Nonuniform Static Field Research Articles

The cloaking method in non-uniform static fields of cylindrical and spherical invisibility cloaks

The cloaking methods of infinite length coaxial cylindrical and concentric spherical invisibility cloaks in the non-uniform static field are investigated. First, the arbitrary potential function at the infinite boundary is expanded into a series of orthogonal potential functions. When there is only the nth order orthogonal potential function distribution at the infinite boundary, the analytical cloaking conditions of both the infinite length coaxial cylindrical and concentric spherical cloaks are derived. Furthermore, based on the analytical cloaking conditions, the approaching cloaking method is proposed in a complex non-uniform static field which composed of multiple orders potential functions. This method can acquire the optimal cloaking condition and avoid the iteration process by using numerical methods such as the finite element method etc. Finally, three examples are given to verify the correctness of the analytical cloaking conditions and the effectiveness of the approaching cloaking method proposed in this paper.

Three-dimensional FDTD Modeling and Analysis of Microwave Circulators

Abstract An extended finite difference time domain (FDTD) scheme for three-dimensional modeling of ferrite loaded microwave circuits is applied to the analysis of microwave circulator structures. An FDTD method for the simulation of magnetized ferrite material is introduced, and a brief stability analysis is given. The effectiveness of the algorithm is demonstrated by the analysis of several examples and a comparison with measurements. The differences that arise from the assumption of either a uniform or non-uniform static field distribution inside the ferrite are analyzed and evaluated.

Propagation of magnetostatic backward surface waves in ferrite-insulator-metal structures magnetized by linearly nonuniform magnetic fields

A theoretical study is made of the trajectories and of the changes in magnitude and direction of the wave vectors of magnetostatic backward surface waves with different frequencies propagating in ferrite-insulator-metal structures with different insulating layer thicknesses and magnetized by a linearly nonuniform static field. It is shown that both forward and backward magnetostatic surface waves (MSSWs) propagate in a waveguide channel, on one side of which MSSWs undergo mirror reflection and on the other side of which their propagation direction is rotated, independently of the thickness of the insulator in the structure. It is shown that when MSSWs propagate in a nonuniform field, the forward wave is converted into a backward wave and, under certain conditions, the backward wave is converted into a forward wave. Some features of the propagation characteristics of magnetostatic backward surface waves are determined.

Image restoration from nonuniform static field influence in modified echo-planar imaging.

Experimental results of images restored from the influence of nonuniform static fields in modified echo-planar imaging are presented. The same restoration technique is used as that proposed for Fourier imaging. Experimental results show that both density variation and geometrical distortion can almost completely be eliminated in the restored image. Image restoration can relax the stringent requirement on the amplitude of the time-modulated gradient needed in modified echo-planar imaging.

Reversible, field induced agglomeration in magnetic colloids

Reversible agglomeration of magnetic particles (20–300-Å diameter) in a colloidal suspension has been induced by applying various magnetic fields: nonuniform static fields and uniform ac or dc fields of magnitudes from 2.5 to 230 Oe. A sensitive Colpitts oscillator circuit monitored the spatial and temporal variations in magnetization in a vertical 11-cm column of magnetic fluid. The results are consistent with the hypothesis that spherical agglomerates of 10 7–10 9 particles are formed and settle gravitationally. The agglomeration was most pronounced in a commercially available water-base, 200-G ferrofluid; 20% of the particles formed agglomerates of ≥ 10 7 particles and settled out in 1 hr in a 230-Oe uniform ac or dc field. This effect was present to a lesser extent in other fluids (200-G ester base, 3% of the particles agglomerate in 1 hr; 200-G hydrocarbon base, ∼ 0.05% in 2 hr). Electron micrograph studies indicate that all sizes of particles participate in the agglomeration. Centrifuge experiments show that the agglomerates are not limited by close packing upon settling to the bottom of the tube. The Colpitts oscillator should be useful in determining the stability of magnetic fluids used in applications such as magnetic ink printers, metal separation, etc. It may also be useful in experimental investigation of agglomeration hypotheses because the extent of the agglomeration can be conveniently controlled via the applicatioa of magnetic fields.

Equilibrium of a toroidal pinch in a static magnetic field and in a high-frequency large-amplitude field

The authors consider the equilibrium confinement of a toroidal plasma pinch in a static magnetic field and in a high-frequency large-amplitude field in the general case where the gas-kinetic plasma pressure NT is comparable with the pressure of the high-frequency field . It is assumed that the high-frequency field in the range of frequencies 1-10 MHz has a helical or multipole configuration and is excited by given external high-frequency currents passing through the current sheath surrounding the pinch.At high amplitudes of the high-frequency field, it is necessary to allow for the effect of the toroidicity of the system on the equilibrium conditions. In a first approximation with respect to the small parameters Δ/rp and rp/R (Δ is the displacement, rp is the radius of the pinch, R is the major radius of the torus) expressions are found for the high-frequency fields in a system with a displaced pinch and the condition for equilibrium of the pinch as a whole along the major radius is obtained. The equilibrium displacement of the pinch is determined from this condition, making use of the field values of the first approximation at the plasma boundary.It is shown that the use of high-frequency large-amplitude fields offers additional possibilities of controlling the equilibrium displacement. In particular, the effect of the toroidicity of the high-frequency field leads to the occurrence of an additional foree, which in a number of cases causes the pinch to be displaced to the inner wall of the toroidal chamber. When , it is also possible to have a paramagnetic effect when the pinch is forced out by a non-uniform static field towards the side where the intensity of the field is higher, i.e. to the inner wall of the torus. Under certain conditions the phenomena making positive or negative contributions to the equilibrium displacement of the pinch may be mutually compensating, so that the displacement can be made fairly small or even eliminated by suitable choice of the system parameters.The minimum displacement of the pinch for a given amplitude of the high-frequency current in the circuit practically always occurs in the case of quadrupole systems.It is shown that equilibrium confinement of plasma by high-frequency fields is also possible in systems where the plasma pinch is surrounded by a continuous conducting shell (so-called "high-frequency tokamak").The theoretical results obtained are in agreement with the results of experimental investigations.

Theory of Macroscopic Excitations of Magnons

The excitation of ferromagnetic magnons by a macroscopic time-varying magnetic field is analyzed quantum mechanically. The quantization of the spin excitations is made by the method of Holstein and Primakoff, generalized for the case of a general nonuniform static field. Both linear and nonlinear excitation mechanisms are considered, with the driving field being either perpendicular or parallel to the static field. Particular attention is given to the coherence properties of the magnon states generated.

Radiation of Hydromagnetic Waves

The dyadic Green's function for hydromagnetic waves in a uniform, fully ionized, perfectly conducting, pressureless fluid is obtained. The electromagnetic field consists of two modes, one that propagates isotropically and one that propagates along the field lines. The radiation rate from some simple model current distributions is derived from the Green's functions. If the source contains a current along the constant static field, then the radiation along the field lines through the source is very intense. This last result also holds in the nonuniform static field of an infinitely long straight wire.