Action Of Strong Magnetic Field Research Articles

Defect evolution in Y<sub>0.5</sub>Gd<sub>0.5</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7–<i>δ</i></sub> superconducting layer irradiated by H<sup>+</sup> ions

<sec>In order to further improve the superconducting current carrying capacity of RE-Ba-Cu-O coated conductor under the action of strong magnetic field, ion irradiation is used to generate the pinning centers of introduced magnetic flux in the RE-Ba-Cu-O coated conductor. In this work, the H<sup>+</sup>-ion irradiation of second-generation high-temperature superconductor RE-Ba-Cu-O strip is carried out by using the 320 kV high charge state ion synthesis research platform. Doppler broadened slow positron beam analysis combined with Raman spectroscopy is used to measure the change of microstructure in Y<sub>0.5</sub>Gd<sub>0.5</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7–<i>δ</i> </sub>(YBCO) sample irradiated by H<sup>+</sup> ions in a range of 5.0 × 10<sup>14</sup>–1.0 × 10<sup>16</sup> ions/cm<sup>2</sup>. The positron annihilation parameters in YBCO before and after irradiation are analyzed. It is found that after 100 keV H<sup>+</sup> ion irradiation, a large number of defects including vacancies, vacancy groups or dislocation groups are produced in the superconducting layer.</sec><sec>The larger the irradiation dose, the more the produced vacancy type defects are and the more complex the defect types, and the annihilation mechanism of positrons in the defects changes. Raman spectroscopy results show that with the increase of H<sup>+</sup> ion irradiation dose, the oxygen atoms in the coating rearrange, the plane spacing increases, the orthogonal phase structure of the coating is destroyed, and the degree of order decreases. The defects produced by such an ion irradiation lay a foundation for the introduction of flux pinning centers. Further research can be carried out in combination with X-ray diffractometer, transmission electron microscope, superconductivity and other testing methods to provide theoretical and practical reference for the optimization of material properties.</sec>

Inner dusty envelope of the AGB stars W Hydrae, SW Virginis, and R Crateris using SPHERE/ZIMPOL

Context. The asymptotic giant branch (AGB) marks the final evolutionary stage of stars with initial masses between ~0.8 and 8 M⊙. During this phase, stars undergo copious mass loss, which contributes significantly to the enrichment of the interstellar medium. The well-accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the density-enhanced and extended AGB stellar atmospheres. The details of the mass-loss process are not yet well understood, however. For oxygen-rich AGB stars, which are the focus of this study, the dust grains that drive the wind are expected to scatter visible light very efficiently because their sizes are relative large. Aims. We study the distribution of dust in the inner wind of oxygen-rich AGB stars to advance our understanding of the wind-driving process. Methods. We observed light scattered off dust grains that form around three oxygen-rich AGB stars (W Hya, SW Vir, and R Crt) with mass-loss rates between 10−7 and 10−6 M⊙ yr−1 using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL with three filters centred at 0.65, 0.75 and 0.82 μm. We compared the observed morphologies and the spectral dependence of the scattered light between the three sources and determined the radial profile, per image octant, of the dust density distribution around the closest target, W Hya. Results. We find the distribution of dust to be asymmetric for the three targets. A biconical morphology is seen for R Crt, with a position angle that is very similar to those inferred from interferometric observations of maser emission and of mid-infrared continuum emission. The cause of the biconical outflow cannot be inferred from the ZIMPOL data, but we speculate that it might be the consequence of a circumstellar disc or of the action of strong magnetic fields. The dust grains polarise light more efficiently at 0.65 μm for R Crt and SW Vir and at 0.82 μm for W Hya. This indicates that at the time of the observations, the grains around SW Vir and R Crt had sizes <0.1 μm, while those around W Hya were larger, with sizes ≳0.1 μm. The asymmetric distribution of dust around R Crt makes the interpretation more uncertain for this star, however. We find that polarised light is produced already from within the visible photosphere of W Hya, which we reproduce using models with an inner dust shell that is optically thick to scattering. We fit radiative transfer models to the radial profile of the polarised light observed around W Hya and find a steep dust density profile, with steepness varying considerably with direction. We find the wind-acceleration region of W Hya to extend to at least ~7 R⋆. This is in agreement with theoretical predictions of wind acceleration up to ~12 R⋆, and highlights that ZIMPOL observations probe the crucial region around AGB stars where dust forms and is accelerated.

Structure and physicomechanical properties of NbN-based protective nanocomposite coatings: A review

This review summarizes the present-day achievements in the study of the structure and properties of protective nanocomposite coatings based on NbN, NbAlN, and NbSiN prepared by a variety of modern deposition techniques. It is shown that a change in deposition parameters has a significant effect on the phase composition of the coatings. Depending on the magnitude of negative potential on the substrate, the pressure of nitrogen or a nitrogen–argon mixture in the chamber, and the substrate temperature, it is possible to obtain coatings containing different phases, such as NbN and SiN x (Si3N4), AlN, and NbAl2N. It is found that, in the case of formation of the e-NbN phase, the coatings become very hard; their hardness achieves values on the order of 53 GPa. At the same time, they remain thermally stable at temperatures of up to 600°C, chemically inert, and resistant to wear. The effect of the nanograin size, the volume fraction of boundaries and interfaces, and the point defect concentration on the physicomechanical properties of these coatings is described. Niobium nitride-based coatings can be used in superconducting systems and single-photon detectors; they are capable of operating under the action of strong magnetic fields of up to 20 T; they can be used in integrated logic circuits and applied as protective coatings of machine parts, edges of cutting tools, etc.

High magnetic field equilibria for the Fokker–Planck–Landau equation

The subject matter of this paper concerns the equilibria of the Fokker–Planck–Landau equation under the action of strong magnetic fields. Averaging with respect to the fast cyclotronic motion when the Larmor radius is supposed to be finite leads to an integro-differential version of the Fokker–Planck–Landau collision kernel, combining perpendicular space coordinates (with respect to the magnetic lines) and velocity. We determine the equilibria of this gyroaveraged Fokker–Planck–Landau kernel and derive the macroscopic equations describing the evolution around these equilibria, in the parallel direction.

MHD Free Convection and Mass Transfer Flow from a Vertical Plate in the Presence of Hall and Ion-Slip Current

MHD free convection and mass transfer flow over a continuously moving vertical porous plate under the action of strong magnetic field is investigated. In this analysis the hall and ion-slip current in the momentum equation are considered for high speed fluid flows and the level of concentration of foreign mass have been taken very high. The governing equations of the problem contain a system of partial differential equations. The coupled partial differential equations are solved numerically by explicit finite difference method. The results of this investigation are discussed for the different values of the well-known parameters with different time steps. Finally, the obtained finite difference solutions are compared with analytical solution by the graphical representation.

Mathematical models for strongly magnetized plasmas with mass disparate particles

The controlled fusion is achieved by magnetic confinement : the plasma is confined into toroidal devices called tokamaks, under the action of strong magnetic fields. The particle motion reduces to advection along the magnetic lines combined to rotation around the magnetic lines. The rotation around the magnetic lines is much faster than the parallel motion and efficient numerical resolution requires homogenization procedures. Moreover the rotation period, being proportional to the particle mass, introduces very different time scales in the case when the plasma contains disparate particles; the electrons turn much faster than the ions, the ratio between their cyclotronic periods being the mass ratio of the electrons with respect to the ions. The subject matter of this paper concerns the mathematical study of such plasmas, under the action of strong magnetic fields. In particular, we are interested in the limit models when the small parameter, representing the mass ratio as we ll as the fast cyclotronic motion, tends to zero.

Transport equations with disparate advection fields. Application to the gyrokinetic models in plasma physics

The subject matter of this paper concerns the asymptotic regimes for transport equations with advection fields having components of very disparate orders of magnitude. The main purpose is to derive the limit models: we justify rigorously the convergence towards these limit models and we investigate the well-posedness of them. Such asymptotic analysis arises in the magnetic confinement context, where charged particles move under the action of strong magnetic fields. In these situations we distinguish between a slow motion driven by the electric field and a fast motion around the magnetic lines.

Weather in stellar atmosphere revealed by the dynamics of mercury clouds in α Andromedae

The formation of long-lasting structures at the surfaces of stars is commonly ascribed to the action of strong magnetic fields. This paradigm is supported by observations of evolving cool spots in the Sun and active late-type stars, and stationary chemical spots in the early-type magnetic stars. However, results of our seven-year monitoring of mercury spots in non-magnetic early-type star alpha Andromedae show that the picture of magnetically-driven structure formation is fundamentally incomplete. Using an indirect stellar surface mapping technique, we construct a series of 2-D images of starspots and discover a secular evolution of the mercury cloud cover in this star. This remarkable structure formation process, observed for the first time in any star, is plausibly attributed to a non-equilibrium, dynamical evolution of the heavy-element clouds created by atomic diffusion and may have the same underlying physics as the weather patterns on terrestrial and giant planets.

Standardization of the strength of metals under conditions of low-temperature instability of plastic deformation and the action of strong magnetic fields

We propose an approach to determination of allowable stresses by a static-strength criterion for materials of structures that operate at very low temperatures (T≤20 K). This approach takes into account the qualitative change in the nature of deformation of the material, which has a significant effect on the mechanical characteristics and the specific work of deformation. The choice of critical stresses for these conditions is substantiated. The calculations are performed by introducing corrections for low-temperature hardening of the material in the presence of the intermittent-flow effect. We discuss the possibility of taking into account the influence of strong magnetic fields on the magnitude of allowable stresses.

Oscillation of the transport lifetime due to LO phonon scattering in quasi-one-dimensional channels.

The transport properties of quantum wires (QW's), mainly under the action of strong magnetic field, have been studied both theoretically and experimentally. In this report we use the memory-function projection-operator theory to obtain the resistivity of a QW built by structural and electric confinement and show that, in the absence of a magnetic field, oscillations in the transport lifetime appear due to scattering by LO phonons. These oscillations describe the harmonic confinement and are weakly dependent on the carrier concentration in the quantum limit. We suggest that this could be an easy way to characterize that structure using dc transport.

Low-temperature strengthening of structural alloys under the action of strong magnetic fields

Results are presented for an experimental study of the effect of strong constant and pulsed magnetic fields on the mechanical properties and deformation features of steels 03Kh20N16AG6, 03KM3AG19, and 12Kh19N10T at temperatures of 293-4.2 K. Relationships are presented connecting the level of critical stresses with magnetic field parameters and the possibility is substantiated of using the low-temperature strengthening effect for steels in determining permissible stresses for structural elements subjected to the effect of strong magnetic fields.

Variations of the Low l Solar Acoustic Spectrum Correlated with the Activity Cycle

AbstractSolar cycle variation of the frequencies and of the power of solar acoustic oscillations are investigated. Integrated sunlight data from 1977 to 1988 obtained at the Observatorio del Teide (Izaña, Tenerife), using a resonant scattering spectrophotometer, is analyzed in 60 day time strings and their power spectra are calculated from 2 to 3.8 mHz. To study the frequency variation, each power spectrum is cross-correlated with the one corresponding to the 1981 series and the shifts of the centroids of the cross-correlation peaks are calculated. The results show a clear variation in frequency of the cross-correlation peaks of −0.37 ± 0.04 μHz peak to peak as solar activity cycle goes from maximum to minimum. Moreover, this effect is found to depend on the l value of the modes, being absent for l = 0 and of 0.42 ± 0.06 μHz for l = 1. These results can be interpreted as an amplitude modulation between modes of the same multiplet, probably as a consequence of the action of strong magnetic fields. As low l modes penetrate deeply into the Sun’s interior, these observations suggest changes in its structure correlated with the solar activity cycle. When the power of the modes is calculated, using the same series as before, and its change along the solar cycle is studied, a variation of ~ 40% is found, the power being higher when solar activity is at its minimum. If this effect is independent of the l value of the p-modes, the results can be interpreted in terms of a change in the efficiency of the excitation mechanism of such modes. Indeed, if turbulent convection is such a mechanism, a change in the characteristic size of the granulation would account for the observed effect. Alternatively, another explanation could be a selective change in the efficiency of the excitation and/or damping mechanisms of the l ≤ 3 modes in front of other l value modes.

Deformation of copper under the action of strong magnetic fields and cryogenic temperatures

Results are presented for an experimental study of the effect of a strong constant and pulsed magnetic field on the mechanical characteristics and deformation features of copper M1 at 293–4.2 K. Dependences have been obtained for the increase in specimen strain on the intensity of a pulsed magnetic field and flow stress, and possibilities have been studied for increasing the effectiveness of pulsedmagnetic deformation of metals. A calculation relationship is suggested for determining the critical stress level under strong skin-effect conditions of a magnetic field.

Resistance to deformation of titanium, alloy under the action of strong magnetic fields in the temperature range 293-4.2�K

In order to study the effect of a pulsed magnetic field (MF)on the material deformation mechanisms in the temperature range 293-4.2 K, UMN-10 equipment was used making it possible to carry out specimen testing with application of a pulsed magnetic field with an intensity up to 32.0 MA/m. Practical use of the effect of superconductivity in turbogenerators with a superconducting exciting winding and other devices requires consideration of the effect of powerful electromagnetic fields on the strength and deformation features of structural materials used for their manufacture. The approach described for estimating the effect of a pulsed MF on deformation and failure of metals may be used in working out the strength of elements for cryogenic structures operating under the action of pulsed MF. With repeated action of an MF it is necessary to take account of the capacity of the material to resist thermal cyclic loading.