73 research outputs found
Nonlinearity in a dynamo
Using a rotating flat layer heated from below as an example, we consider
effects which lead to stabilizing an exponentially growing magnetic field in
magnetostrophic convection in transition from the kinematic dynamo to the full
non-linear dynamo. We present estimates of the energy redistribution over the
spectrum and helicity quenching by the magnetic field. We also study the
alignment of the velocity and magnetic fields. These regimes are similar to
those in planetary dynamo simulations.Comment: Accepted to Geophys. Astrophys. Fluid Dyna
Initial conditions, Discreteness and non-linear structure formation in cosmology
In this lecture we address three different but related aspects of the initial
continuous fluctuation field in standard cosmological models. Firstly we
discuss the properties of the so-called Harrison-Zeldovich like spectra. This
power spectrum is a fundamental feature of all current standard cosmological
models. In a simple classification of all stationary stochastic processes into
three categories, we highlight with the name ``super-homogeneous'' the
properties of the class to which models like this, with , belong. In
statistical physics language they are well described as glass-like. Secondly,
the initial continuous density field with such small amplitude correlated
Gaussian fluctuations must be discretised in order to set up the initial
particle distribution used in gravitational N-body simulations. We discuss the
main issues related to the effects of discretisation, particularly concerning
the effect of particle induced fluctuations on the statistical properties of
the initial conditions and on the dynamical evolution of gravitational
clustering.Comment: 28 pages, 1 figure, to appear in Proceedings of 9th Course on
Astrofundamental Physics, International School D. Chalonge, Kluwer, eds N.G.
Sanchez and Y.M. Pariiski, uses crckapb.st pages, 3 figure, ro appear in
Proceedings of 9th Course on Astrofundamental Physics, International School
D. Chalonge, Kluwer, Eds. N.G. Sanchez and Y.M. Pariiski, uses crckapb.st
Proton Driven Plasma Wakefield Acceleration
Plasma wakefield acceleration, either laser driven or electron-bunch driven,
has been demonstrated to hold great potential. However, it is not obvious how
to scale these approaches to bring particles up to the TeV regime. In this
paper, we discuss the possibility of proton-bunch driven plasma wakefield
acceleration, and show that high energy electron beams could potentially be
produced in a single accelerating stage.Comment: 13 pages, 4 figure
Big bang simulation in superfluid 3He-B -- Vortex nucleation in neutron-irradiated superflow
We report the observation of vortex formation upon the absorption of a
thermal neutron in a rotating container of superfluid He-B. The nuclear
reaction n + He = p + H + 0.76MeV heats a cigar shaped region of the
superfluid into the normal phase. The subsequent cooling of this region back
through the superfluid transition results in the nucleation of quantized
vortices. Depending on the superflow velocity, sufficiently large vortex rings
grow under the influence of the Magnus force and escape into the container
volume where they are detected individually with nuclear magnetic resonance.
The larger the superflow velocity the smaller the rings which can expand. Thus
it is possible to obtain information about the morphology of the initial defect
network. We suggest that the nucleation of vortices during the rapid cool-down
into the superfluid phase is similar to the formation of defects during
cosmological phase transitions in the early universe.Comment: 4 pages, LaTeX file, 4 figures are available at
ftp://boojum.hut.fi/pub/publications/lowtemp/LTL-95009.p
Influence of epithermal muonic molecule formation on kinetics of the CF processes in deuterium
The non-resonant formation of molecules in the loosely bound state in
collisions of non-thermalized atoms with deuterium molecules D has
been considered. The process of such a type is possible only for collision
energies exceeded the ionization potential of D. The calculated rates of
formation in the above-threshold energy region are about one order of
magnitude higher than obtained earlier.
The role of epithermal non-resonant -molecule formation for the kinetics
of CF processes in D gas was studied. It was shown that the
non-resonant formation by atoms accelerated during the cascade
can be directly observed in the neutron time spectra at very short initial
times.Comment: 6 pages, 5 figures, Proceedings of the International Conference on
Exotic Atoms and Related Topics EXA-2011, Vienna, Sep 5-9, 201
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel.In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime: we compute the two-point
correlation functions for the linearized Einstein tensor and for the metric
perturbations. Second, we discuss structure formation from the stochastic
gravity viewpoint. Third, we discuss the backreaction of Hawking radiation in
the gravitational background of a quasi-static black hole.Comment: 75 pages, no figures, submitted to Living Reviews in Relativit
A reconstruction of the initial conditions of the Universe by optimal mass transportation
Reconstructing the density fluctuations in the early Universe that evolved
into the distribution of galaxies we see today is a challenge of modern
cosmology [ref.]. An accurate reconstruction would allow us to test
cosmological models by simulating the evolution starting from the reconstructed
state and comparing it to the observations. Several reconstruction techniques
have been proposed [8 refs.], but they all suffer from lack of uniqueness
because the velocities of galaxies are usually not known. Here we show that
reconstruction can be reduced to a well-determined problem of optimisation, and
present a specific algorithm that provides excellent agreement when tested
against data from N-body simulations. By applying our algorithm to the new
redshift surveys now under way [ref.], we will be able to recover reliably the
properties of the primeval fluctuation field of the local Universe and to
determine accurately the peculiar velocities (deviations from the Hubble
expansion) and the true positions of many more galaxies than is feasible by any
other method.
A version of the paper with higher-quality figures is available at
http://www.obs-nice.fr/etc7/nature.pdfComment: Latex, 4 pages, 3 figure
Did smokefree legislation in England reduce exposure to secondhand smoke among nonsmoking adults? Cotinine analysis from the Health Survey for England.
Background: On 1 July 2007, smokefree legislation was implemented in England, which made virtually all enclosed public places and workplaces smokefree.
Objectives: We examined trends in and predictors of secondhand smoke exposure among nonsmoking adults to determine whether exposure changed after the introduction of smokefree legislation and whether these changes varied by socioeconomic status (SES) and by household smoking status.
Methods: We analyzed salivary cotinine data from the Health Survey for England that were collected in 7 of 11 annual surveys undertaken between 1998 and 2008. We conducted multivariate regression analyses to examine secondhand smoke exposure as measured by the proportion of nonsmokers with undetectable levels of cotinine and by geometric mean cotinine.
Results: Secondhand smoke exposure was higher among those exposed at home and among lower-SES groups. Exposure declined markedly from 1998 to 2008 (the proportion of participants with undetectable cotinine was 2.9 times higher in the last 6 months of 2008 compared with the first 6 months of 1998 and geometric mean cotinine declined by 80%). We observed a significant fall in exposure after legislation was introduced—the odds of having undetectable cotinine were 1.5 times higher [95% confidence interval (CI): 1.3, 1.8] and geometric mean cotinine fell by 27% (95% CI: 17%, 36%) after adjusting for the prelegislative trend and potential confounders. Significant reductions were not, however, seen in those living in lower-social class households or homes where smoking occurs inside on most days.
Conclusions: We found that the impact of England’s smokefree legislation on secondhand smoke exposure was above and beyond the underlying long-term decline in secondhand smoke exposure and demonstrates the positive effect of the legislation. Nevertheless, some population subgroups appear not to have benefitted significantly from the legislation. This finding suggests that these groups should receive more support to reduce their exposure
Quasistationary Stabilization of the Decay of a Weakly-Bound Level and Its Breakdown in a Strong Laser Field
Although it was pointed out about 10 years ago that an atomic decay rate might decrease as the intensity of a high-frequency laser field increases, there still does not exist any complete understanding of either the physical origin of this interesting nonlinear phenomenon or its dependence on the atomic and field parameters. Essentially, the problem consists in that the phenomenon requires a major modification of the standard picture of photoeffect in a strong laser field. In Reference #1 the origin of this stabilization is related to a particular distortion of an atomic potential by an intense monochromatic high-frequency field. This phenomenon is called adiabatic or quasistationary stabilization (QS). For the case of Rydberg levels, another (interference) mechanism of QS was suggested. Both theories predict an unlimited decrease of the decay rate (or of the width Γ of an atomic level, i.e., of the imaginary part of the complex quasienergy, ε = Re ε – iΓ/2 ) as the laser field amplitude increases. In recent years the idea of “dynamic stabilization” (DS) has become popular. It originates from the pulse form of a laser field rather than from any intrinsic property of the atom in a strong monochromatic field. Within this model the numerous simulations point also to the possibility of a breakdown of stabilization for the case of superintense short laser pulses. However, a recent paper, using the quasistationary quasienergy states (QQES) as an adiabatic basis for the laser pulse has shown that DS has the same (quasistationary) origin as QS. Finally, a number of authors deny the existence of stabilization, in particular, of QS for ionization from a short-range potential and of DS in pulsed fields. Obviously, these controversies and ambiguities are caused by the complexity of the numerical solution of the Cauchy problem for the time-dependent Schrödinger equation in a strong field and by the absence of analyses for exactly solvable analytical models. We analyze the exactly solvable problem of an electron in a three-dimensional, short-range potential and consider the two questions: does a QS-like behavior of the decay rate exist for this model, and, if so, is there an upper intensity limit of the QS regime
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