Pair separation of magnetic elements in the quiet Sun

The dynamic properties of the quiet Sun photosphere can be investigated by analyzing the pair dispersion of small-scale magnetic fields (i.e., magnetic elements). By using $25$ hr-long Hinode magnetograms at high spatial resolution ($0".3$), we tracked $68,490$ magnetic element pairs within a supergranular cell near the disk center. The computed pair separation spectrum, calculated on the whole set of particle pairs independently of their initial separation, points out what is known as a super-diffusive regime with spectral index $\gamma=1.55\pm0.05$, in agreement with the most recent literature, but extended to unprecedented spatial and temporal scales (from granular to supergranular). Furthermore, for the first time, we investigated here the spectrum of the mean square displacement of pairs of magnetic elements, depending on their initial separation $r_0$. We found that there is a typical initial distance above (below) which the pair separation is faster (slower) than the average. A possible physical interpretation of such a typical spatial scale is also provided

Occurrence and persistence of magnetic elements in the quiet Sun

Turbulent convection efficiently transports energy up to the solar photosphere, but its multi-scale nature and dynamic properties are still not fully understood. Several works in the literature have investigated the emergence of patterns of convective and magnetic nature in the quiet Sun at spatial and temporal scales from granular to global. Aims. To shed light on the scales of organisation at which turbulent convection operates, and its relationship with the magnetic flux therein, we studied characteristic spatial and temporal scales of magnetic features in the quiet Sun. Methods. Thanks to an unprecedented data set entirely enclosing a supergranule, occurrence and persistence analysis of magnetogram time series were used to detect spatial and long-lived temporal correlations in the quiet Sun and to investigate their nature. Results. A relation between occurrence and persistence representative for the quiet Sun was found. In particular, highly recurrent and persistent patterns were detected especially in the boundary of the supergranular cell. These are due to moving magnetic elements undergoing motion that behaves like a random walk together with longer decorrelations ($\sim2$ h) with respect to regions inside the supergranule. In the vertices of the supegranular cell the maximum observed occurrence is not associated with the maximum persistence, suggesting that there are different dynamic regimes affecting the magnetic elements

Lattice Gauge Fixing for Parameter Dependent Covariant Gauges

We propose a non-perturbative procedure to fix generic covariant gauges on the lattice. Varying the gauge parameter, this gauge fixing provides a concrete method to check numerically the gauge dependence of correlators measured on the lattice. The new algorithm turns out to converge with a good efficiency. As a preliminary physical result, we find a sensitive dependence of the gluon propagator on the gauge parameter.Comment: 10 pages (LaTeX2e), 5 eps figure

Three-body description of direct nuclear reactions: Comparison with the continuum discretized coupled channels method

The continuum discretized coupled channels (CDCC) method is compared to the exact solution of the three-body Faddeev equations in momentum space. We present results for: i) elastic and breakup observables of d-12C at E_d=56 MeV, ii) elastic scattering of d-58Ni at E_d=80 MeV, and iii) elastic, breakup and transfer observables for 11Be+p at E_{11Be}/A=38.4 MeV. Our comparative studies show that, in the first two cases, the CDCC method is a good approximation to the full three-body Faddeev solution, but for the 11Be exotic nucleus, depending on the observable or the kinematic regime, it may miss out some of the dynamic three-body effects that appear through the explicit coupling to the transfer channel.Comment: 12 pages, 10 figures, accepted for publication in Physical Review

Variation of Creep Resistance in Ferritic Steels by a Heat Treatment

AbstractIn the power plants, boiler pipes and heaters, are made with ferritic steels low alloy. These steels have a microstructure with fine stable alloy carbides that impede the movement of the dislocations, however it is inevitable that during long periods of service or very critical conditions, microstructural changes occur that are responsible for the loss of material strength. In the past decades the 1Cr-0.5Mo steel was used, but it has been replaced by ferritc steels containing higher amounts of Cr and Mo, with the addition of other micro alloying elements such as niobium, titanium and vanadium to increase their mechanical strength. The objective of this work is to study the creep behavior of 1Cr-0.5Mo steel and to compare its strength when prior to service it is subjected to different heat treatments that improve its conditions of service, as that is beneficial from the economical point of view. Tensile creep tests were performed at a temperature range between 843 and 893K, and applied stresses between 131 and 205MPa in the material reception conditions comparing its behavior with others that previously has undergone different heat treatments. From experimental data the characteristic parameters were calculated such as the creep coefficient of stress and activation energy. The microstructural variation of the original material was also analyzed, after heat treatment and creep samples were characterized by optical microscopy, scanning electron microscopy and analysis by dispersive X- ray spectroscopy, to evaluate the effects of kinetics changes occurred in the precipitated phases and the presence of microstructural damage, such as nucleation, growth and coalescence of micro cavities. The microhardness of the phases present in the different samples were also measured

Ag-based thick-film front metallization of silicon solar cells

The evolution of microstructure and electrical properties of silver-based thick-film metallizations of silicon solar cells prepared by infrared firing processes has been ivestigated. The performance of the cells are shown to be dependent on several dynamical and diffusive phenomena. In particular, the sintering of silver grains, silver diffusion in the glassand the flow of gas at the metal/silicon interface strongly affect important characteristics of the cells such as sheet and contatct resistivities and the adhesion of fingers and bus bars. The existance of an optimum value of the peak firing temperature is observed and explained in terms of competitive phenomena occuring at the metal/silicon interface. Moreover it is shown that IR firing treatments require aa careful consideration of the sequence of printing anf firing steps. The features of heat treatments performed in conveyor belt furnaces using Joule and infrared sources are compared

Spreading of thin films assisted by thermal fluctuations

We study the spreading of viscous drops on a solid substrate, taking into account the effects of thermal fluctuations in the fluid momentum. A nonlinear stochastic lubrication equation is derived, and studied using numerical simulations and scaling analysis. We show that asymptotically spreading drops admit self-similar shapes, whose average radii can increase at rates much faster than these predicted by Tanner's law. We discuss the physical realizability of our results for thin molecular and complex fluid films, and predict that such phenomenon can in principal be observed in various flow geometries.Comment: 5 pages, 3 figure