Изучение каталитической активности ультрадисперсных порошков железа в процессе получения жидких углеводородов из синтез-газа

The propagation of linearly polarized large-amplitude electromagnetic waves in critical density plasmas is studied in the framework of the Akiezer-Polovin model. A new mechanism of mode conversion is presented. The well-known periodic solutions are generalized to quasiperiodic solutions taking into account additional electrostatic oscillations. Nearly periodic circle-like solutions are found to be stabilized by intrinsic mode coupling whereas for nearly periodic eight-like solutions an effective mode conversion mechanism is discovered. Finally, the modulation timescales are considered

Non-stationary Rayleigh-Taylor instability in supernovae ejecta

The Rayleigh-Taylor instability plays an important role in the dynamics of several astronomical objects, in particular, in supernovae (SN) evolution. In this paper we develop an analytical approach to study the stability analysis of spherical expansion of the SN ejecta by using a special transformation in the co-moving coordinate frame. We first study a non-stationary spherical expansion of a gas shell under the pressure of a central source. Then we analyze its stability with respect to a no radial, non spherically symmetric perturbation of the of the shell. We consider the case where the polytropic constant of the SN shell is $\gamma=5/3$ and we examine the evolution of a arbitrary shell perturbation. The dispersion relation is derived. The growth rate of the perturbation is found and its temporal and spatial evolution is discussed. The stability domain depends on the ejecta shell thickness, its acceleration, and the perturbation wavelength.Comment: 16 page

Assessing regional-scale variability in deforestation and forest degradation rates in a tropical biodiversity hotspot

Deforestation and forest degradation are major drivers of global environmental change and tropical forests are subjected to unprecedented pressures from both.For most tropical zones, deforestation rates are averaged across entire countries,often without highlighting regional differentiation. There are also very few estimates of forest degradation, either averaged or localized for the tropics. We quantified regional and country-wide changes in deforestation and forest degradation rates for Madagascar from Landsat temporal data (in two intervals,1994–2002 and 2002–2014). To our knowledge, this is the first country-wide estimate of forest degradation for Madagascar. We also performed an intensity analysis to categorize the magnitude and speed of transitions between forest,vegetation matrix, cultivated land and exposed surface. We found significant regional heterogeneity in deforestation and forest degradation. Deforestation rates decreased annually in lowland evergreen moist forest by -0.24% and in all other vegetation zones. Forest degradation rates had annual increases in the same period in lowland evergreen moist forest (0.09%), littoral forest (0.06%)but decreased in medium altitude moist evergreen forest (-0.25%), dry deciduous forest (-0.23%) and scelrophyllous woodland (-0.61%) in the same period. Despite these regional differences, higher rates of deforestation and forest degradation were consistently driven by rapid and large-sized conversions of largely intact forest to cultivated lands and exposed surfaces, most of which occurred between 1994 and 2002. These results suggest that while targeted conservation projects may have reduced forest degradation rates in some areas (e.g.medium altitude moist evergreen forest), the drivers of land cover change remain intense in relatively neglected regions. We advocate a more balanced approach to future conservation initiatives, one recognizing that deforestation and forest degradation, particularly in tropical Africa, are often driven by region-specific conditions and therefore require conservation policies tailored for local conditions

Phase transitions in self-gravitating systems. Self-gravitating fermions and hard spheres models

We discuss the nature of phase transitions in self-gravitating systems both in the microcanonical and in the canonical ensemble. We avoid the divergence of the gravitational potential at short distances by considering the case of self-gravitating fermions and hard spheres models. Three kinds of phase transitions (of zeroth, first and second order) are evidenced. They separate a ``gaseous'' phase with a smoothly varying distribution of matter from a ``condensed'' phase with a core-halo structure. We propose a simple analytical model to describe these phase transitions. We determine the value of energy (in the microcanonical ensemble) and temperature (in the canonical ensemble) at the transition point and we study their dependance with the degeneracy parameter (for fermions) or with the size of the particles (for a hard spheres gas). Scaling laws are obtained analytically in the asymptotic limit of a small short distance cut-off. Our analytical model captures the essential physics of the problem and compares remarkably well with the full numerical solutions.Comment: Submitted to Phys. Rev. E. New material adde

Enhanced inverse bremsstrahlung heating rates in a strong laser field

Test particle studies of electron scattering on ions, in an oscillatory electromagnetic field have shown that standard theoretical assumptions of small angle collisions and phase independent orbits are incorrect for electron trajectories with drift velocities smaller than quiver velocity amplitude. This leads to significant enhancement of the electron energy gain and the inverse bremsstrahlung heating rate in strong laser fields. Nonlinear processes such as Coulomb focusing and correlated collisions of electrons being brought back to the same ion by the oscillatory field are responsible for large angle, head-on scattering processes. The statistical importance of these trajectories has been examined for mono-energetic beam-like, Maxwellian and highly anisotropic electron distribution functions. A new scaling of the inverse bremsstrahlung heating rate with drift velocity and laser intensity is discussed.Comment: 12 pages, 12 figure

Overweight in relation to allergic disease in childhood and adolescence

The prevalences of childhood overweight and allergic diseases have increased in parallel during the last decades. The overall aim of this thesis was to investigate the associations between overweight (maternal and childhood) and allergic diseases, as well as lung function, throughout childhood up to adolescence. In addition, we investigated the validity of self-reported height, weight and corresponding body mass index (BMI) among Swedish adolescents. All studies were based on the BAMSE study, a population-based birth cohort of 4,089 children followed until age 16 years. Maternal BMI was obtained from the Swedish medical birth register, while childhood BMI was measured at clinical investigations, collected from child and school health care records and self-reported. Allergic diseases were assessed by repeated questionnaires regarding symptoms and medications, while allergic sensitization to inhalant allergens was defined by the presence of specific immunoglobulin E (IgE)-antibodies in blood. Lung function was measured by spirometry at 8 and 16 years and by impulse oscillometry (IOS) at 16 years. The results of Study I showed that maternal BMI in early pregnancy was associated with asthma, but not rhinitis, eczema or allergic sensitization in the offspring up to 16 years. The association was strongest for persistent asthma, while no increased risk was observed for transient asthma. Categorization of maternal BMI showed that maternal obesity, but not overweight, was significantly associated with childhood asthma. However, the child’s own weight status could partly explain the observed association between maternal BMI and asthma in the offspring. In Study II, we found that girls with persistent asthma had a higher BMI and an increased risk of overweight throughout childhood, compared to girls without asthma. Girls with transient asthma had an increased risk of overweight at ages 4-7.9 years, whereas girls with late-onset asthma had a tendency towards an increased risk of overweight at age ≥15 years. In boys, the difference in BMI between children with and without asthma was smaller, and no consistent association was observed between asthma phenotypes and overweight. In Study III, we observed that overweight and obesity at age 8 years were associated with increased forced vital capacity (FVC) and to some extent forced expiratory volume in one second (FEV1), but reduced FEV1/FVC ratios at 8 and 16 years. The strongest association with FEV1/FVC was observed for persistent overweight at both 8 and 16 years, whereas no significant association was found for transient overweight. Cross-sectional analyses of IOS showed that overweight and obesity were associated with higher peripheral airway resistance and reactance at 16 years. The result of Study IV showed that self-reported and measured height and weight were highly correlated at 16 years (r=0.98 for height, r=0.96 for weight). On average, self-reported weight was underreported by 1.1 kg and height was overreported by 0.5 cm, leading to an underestimation of BMI by 0.5 kg/m2. The accuracy of self-reported BMI was somewhat lower among girls and among overweight and obese participants, compared to normal weight participants. Our results suggest that maternal and childhood overweight and obesity are associated with asthma and evidence of airway obstruction in children and adolescents. The association between maternal BMI and asthma may, to some extent, be mediated through childhood overweight and seems to be explained by non-allergic mechanisms. In addition, we conclude that web-based self-reported BMI can be used as a valid, quick and cost-effective alternative to measured BMI among Swedish adolescents. The accuracy however declines with increasing BMI, and is somewhat lower among girls compared to boys

Leaf venation, as a resistor, to optimize a switchable IR absorber

Leaf vascular patterns are the mechanisms and mechanical support for the transportation of fluidics for photosynthesis and leaf development properties. Vascular hierarchical networks in leaves have far-reaching functions in optimal transport efficiency of functional fluidics. Embedding leaf morphogenesis as a resistor network is significant in the optimization of a translucent thermally functional material. This will enable regulation through pressure equalization by diminishing flow pressure variation. This paper investigates nature’s vasculature networks that exhibit hierarchical branching scaling applied to microfluidics. To enable optimum potential for pressure drop regulation by algorithm design. This code analysis of circuit conduit optimization for transport fluidic flow resistance is validated against CFD simulation, within a closed loop network. The paper will propose this self-optimization, characterization by resistance seeking targeting to determine a microfluidic network as a resistor. To advance a thermally function material as a switchable IR absorber

Laser-Cluster-Interaction in a Nanoplasma-Model with Inclusion of Lowered Ionization Energies

The interaction of intense laser fields with silver and argon clusters is investigated theoretically using a modified nanoplasma model. Single pulse and double pulse excitations are considered. The influence of the dense cluster environment on the inner ionization processes is studied including the lowering of the ionization energies. There are considerable changes in the dynamics of the laser-cluster interaction. Especially, for silver clusters, the lowering of the ionization energies leads to increased yields of highly charged ions.Comment: 10 pages, 11 figure

Cold collapse and the core catastrophe

We show that a universe dominated by cold dark matter fails to reproduce the rotation curves of dark matter dominated galaxies, one of the key problems that it was designed to resolve. We perform numerical simulations of the formation of dark matter halos, each containing \gsim 10^6 particles and resolved to 0.003 times the virial radius, allowing an accurate comparison with rotation curve data. A good fit to both galactic and cluster sized halos can be achieved using the density profile rho(r) \propto [(r/r_s)^1.5(1+(r/r_s)^1.5)]^-1, where r_s is a scale radius. This profile has a steeper asymptotic slope, rho(r) \propto r^-1.5, and a sharper turnover than found by lower resolution studies. The central structure of relaxed halos that form within a hierarchical universe has a remarkably small scatter (unrelaxed halos would not host disks). We compare the results with a sample of dark matter dominated, low surface brightness (LSB) galaxies with circular velocities in the range 100-300 km/s. The rotation curves of disks within cold dark matter halos rise too steeply to match these data which require a constant mass density in the central regions. The same conclusion is reached if we compare the scale free shape of observed rotation curves with the simulation data. It is important to confirm these results using stellar rather than HI rotation curves for LSB galaxies. We test the effects of introducing a cut-off in the power spectrum that may occur in a universe dominated by warm dark matter. In this case halos form by a monolithic collapse but the final density profile hardly changes, demonstrating that the merger history does not play a role in determining the halo structure.Comment: Latex 13 pages, 4 figures. Submitted to MNRAS. High resolution colour version of figure 4 and other N-body images here: http://star-www.dur.ac.uk:80/~moore/images