交直流载流矩形线圈相互作用演示仪

设计制作了交直流截流矩形线圈相互作用演示仪,采用多匝矩形线圈替代单一导线,演示2根载流导线之间通过磁场存在着相互作用,实验仪器制作简单,使用方便,性能可靠,现象直观.     

Perpendicular susceptibility of completely shielded elliptical and rectangular superconducting films

The perpendicular susceptibility -χ₀$-{\chi }_0$ of completely shielded elliptical and rectangular superconducting films with different aspect ratios has been calculated accurately. The obtained χ₀${\chi }_0$ may be compared with the measured low-field limit of ac susceptibility to check the quality of superconducting films, and used as a scaling parameter to obtain field amplitude dependent complex critical-state ac susceptibility.     

Some extensions of the Gaussian beam expansion: radiation fields of the rectangular and the elliptical transducer

A straightforward extension of Gaussian beam expansion is presented for calculation of the Fresnel field integral [J. J. Wen and M. A. Breazeale, J. Acoust. Soc. Am. 83, 1752-1756 (1988)]. The source distribution function is expanded into the superposition of a series of two-dimensional Gaussian functions. The corresponding radiation field is expressed as the superposition of these two-dimensional Gaussian beams and is then reduced to the computation of these simple functions. This treatment overcomes the limit that the shape of source is of circular axial-symmetry. The numerical examples are presented for the field of the (uniform) elliptical and the rectangular piston transducers and agree well with the results given by complicated computation.     

Eigenfrequency of a Helmholtz resonator at the wall of a rectangular duct

The eigenfrequency of a Helmholtz resonator fixed at the wall of a rectangular duct is estimated. Special attention is given to determining the attached length of the resonator neck as viewed from the duct. The dependences of the attached neck length and the eigenfrequency on the duct configuration are analyzed. Theoretical results are compared with the data of numerical calculations performed by the finite-element method.     

矩形波导近似计算椭圆应力包层单模光纤的双折射

提出矩形波导近似,并分别采用Marcatili法和有效折射率法进行计算,得出了数值结果,并将其与大块光学材料近似、修正耦合波理论和微扰法以及精确求解特征方程的结果进行比较.     

Comment on "Exact solution of resonant modes in a rectangular resonator"

We comment on the recent Letter by J. Wu and A. Liu [Opt. Lett. 31, 1720 (2006)] in which an exact scalar solution to the resonant modes and the resonant frequencies in a two-dimensional rectangular microcavity were presented. The analysis is incorrect because (a) the field solutions were imposed to satisfy simultaneously both Dirichlet and Neumann boundary conditions at the four sides of the rectangle, leading to an overdetermined problem, and (b) the modes in the cavity were expanded using an incorrect series ansatz, leading to an expression for the mode fields that does not satisfy the Helmholtz equation.     

Inertial focusing and rotating characteristics of elliptical and rectangular particle pairs in channel flow

The lattice Boltzmann method is used to study the inertial focusing and rotating characteristics of two-dimensional elliptical particles and rectangular particles in channel flow. The results show that both elliptical particles and rectangular particles initially located on one side and two sides of channel centerline migrate first towards the equilibrium position. Then, the single-line particle train with an increasing spacing and the staggered particle train with stable spacing are formed. The axial spacing of the staggered particle pair increases with aspect ratio and Reynolds number increasing. The staggered elliptical or rectangular particle pairs form perpendicular orientation angles, which will be more obvious at larger aspect ratio and lower Reynolds number. The single-line particle trains with different shapes seldom form the perpendicular orientation angle.     

High-sensitivity refractive index sensors based on Fano resonance in a metal-insulator-metal based arc-shaped resonator coupled with a rectangular stub

A metal-insulator-metal (MIM)-based arc-shaped resonator coupled with a rectangular stub (MARS) structure is proposed. This structure can generate two tunable Fano resonances originating from two different mechanisms. The structure has the advantage of being sensitive to the refractive index, and this feature makes it favorable for application in various microsensors. The relationship between the structural parameters and Fano resonance is researched using the finite element method (FEM) based on the software COMSOL Multiphysics 5.4. The simulation reveals that the sensitivity reaches 1900 nm/refractive index unit (RIU), and the figure of merit (FOM) is 23.75.     

Quality enhancement and insertion loss reduction of a rectangular resonator by employing an ultra-wide band gap diamond phononic crystal

We report on an ‘in plane mode’ band gaps investigation of a novel diamond CRS (circle-rectangle-square) shaped holey phononic crystals in the desired operating frequency ranges. An ultra-wide band gap for diamond in the ‘in plane mode’ is observed. We also investigate an ultra-wide acoustic band gap for a finite one-dimensional (1D) diamond CRS phononic crystal (PnC) in the ‘out of plane mode’, and an ultra-wide acoustic band gap of a finite two-dimensional (2D) diamond CRS phononic crystal (PnC) in the ‘out of plane mode’ based on the FEA (Finite Element Analysis) method. We analyze that the transmission response of diamond in the length extension and width extension manner is more reasonable. Ultra-wide peak attenuations in the transmission spectra of a CRS shaped diamond phononic crystal successfully reveal the complete band gaps. The wide band gap of a CRS shaped diamond phononic crystal and the wide peak attenuation strongly agree in the same frequency region. It is analyzed that when a CRS-diamond phononic crystal is employed for MEMS resonators with different tether widths the quality Q of the resonators improved, and the energy losses decrease with extremely low insertion loss. In addition, it is observed that the vibrational displacement of a resonator is reduced by employing a diamond phononic crystal.     

Computer-assisted design of surface coils used in magnetic resonance imaging. II. Rotational discrimination nonlinear regression analysis and the design of surface coils

For a number of reasons, it is desirable to fabricate coils which, for a known current, shall produce predetermined values of the magnetic field intensity at a number of points within a nuclear magnetic resonance imager.

The calculation of the magnetic field intensity at a set of points involves the integration of the Biot-Savart equation for all components of the segments of conductor which make up the coil. This process in itself is a rather formidable task. When this process is parameterized in terms of coil diameter, coil spacing, etc. the problem is to determine the values of these parameters to match values of magnetic field intensities which are desired. The problem thereby increases in complexity to the point where, by ordinary methods, the problem becomes intractable.

A generalized solution technique has been developed on a digital computer to implement the rotational discrimination nonlinear regression techniques of Faris, Law and Letcher to find the best solution to this problem. The problem is posed by integrating the Biot-Savart equation. This produces algebraic expressions for incorporation into the optimization program which is executed on a computer in a conversational mode.

This technique was employed to specify the dimensions of a rectangular surface coil for the investigation of the whole human spine.     

Dual superconductivity. Variations on a theme

Its is pointed out that the low-energy effective theory that describes the low-lying glueballs of the pure Yang-Mills theory sustains static classical stringlike solutions. We suggest that these objects can be identified with the QCD flux tubes and their energy per unit length with the string tension.     

Acoustic flow in the resonator throat: Experiment and computational modeling

This paper is devoted to the study of acoustic flow in the Helmholtz resonator throat. The presented results of a complex investigation on the basis of data of a physical experiment and a mathematical modeling, their comparative analysis, and comparison with theoretical estimations provide a rather complete image of the acoustic flow formation and, in particular, enable one to obtain qualitative and quantitative dependences of the averaged velocity component in the resonator throat at different powers of an exciting sound wave.     

Active shielding of cylindrical saddle-shaped coils: application to wire-wound RF coils for very low field NMR and MRI

We provide an exact expression for the magnetic field produced by cylindrical saddle-shaped coils and their ideal shield currents in the low-frequency limit. The stream function associated with the shield surface current is also determined. The results of the analysis are useful for the design of actively shielded radio-frequency (RF) coils. Examples pertinent to very low field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are presented and discussed.     

Unstable resonator alignment based on the fringe analysis

The existing cavity mirrors alignment method aligns cavities according to the symmetry of the near-field interference fringes of the guided lights by the unstable resonator. Its precision is restricted due to the lack of objective criterion for the fringe symmetry. This article proposes a high-precision cavity mirrors alignment method using the grey projection of fringe patterns as a solution to the problem. First, vertical and horizontal grey projection on the concerned fringe patterns; then an evaluation function for the cavity mirrors alignment is established using the distances between peak/valley values of all the levels in the projection curve as variables; finally, the function value is employed to characterize the fringe symmetry and thus to provide an objective criterion for the alignment effect of the stable resonator cavity mirrors. Experiments show that the proposed method is able to give effective evaluation for the symmetry of the near-field interference fringes of the guided lights and improve the alignment precision greatly.     

Variations on the Kepler problem

The elliptical orbits resulting from Newtonian gravitation are generated with a multifaceted symmetry, mainly resulting from their conservation of both angular momentum and a vector fixing their orientation in space—the Laplace or Runge-Lenz vector. From the ancient formalisms of celestial mechanics, I show a rather counterintuitive behavior of the classical hydrogen atom, whose orbits respond in a direction perpendicular to a weak externally-applied electric field. I then show how the same results can be obtained more easily and directly from the intrinsic symmetry of the Kepler problem. If the atom is subjected to an oscillating electric field, it enjoys symmetry in the time domain as well, which is manifest by quasi-energy states defined only modulo ħω. Using the Runge-Lenz vector in place of the radius vector leads to an exactly-solvable model Hamiltonian for an atom in an oscillating electric field—embodying one of the few meaningful exact solutions in quantum mechanics, and a member of an even more exclusive set of exact solutions having a time-dependent Hamiltonian. I further show that, as long as the atom suffers no change in principal quantum number, incident radiation will produce harmonic radiation with polarization perpendicular to the incident radiation. This unusual polarization results from the perpendicular response of the wavefunction, and is distinguished from most usual harmonic radiation resulting from a scalar nonlinear susceptibility. Finally, I speculate on how this radiation might be observed.     

Variations on the deuteron theme

Several problems concerning the deuteron and having simple analytic solutions are considered. The relation between the electric quadrupole moment of the deuteron and the np scattering amplitude is established. The degree of the circular polarization of the photon emitted in the radiative capture of longitudinally polarized thermal neutrons is found. The anapole, electric dipole, and magnetic quadrupole moments of the deuteron are calculated.