Effects of an external magnetic field on thermo-acoustical waves in a linear isotropic thermo-elastic dielectric material

Summary Thermo-mechanico-electromagnetic coupled waves propagating in a linear isotropic thermo-elastic dielectric material are theoretically investigated, in case an external magnetic field is applied to the material. Here the constitutive equations derived from the Clausius-Duhem inequality and Vernotte's heat conduction law are adopted. There are three types of coupled waves: the predominantly electromagnetic wave, the predominantly mechanical transverse wave and the predominantly thermo-mechanical longitudinal wave. The first and second waves have no thermal coupling. The third wave has thermal coupling and its propagation velocity and attenuation constant are perturbed by the external magnetic field.     

Off-diagonal impedance in amorphous wires and its application to linear magnetic sensors

We investigated the magnetic-field behavior of the off-diagonal impedance in Co-based amorphous wires under sinusoidal (50 MHz) and pulsed (5 ns rise time) current excitations. For comparison, we measured the field characteristics of the diagonal impedance as well. In general, when an alternating current is applied to a magnetic wire, the voltage signal is generated not only across the wire but also in a pickup coil wound on it. These voltages are related to the diagonal and off-diagonal impedances, respectively. We demonstrate that these impedances have a different behavior as functions of axial magnetic field: the diagonal impedance is symmetrical, whereas the off-diagonal one is antisymmetrical with a near-linear portion within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate circular domains. In the case of the sinusoidal excitation without a dc bias current, the off-diagonal response is very small and irregular. In contrast, the pulsed excitation, combining both high- and low-frequency harmonics, produces the off-diagonal voltage response without additional biasing. This behavior is ideal for a practical sensor circuit design. We discuss the principles of operation of a linear magnetic sensor based on a complementary metal-oxide-semiconductor transistor circuit.     

Study of the direct current breakage process in an external nonuniform magnetic field

We study experimentally the influence of an external nonuniform magnetic field with transverse and longitudinal components on the electric characteristics, plasma configuration, and cathode spot arrangement of vaccum-arc discharge. It is revealed that for a cylindrical cathode, cathode spots are nonuniformly distributed on the cathode surface, the spot configuration has no axial symmetry, and the arrangement of spots changes in time depending on the induction value of the external pulsed magnetic field. With an increase in induction, spots (on average) are arranged closer to the cathode, i.e., displaced to the region of weaker transverse field. For two cathode geometries, the probabilities of direct current breakage depending on the induction of the external nonuniform magnetic field are experimentally determined. To determine the conditions of current breakage, a three-dimensional mathematical model of ion motion is suggested and the current breakage criterion is formulated. The trajectories of fast cathode ions in an electrode system with a ring cathode have been calculated using the model. Calculations were performed in a nonuniform magnetic field, the radial and axial components of which have been measured experimentally. It is shown that conditions of current breakage determined with this criterion agree with the results of experiment.     

Off-diagonal magnetoimpedance in NiFe-Au-NiFe layered film and its application to linear magnetic sensors

We have measured the field dependence of the off-diagonal impedance in the megahertz frequency range for a NiFe-Au-NiFe layered film using a helical microcoil. The film and the coil were deposited by means of radio-frequency sputtering, and a transverse anisotropy in magnetic layers was established by applying a dc magnetic field during the deposition and by postproduction annealing. The film had 5 mm length, 50 /spl mu/m width, and 1.5 /spl mu/m total thickness. The helical microcoil had 23 turns with a 50 /spl mu/m turn width. We applied high-frequency excitation by means of the coil current and measured the induced voltage across the film stripe. This voltage response is directly proportional to the off-diagonal component of the total impedance tensor. We found that the plots of the real and imaginary parts of the off-diagonal impedance, as functions of the applied dc magnetic field, are antisymmetrical with respect to the field direction. The dc bias current through the film plays an important role: without the bias current, the measured signal is very small and irregular. The field antisymmetry demonstrated by the off-diagonal impedance can be utilized in highly sensitive and linear magnetic sensors, and we discuss the principles of operation of such sensors here.     

Nonholonomic relation between the polarization state of light and the angle of twist of a single-mode optical fiber with linear birefringence

The relation between the polarization state of light in a twisted single-mode optical fiber possessing linear birefringence and the twist angle of the fiber is considered for the case of a fiber stretched in a straight line. It is shown that this reflection is of a nonholonomic character. Pis’ma Zh. Tekh. Fiz. 24, 22–25 (June 12, 1998)     

Using magnetic field pulses with a slow sweep rate to produce uniform current distribution in multifilamentary superconducting wire

A detailed discussion is presented on the change in current distribution which takes place in a twisted multifilamentary superconducting wire induced by successive magnetic field pulses with a slow sweep-rate. The number of the field pulses required for a localized current-distribution to become uniform is estimated. Some modification of existing theories is necessary to describe the phenomenon. The results obtained are confirmed by systematic measurements of the net flux penetration into the wire during each cycle of field pulses.     

Complete polarization state generator with one variable retarder and its application for fast and sensitive measuring of two-dimensional birefringence distribution

The complete polarization state generator (PSG), which consists of one rotatable polarizer and one variable retarder with a quarter-wave plate, is introduced. The orientation angle of its output polarization ellipse equals half of the retardance of the variable retarder, and the ellipticity angle corresponds to the polarizer azimuth. The PSG is employed in the quantitative orientation-independent differential polarization microscope, which uses polarized light states with the same ellipticity and different orientation angles. Image processing algorithms using three or four frames are described.     

Decimeter-wave impedance measurement of Y- and Tl-based superconductors with and without an external magnetic field

The real part of low-temperature surface impedance of samples of high-T _c superconductors in the YBa₂ Cu₃O_6+x ceramic system was studied by the method of registration of the ownQ-factor of coaxial shortringed cylindrical resonator constructed entirely from examined material. For ceramic samples of high-T _c superconductors in the Tl₂Ba₂Ca₂Cu₃O system the same measurements were made with the help of coaxial Al resonator. The wavelength of radio frequency field equals 70cm. The dependence of the impedance of Y-based superconductors on weak magnetic field at helium temperature is close to the square law. The magnitude of impedance at temperature of liquid helium for Y-based superconductors composes 0,04 ohm, for Tl-based superconductors −0,01 ohm respectively which correlates with the statement of the presence of giant resonant absorption of radio frequency field power.     

Spatial variations of field polarization and phase in microwavecavities: application to the cesium fountain cavity

Wall losses in microwave cavities will generate a phase difference between the components of the field, and give rise to spatial phase variations which are related to the geometry of complicated shaped metal boundaries. Such effects are important to the design of the cavities used in the field of atomic frequency standards. Past attempts at calculating spatial phase variations in microwave cavities have been either limited to 1-D models or based upon an idealized model of the cavity, which simplifies its boundary shapes and neglects the effect of the source. In this paper, a numerical implementation of an electromagnetic field approach is used to overcome these limitations. The finite element method (FEM) is used to solve the driven form of the electromagnetic wave equation. The results show good agreement with transmission line predictions for a structure having simply shaped metal walls. The spatial phase distribution is then calculated for a 2-D approximation of the fountain cavity operating in the cylindrical TE₀₁₁ mode, which has been recommended for use in a Cs fountain frequency standard. A physical interpretation of the gradient of the phase in the cavity is presented, which shows it to be proportional to power flow     

Influence of an applied magnetic field on shielding current paths in a high TC superconductor

The influence of an applied magnetic field on shielding current paths in a bulk high T_c superconductor was investigated. This issue is very important for the quantitative analysis of levitation force, since the distortion and localization of the current paths have been found to be a major source of discrepancy between computed and experimental results. Furthermore, it has been speculated that the current paths vary, depending on the applied magnetic field. The magnetic field due to superconducting currents was measured by a Hall sensor when a permanent magnet was positioned at different gaps from a superconductor. The current paths were then reconstructed from the measured field data by an inverse analysis. A genetic algorithm, which is a robust probabilistic optimizing method, was used for the inverse analysis. It was found that the current paths merged into larger loops as the permanent magnet was retreating from the superconductor. It implies that a stronger applied magnetic field locally degrades the current density (J_c) and the shielding current paths are localized by these low-J_c areas which act as insulating boundaries.     

Evolution of an N-level system via automated vectorization of the Liouville equations and application to optically controlled polarization rotation

The Liouville equation governing the evolution of the density matrix for an atomic/molecular system is expressed in terms of a commutator between the density matrix and the Hamiltonian, along with terms that account for decay and redistribution. To find solutions of this equation, it is convenient first to reformulate the Liouville equation by defining a vector corresponding to the elements of the density operator, and determining the corresponding time-evolution matrix. For a system of N energy levels, the size of the evolution matrix is N^2?×^?N². When N is very large, evaluating the elements of these matrices becomes very cumbersome. We describe a novel algorithm that can produce the evolution matrix in an automated fashion for an arbitrary value of N. As a non-trivial example, we apply this algorithm to a 15-level atomic system used for producing optically controlled polarization rotation. We also point out how such a code can be extended for use in an atomic system with arbitrary number of energy levels.     

Stressed state of a matrix and an inclusion made from a twisted composite superconductor in cladding when a circular current interacts with a magnetic field

This paper discusses the stressed state of the matrix and a long wire made from a twisted multicore composite superconductor and encased in cladding when the circular component of the transport current interacts with a magnetic field. Pis’ma Zh. Tekh. Fiz. 24, 1–6 (January 26, 1998)     

Highly conductive and transparent ZnO thin films prepared by r.f. magnetron sputtering in an applied external d.c. magnetic field

Highly conductive films of undoped and aluminum-doped ZnO were prepared by r.f. magnetron sputtering in an applied external magnetic field in pure argon gas. Films with a resistivity as low as $\text{2 × 10}{}^{-4}\text{Ω}\text{cm}$ and a transmittance above 80% at wavelengths between 400 and 800 nm can be produced on a non-intentionally heated substrate by the sputtering of a ZnO target to which 2 wt.% Al₂O₃ had been added. The characteristic features of aluminum-doped ZnO films are their high carrier concentration, low mobility and blue shift in the absorption edge in comparison with the properties of undoped ZnO films.     

On the mechanism of the external magnetic field action on the electron temperature and ion charge state distribution in a vacuum arc plasma

When an external axial magnetic field is applied to a vacuum arc, the radial expansion of plasma from cathode spots transforms into a plasma flow along the magnetic field, provided that the electron-ion collision frequency is smaller than the Larmor frequency. As the magnetic field strength increases, the diameter of the resulting cylindrical channel decreases. This leads to an increase in the electron temperature and the ion charge due to enhanced Joule heating of the plasma. Unlike the intrinsic azimuthal magnetic field, the external axial field only restricts expansion of the plasma, rather than compressing the plasma jet.     

Radiation spectrum of an electron moving in a spiral in magnetic field in transparent media and in vacuum

Using the exact integral relationship for the spectral distribution of radiation power of an electron moving along a spiral in vacuum, the fine structure of synchrotron radiation spectrum at the first harmonics was investigated. The variation of the power spectral distribution of synchrotron-Cherenkov radiation for an electron moving in magnetic field in a transparent medium is studied.     

Boundary element method solution of magnetohydrodynamic flow in a rectangular duct with conducting walls parallel to applied magnetic field

The magnetohydrodynamic (MHD) flow of an incompressible, viscous, electrically conducting fluid in a rectangular duct with one conducting and one insulating pair of opposite walls under an external magnetic field parallel to the conducting walls, is investigated. The MHD equations are coupled in terms of velocity and magnetic field and cannot be decoupled with conducting wall boundary conditions since then boundary conditions are coupled and involve an unknown function. The boundary element method (BEM) is applied here by using a fundamental solution which enables to treat the MHD equations in coupled form with the most general form of wall conductivities. Also, with this fundamental solution it is possible to obtain BEM solution for values of Hartmann number (M) up to 300 which was not available before. The equivelocity and induced magnetic field contours which show the well-known characteristics of MHD duct flow are presented for several values of M.     

Porous soft magnetic material: The maghemite microsphere with hierarchical nanoarchitecture and its application in water purification

Porous soft magnetic material is a member of soft magnetic material family having large surface area. This kind of material has vast potential in the surface-related applications. In this paper, a kind of maghemite (γ-Fe₂O₃) with large surface area, 82.7 m²/g, specifically, was obtained by the solvothermal method and subsequent calcining process. As shown in the test results, the maghemite microsphere has no magnetic memory in the magnetic field since the residual magnetization is 0.7 emu/g. In the waste water treatment process, Rhodamine B, a model organic pollutant in the water, was removed through the adsorption and desorption cycles by the maghemite microspheres. The maghemite microspheres in the water can be recycled easily by a magnetic separation procedure and regenerated in ethanol at room temperature.     

Linear and nonlinear optical properties in an asymmetric double quantum well under intense laser field: Effects of applied electric and magnetic fields

In the present study, the effects of electric and magnetic fields on the linear and third-order nonlinear optical absorption coefficients and relative change of the refractive index in asymmetric GaAs/GaAlAs double quantum wells under intense laser fields are theoretically investigated. The electric field is oriented along the growth direction of the heterostructure while the magnetic field is taken in-plane. The intense laser field is linear polarization along the growth direction. Our calculations are made using the effective-mass approximation and the compact density-matrix approach. Intense laser effects on the system are investigated with the use of the Floquet method with the consequent change in the confinement potential of heterostructures. Our results show that the increase of the electric and magnetic fields blue-shifts the peak positions of the total absorption coefficient and of the total refractive index while the increase of the intense laser field firstly blue-shifts the peak positions and later results in their red-shifting.     

Complexity reduction and kaizen events to balance manual assembly lines: an application in the field

Notwithstanding the existence of a broad research base on assembly line balancing (ALB), companies do not use the mathematical approaches developed in the literature to configure assembly lines. This article aims to fill the gap between research and application by presenting and testing in a real industrial context a methodology based on complexity reduction and kaizen events. First, the methodology supports reducing the complexity that affects real-life assembly systems in terms of the variety of, e.g. finished products, materials and parts. Next, the methodology proposes the conduction of kaizen events by using lean manufacturing tools, such as process analysis, time observation, waste identification, workstation standard documents and yamazumi charts. The methodology is successfully applied to a case study that describes its use in the confectionery process for a major chocolatier company along with the results of the application. The main contribution of this paper consists in presenting a method to manage the line balancing activity within everyday industrial realities, helping practitioners to improve and maintain the performance over time.     

On the electromagnetic field of one- and two-dimensional gratings of large cylinders--Part II: Permeable cylinders in uniform field

The field pattern due to gratings of large ferromagnetic or conducting circular cylinders in a uniform external magnetic or electric field is calculated. The method used involves the relation between the size of the cylinder and the spacing of line charges forming dipoles codirectional with the external field. Solutions are provided for square as well as rectangular unit cells, including the one-dimensional (array) case. In the same context, a uniform field obliquely oriented towards the grating is considered, as is the flux pattern for the different configurations.