Formation and dynamics of a virtual cathode in a tubular electron beam placed in a magnetic field

Mechanisms underlying the formation and dynamics of a virtual cathode (VC) in a tubular electron beam subjected to a magnetic field are studied using a numerical 2D model. Two qualitatively different competing types of space charge dynamics near the VC are discovered. Which of them predominates depends on the magnetic field strength. The beam current critical density at which a nonstationary VC forms in the system is also strongly dependent on the magnetic field. It is shown that the optimal strength of the magnetic field that minimizes the beam current critical density depends on the Brillouin magnetic field.     

Pattern of the induced magnetic field in high-temperature superconducting ceramics

Our experiments performed earlier have shown that, when an external magnetic field is absent, the transport critical current in 3-d superconducting ceramics is a homogeneous function of the sample transverse sizes. The transport critical current density and magnetic field induced by the current are homogeneous functions of a point on the sample cross-section. Using these experimental results equations describing the induced magnetic field pattern in ceramic sample have been derived. The distributions of the transport critical current density and induced magnetic field in the samples having polygonal, diamond-shaped cross-sections illustrate the results.     

Anomalous behavior of textures in a magnetic field

It is shown that a qualitatively new type of transition to a nonlinear phase is possible in two-dimensional two-phase textures in a magnetic field. The anomalous growth of the effective response (higher-order correlation functions of the current) near the critical magnetic field is studied. The dependence of the critical magnetic field on the angles of microinclusions is calculated. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 115–119 (25 July 1997)     

Oscillations of a three-component assembly with or without magnetic field

The plasma is taken to be composed of singly ionized molecules, free electrons and neutral molecules, each of the component being described by the hydromagnetic equations, modified to take into account the displacement current, existence of free charge in the medium, and the modified current equation without involving the scalar conductivity. The basic equations are linearized and only small amplitude waves are considered. In the absence of any external magnetic field, the transverse and longitudinal modes of oscillation separate out. In the transverse part a coupled plasma oscillation occurs which could be propagated only above a certain critical frequency and in the longitudinal part one extraordinary mode of propagation occurs having a forbidden range of frequencies. When there is an external applied magnetic field, ordinary and extraordinary waves are propagated along the direction of the magnetic field, whereas only ordinary waves are propagated transverse to the magnetic field. The critical frequencies above which these waves are propagated are evaluated and, the possible explanation of this medium like behaviour could be the implicit assumption of conductivity being not a scalar.     

Effect of longitudinal applied magnetic field on the self-pinched critical current in intense electron beam diode

The effect of applied longitudinal magnetic field on the self-pinched critical current in the intense electron beam diode is discussed. The self-pinched critical current is derived and its validity is tested by numerical simulations. The results shows that an applied longitudinal magnetic field tends to increase the self-pinched critical current. Without the effect of anode plasma, the maximal diode current approximately equals the self-pinched critical current with the longitudinal magnetic field applied; when self-pinched occurs, the diode current approaches the self-pinched critical current.     

Resonant tunneling through a double-barrier quantum well in a transverse magnetic field

We theoretically analyze the tunneling of electrons through a heterostructure with two barriers and a quantum well between them in a magnetic field perpendicular to the current. We take into account the contribution from electrons with various positions of the magnetic oscillator center to the current. The region of the Z-shaped current-voltage characteristic for the heterostructure is shown to narrow as the magnetic field strengthens. Our analysis reveals a critical magnetic field strength at which the Z-shaped current-voltage characteristic transforms into an N-shaped one. We compare our results with experimental data.     

Destruction of defective conductors with a current in a magnetic field

A mechanism of destruction of conductors with a crack in an external magnetic field is proposed. In this case, ponderomotive forces form a complex strained state in the crack tip, which is estimated from the equations of the linear theory of elasticity. The critical ponderomotive forces are found, which depend on the current and magnetic field parameters and induce stresses in the defect top, which are comparable with the yield strength of the material.     

Description of the transport critical current density behavior of polycrystalline superconductors as a function of the applied magnetic field

A model to describe the critical current density behavior of high-T_c polycrystalline superconductors is proposed for all magnetic field values. The main features of the model are as follows: the transport critical current density is controlled by the weak-link network at grain boundaries. The size distribution of weak links is well represented by a Gamma-type distribution. Finally, the tunneling critical current between grains follows a Fraunhofer diffraction pattern or a modified pattern if the applied magnetic field is lower or higher than the first critical field H_c1.     

Properties of Josephson junctions in the inhomogeneous magnetic field of a system of ferromagnetic particles

The effect of an array of ferromagnetic nanoparticles on the field-dependent critical current of the short overlap Josephson junction is experimentally studied. Large reversible variations of the maximum critical current are observed depending on the magnetic state of the particles. The pronounced commensurability effects are detected which are proved by the additional peaks of magnetic field induced diffraction pattern.     

外场中超导结的直流约瑟夫逊电流的计算

外加磁场对超导体/半导体/超导体结的临界电流有一定影响。超导结的临界电流与磁场的曲线非常类似于单色光在单狭缝衍射的夫琅和费图样。该文在未考虑外加磁场的超导体/半导体/超导体结的临界电流的基础上,进一步对外加磁场的情形进行了研究,并对其结果借助M athem atica软件做了进一步的讨论。     

Properties of Josephson junctions in the inhomogeneous magnetic field of a system of ferromagnetic particles

The effect of a system of ferromagnetic particles on the field-dependent critical current of a Josephson junction is experimentally studied for junctions of different geometries. For edge junctions, the effect of commensurability between the periodic magnetic field of the particles and the Josephson vortex lattice is observed. The effect manifests itself in additional maxima of the field-dependent critical current. For overlap junctions, giant (greater than sixfold) variations of the maximum critical current are observed depending on the magnetic state of the particles. The changes in the “Fraunhofer” pattern of the overlaped Josephson junctions are attributed to the formation of Abrikosov vortices due to the effect of uniformly magnetized particles. The effects revealed in the experiments can be used to analyze the inhomogeneous magnetic field of a system of submicron particles and to control the transport properties of Josephson junctions.     

Three-dimensional evolution of a relativistic current sheet: triggering of magnetic reconnection by the guide field

The linear and nonlinear evolution of a relativistic current sheet of pair (e(+/-)) plasmas is investigated by three-dimensional particle-in-cell simulations. In a Harris configuration, it is obtained that the magnetic energy is fast dissipated by the relativistic drift kink instability (RDKI). However, when a current-aligned magnetic field (the so-called "guide field") is introduced, the RDKI is stabilized by the magnetic tension force and it separates into two obliquely propagating modes, which we call the relativistic drift-kink-tearing instability. These two waves deform the current sheet so that they trigger relativistic magnetic reconnection at a crossover thinning point. Since relativistic reconnection produces a lot of nonthermal particles, the guide field is of critical importance to study the energetics of a relativistic current sheet.     

A simple electromagnetic method of cyclic loss measurement for superconducting wires in a combined alternating transverse magnetic field and transport current

We measured cyclic losses in a superconducting wire, carrying alternating transport current, simultaneously exposed to an alternating transverse magnetic field. Samples of Bi-2223 Ag-sheathed tapes have configuration of a double-layer non-inductive coil, which itself is a pickup coil to measure the AC losses. Potential taps were attached to both terminals of the sample coil. The external field was applied along the axis of the sample coil. In this procedure, we can estimate an averaged Poynting's vector on a cylindrical surface between the two layers by means of signals from a pair of the potential taps and from pickup coils for the external magnetic field and the transport current. We can also measure a magnetization and an extended transport-current components of AC losses in addition to a total cyclic loss for a combined alternating external field and transport current. Obtained results are compared with numerical predictions of the critical state model taking into account the magnetic field dependence of critical current density.     

Nonlinear microwave response of YBCO single crystal in constant magnetic field

The generation of a microwave second harmonic by a YBCO single crystal in a dc magnetic field was studied. We found that the signal existed only when there was a direct screening current. As a result, the pinning current as a function of magnetic field can be derived directly from the second harmonic intensity versus the magnetic field. The experimental data were interpreted in terms of a generalized model of the critical state taking into account diffusion of vortices and the absence of a barrier stopping vortices from leaving the sample. We have shown that, in a decreasing dc magnetic field, the current density is considerably lower than both the critical and screening current densities in an increasing dc field. Our experiments indicate that vortices are not the sources of radiation at the double frequency. A relation between the mechanism of harmonic generation in the Meissner phase and modulation of the order parameter by the microwave magnetic field (Ginzburg-Landau nonlinearity) is discussed. It is remarkable that, by measuring the second harmonic intensity in the Meissner state versus temperature, one can obtain the magnetic field penetration depth as a function of temperature with fairly good accuracy. Zh. éksp. Teor. Fiz. 115, 2242–2253 (June 1999)     

Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

The DC magnetization and AC magnetic susceptibilities were measured for MgB₂ single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.06)₂. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.     

Column buckling of doubly parallel slender nanowires carrying electric current acted upon by a magnetic field

Axial buckling of current-carrying double-nanowire-systems immersed in a longitudinal magnetic field is aimed to be explored. Each nanowire is affected by the magnetic forces resulted from the externally exerted magnetic field plus the magnetic field resulted from the passage of electric current through the adjacent nanowire. To study the problem, these forces are appropriately evaluated in terms of transverse displacements. Subsequently, the governing equations of the nanosystem are constructed using Euler–Bernoulli beam theory in conjunction with the surface elasticity theory of Gurtin and Murdoch. Using a meshless technique and assumed mode method, the critical compressive buckling load of the nanosystem is determined. In a special case, the obtained results by these two numerical methods are successfully checked. The roles of the slenderness ratio, electric current, magnetic field strength, and interwire distance on the axial buckling load and stability behavior of the nanosystem are displayed and discussed in some detail.     

Analysis of the magnetic field distribution and estimation of the critical current density based on a magnetooptical study of the superconducting strip composites

The results of magnetic flux penetration into high-temperature superconducting composites of the second generation have been analyzed and described. The distribution of magnetic flux was observed experimentally using the magnetooptical visualization technique at various temperatures of 4.2–80 K. The experimental data have been found to coincide with the results of analytical and semi-numeric calculations of magnetic field profiles carried out for regimes of full and partial field penetration into superconducting composites. The method of estimating the critical current density based on measurements of the topography of the transverse magnetic flux has been considered, and an approach to estimating the critical current density for strips from measurements of the tangential field component has been proposed.     

Measuring transport current loss of BSCCO/Ag tapes exposed to external AC magnetic field

BSCCO/Ag tape superconductors are developed for electrical power applications at liquid nitrogen temperatures. In these applications, e.g., superconducting transformers and power cables, an AC transport current and an AC magnetic field are present at the same time. A set-up to measure the influence of external AC magnetic field on the transport current loss, i.e., the voltage drop across a sample supplied with an AC transport current, has been developed. The magnetic field can be applied both parallel and perpendicular to the broad side of the tape conductor. An increase of the transport current loss due to the external AC magnetic field is observed. When a DC external magnetic field is applied the increase of the self-field loss can be described well by the decrease of the critical current due to the magnetic field. In the case of an AC external magnetic field this is only a minor effect. For magnetic field amplitudes higher than a certain threshold value the transport current loss is described reasonably well by the self-field loss and a dynamic resistance contribution calculated from the DC voltage–current relation in AC magnetic field.     

Electric field induced by magnetic flux motion in superconductor containing fractal clusters of a normal phase

The influence of the fractal clusters of a normal phase, which act as pinning centers, on the dynamics of magnetic flux in percolative type-II superconductor is considered. The voltage–current characteristics of such a superconductor are obtained taking into account the effect of fractal properties of cluster boundaries on the magnetic flux trapping. It is revealed that the fractality reduces the electric field arising from magnetic flux motion and thereby raises the critical current of a superconductor.     

Penetration of magnetic field into a long periodically modulated Josephson contact

A new approach to magnetic field profiling inside a Josephson contact is suggested. Its essence consists in analyzing continuous variation of a current configuration leading to a decrease in the Gibbs potential. With this approach, one can find a configuration into which the Meissner state turns when an external field slightly exceeds the upper boundary of the Meissner regime and trace the evolution of this configuration with increasing field. Calculations show that there exists critical value I _c of the pinning parameter in the range 0.95–1.00. This critical value separates two possible conditions of magnetic field penetration into the contact. At I > I _c, a near-boundary current configuration completely compensating for the external field inside the contact arises irrespective of the external field strength. At I < I _c, such a situation is observed only until the external field strength exceeds certain value H _max. Higher fields penetrate into the contact indefinitely deep. In nearboundary configurations, the magnetic field drops with increasing depth almost linearly. Its slope k has rational values, which remain constant within finite intervals of I. As I goes beyond a given interval, k rises stepwise and takes on another rational value. When an external magnetic field is switched on adiabatically, configurations with a maximal growth rate of the magnetic field are observed.