition probabilities for Cr II
Aims: We measure transition probabilities for Cr II transitions from the z
^4H_J, z ^2D_J, y ^4F_J, and y ^4G_J levels in the energy range 63000 to 68000
cm^{-1}. Methods: Radiative lifetimes were measured using time-resolved
laser-induced fluorescence from a laser-produced plasma. In addition, branching
fractions were determined from intensity-calibrated spectra recorded with a UV
Fourier transform spectrometer. The branching fractions and radiative lifetimes
were combined to yield accurate transition probabilities and oscillator
strengths. Results: We present laboratory measured transition probabilities for
145 Cr II lines and radiative lifetimes for 14 Cr II levels. The
laboratory-measured transition probabilities are compared to the values from
semi-empirical calculations and laboratory measurements in the literature.Comment: 13 pages. Accepted for publication in A&
Multiplicity among chemically peculiar stars II. Cool magnetic Ap stars
We present new orbits for sixteen Ap spectroscopic binaries, four of which
might in fact be Am stars, and give their orbital elements. Four of them are
SB2 systems: HD 5550, HD 22128, HD 56495 and HD 98088. The twelve other stars
are : HD 9996, HD 12288, HD 40711, HD 54908, HD 65339, HD 73709, HD 105680, HD
138426, HD 184471, HD 188854, HD 200405 and HD 216533. Rough estimates of the
individual masses of the components of HD 65339 (53 Cam) are given, combining
our radial velocities with the results of speckle interferometry and with
Hipparcos parallaxes. Considering the mass functions of 74 spectroscopic
binaries from this work and from the literature, we conclude that the
distribution of the mass ratio is the same for cool Ap stars as for normal G
dwarfs. Therefore, the only differences between binaries with normal stars and
those hosting an Ap star lie in the period distribution: except for the case of
HD 200405, all orbital periods are longer than (or equal to) 3 days. A
consequence of this peculiar distribution is a deficit of null eccentricities.
There is no indication that the secondary has a special nature, like e.g. a
white dwarf.Comment: 31 pages, 15 figures, A&A accepte
Towards a new image processing system at Wendelstein 7-X: From spatial calibration to characterization of thermal events
Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors
Forward modeling of collective Thomson scattering for Wendelstein 7-X plasmas: Electrostatic approximation
In this paper, we present a method for numerical computation of collective Thomson scattering (CTS). We developed a forward model, eCTS, in the electrostatic approximation and benchmarked it against a full electromagnetic model. Differences between the electrostatic and the electromagnetic models are discussed. The sensitivity of the results to the ion temperature and the plasma composition is demonstrated. We integrated the model into the Bayesian data analysis framework Minerva and used it for the analysis of noisy synthetic data sets produced by a full electromagnetic model. It is shown that eCTS can be used for the inference of the bulk ion temperature. The model has been used to infer the bulk ion temperature from the first CTS measurements on Wendelstein 7-X
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