Controlled Space Radiation concept for mesh-free semi-analytical technique to model wave fields in complex geometries

Numerical modelling of the ultrasonic wave propagation is important for Structural Heath Monitoring and System Prognosis problems. In order to develop intelligent and adaptive structures with embedded damage detector and classifier mechanisms, detailed understanding of scattered wave fields due to anomaly in the structure is inevitably required. A detailed understanding of the problem demands a good modelling of the wave propagation in the problem geometry in virtual form. Therefore, efficient analytical, semi-analytical or numerical modelling techniques are required. In recent years a semi-analytical mesh-free technique called Distributed Point Source Method (DPSM) is being used for modelling various ultrasonic, electrostatic and electromagnetic wave field problems. In the conventional DPSM approach point sources are placed along the transducer faces, problem boundaries and interfaces to model incident and scattered fields. Every point source emits energy in all directions uniformly. Source strengths of these 360° radiation sources are obtained by satisfying interface and boundary conditions of the problem. In conventional DPSM modelling approach it is assumed that the shadow zone does not require any special consideration. 360° Radiation point sources should be capable of properly modelling shadow zones because all boundary and interface conditions are satisfied. In this paper it is investigated how good this assumption is by introducing the ‘shadow zone’ concept at the point source level and comparing the results generated by the conventional DPSM and by this modified approach where the conventional 360° radiation point sources are replaced by the Controlled Space Radiation (CSR) sources.     

磁光平面波导的单向传播特性

表面磁等离子体(surfacemagnetoplasmons,SMPs)是一种在电介质和偏置磁场作用下磁光材料界面处传播的近场电磁波.其独特的非互易传播特性引起了大量科研工作的关注,但在具体的波导结构设计上仍存在很多问题.本文研究了一种银-硅-磁光材料的3层平面波导结构,SMPs在磁光材料和硅的界面处传播,发现在特定的频率范围内,SMPs的基模及高阶模式均具有正向或反向的单向传播特性.分别计算了旋磁与旋电材料平面波导的色散方程,研究了硅层厚度与外加磁场对能带结构及SMPs单向传播区域的影响,发现无论是旋磁或旋电材料的结构,硅层厚度的增加使高阶模式使高阶模式出现在更低的频率位置,使单向传输带宽变小甚至消失,外加磁场的变大使磁光材料的能带结构频率增大的同时带隙中也引入了高阶模式.计算了2种磁光材料平面波导的正向和反向的单向传播带宽宽度,发现旋磁材料YIG的单向SMPs模式出现在GHz波段,最大单向带宽可达到2.45 GHz;旋电材料InSb的单向SMPs模式出现在THz波段,最大单向带宽达到3.9 THz.     

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. Zh. Tekh. Fiz. 69, 70–77 (February 1999)     

Plane Waves Propagating in Chiral Plasma and Chiral Ferrite Media

Plane wave propagation in chiral plasma and chiral ferrite media is studied in kDB coordinate system. General wave equations and characteristic equations of plane waves propagating along an arbitrary direction in chiral plasma and in chiral ferrites are derived in simple formulations respectively. Four wavenumbers and their corresponding dispersion characteristics are resulted for propagation both along and normal to the biasing magnetic field. When plane wave with negative helicity propagates along the biasing magnetic field in chiral ferrites, backward waves emerge. However backward waves occur with both positive and negative helicities when propagating along the biasing magnetic field in chiral plasma.     

Gaussian-DPSM (G-DPSM) and Element Source Method (ESM) modifications to DPSM for ultrasonic field modeling

In the last few years, Distributed Point Source Method (DPSM) a mesh-free semi-analytical technique has been developed. In spite of its many advantages, one shortcoming of the conventional DPSM method is that the field obtained by conventional DPSM method needs to be scaled to match the theoretical solutions. Two modification techniques called Gaussian-DPSM (G-DPSM) and Element Source Method (ESM) are developed here to avoid the scaling need. G-DPSM technique introduces additional fictitious point sources around every parent point source. Gaussian weight functions determine the strength of these additional fictitious point sources that are denoted as child point sources. ESM replaces discrete point sources used in the conventional DPSM by continuous sources. In the ESM formulation individual point sources are denoted as nodes. Special elements are formed on the boundary by connecting these nodes. The source strength inside the element can vary linearly or non-linearly depending on the order of the interpolation function used inside the element. Results generated by both these methods are compared with the conventional DPSM solution and analytical solution. It is shown that the ultrasonic field in front of the transducer computed by G-DPSM and ESM matches very well with the theory without using any scaling factor.     

Bragg Deflectors of Wave Fluxes for High-Power Relativistic Masers

Technical Physics - Quasi-optical Bragg structures that represent sections of planar waveguides with tilted (with respect to the beam propagation direction) corrugation are studied. It is shown...     

NETWORK CHARACTERIZATION OF STEP DISCONTINUITY WITH GENERALIZED CONSERVATION OF COMPLEX POWER TECHNIQUE AND ITS APPLICATION TO ANALYZING PERIODIC DIELECTRIC WAVEGUIDES

The propagation of electromagnetic waves along open periodic, dielectric waveguides is formulated in the case that surface wave is guided and propagates normally to the corrugation. Our approximate analysis with the propagation characteristics is to consider a corresponding bounded waveguide problem in which perfect electric or magnetic walls are introduced, and the periodic corrugation is regarded as consisting of step discontinuities connected by a length of uniform slab waveguide. By properly taking into account of both surface modes and only a few non-surface-modes, a novel network approach is proposed for characterizing step discontinuity based on the generalized conservation of complex power technique (GCCPT). Employing solution selection rule (SSR), we can readily derive propagation characteristics in the Bragg interaction region. A number of numerical results are shown to demonstrate the usefulness of our approach.     

DPSM technique for ultrasonic field modelling near fluid-solid interface

Distributed point source method (DPSM) is gradually gaining popularity in the field of non-destructive evaluation (NDE). DPSM is a semi-analytical technique that can be used to calculate the ultrasonic fields produced by transducers of finite dimension placed in homogeneous or non-homogeneous media. This technique has been already used to model ultrasonic fields in homogeneous and multi-layered fluid structures. In this paper the method is extended to model the ultrasonic fields generated in both fluid and solid media near a fluid-solid interface when the transducer is placed in the fluid half-space near the interface. Most results in this paper are generated by the newly developed DPSM technique that requires matrix inversion. This technique is identified as the matrix inversion based DPSM technique. Some of these results are compared with the results produced by the Rayleigh-Sommerfield integral based DPSM technique. Theory behind both matrix inversion based and Rayleigh-Sommerfield integral based DPSM techniques is presented in this paper. The matrix inversion based DPSM technique is found to be very efficient for computing the ultrasonic field in non-homogeneous materials. One objective of this study is to model ultrasonic fields in both solids and fluids generated by the leaky Rayleigh wave when finite size transducers are inclined at Rayleigh critical angles. This phenomenon has been correctly modelled by the technique. It should be mentioned here that techniques based on paraxial assumptions fail to model the critical reflection phenomenon. Other advantages of the DPSM technique compared to the currently available techniques for transducer radiation modelling are discussed in the paper under Introduction.     

Ultrahigh-intensity optical slow-wave structure

We report the development of corrugated "slow-wave" plasma guiding structures with application to quasiphase-matched direct laser acceleration of charged particles and generation of a wide spectrum of electromagnetic radiation. These structures support guided propagation at intensities up to 2 x 10(17) W/cm(2), limited by our current laser energy and side leakage. Hydrogen and argon plasma waveguides up to 1.5 cm in length with corrugation period as short as 35 microm are generated in a cryogenic cluster jet. Experimental data are consistent with simulations showing periodic modulations of the laser pulse intensity.     

Spectral and perturbation analysis for ultrasonic guided waves

The influence of damage on waves propagating in complex geometry waveguides is investigated through a numerical model formulated by combining the Spectral Finite Element Method and Perturbation Techniques. The resulting numerical tool allows efficient computation of the wave propagation response and the analysis of the effects of damages of various extent and location. The dynamic behavior of the damaged waveguides is described through a general higher order model which couples different waves thus allowing the prediction of mode conversion phenomena. Arbitrary cross-section can be considered through Finite Element (FE) discretization according to well-established Semi-Analytical Finite Element (SAFE) procedures. Two types of damages which allow the application of perturbation theory are considered: a small localized reduction of the thickness and a reduction of material stiffness and density. A validation by comparison with a Finite Element Model as well as numerical examples are presented to illustrate the model capabilities.     

间接馈电柱-锥级联爆磁压缩发生器数值模拟

为了研究间接馈电、两级级联柱-锥构型的爆磁压缩发生器的物理特性，发展了描述爆磁压缩物理过程的2维爆轰磁流体力学程序——MFCG（Ⅴ），并用其分析、计算了一系列模型。通过将计算结果与柱-柱构型的结果进行比较，表明了柱-锥构型更有利于功率放大:在同样的输出电流条件下，不仅爆磁压缩的时间缩短，而且输出功率也相应地增加。MFCG（Ⅴ）程序的建立以及相应的数值模拟结果有助于加深对柱-锥构型的爆磁压缩发生器物理过程的认识，并为实验设计提供参考。     

Finite element analysis of guided waves in fluid-filled corrugated pipes

Free wave propagation in fluid-filled corrugated pipes is analyzed using finite element methods in combination with a wave-based approach. By combining discretized models with a wave-based approach, complex mechanism of wave motion in the three-dimensional waveguide is fully included. The pipes are treated as waveguides having periodic properties in the direction of wave propagation. The analysis of these guided waves leads to dispersion curves which show the strong frequency-dependency of the different wave modes. The method also allows the inclusion of coupling between fluid-borne and structure-borne wave modes which occur at the acoustic-structure interface. Phase and group velocities of the wave modes are derived in postprocessing steps. Additionally, the energy ratio of the fluid-domain and solid-domain vibrational energies is computed. Finally, linear damping models are included in order to explore wave mode attenuation.     

The influence of magnetic field swept rate on the ultrasonic propagation velocity of magnetorheological fluids

Structure of magnetorheological (MR) fluids depends on the strength of the magnetic field applied and on the mode of its application. The ultrasonic wave propagation velocity changes under the effect of an external magnetic field as a result of formation of clusters arranged along the direction of the field in the MR fluids. Therefore, we propose a qualitative analysis of these clustering structures by measuring properties of ultrasonic propagation. Since the MR fluids are opaque, the non-contact inspection using this ultrasonic technique can be very useful. In this study, we measured ultrasonic propagation velocity in MR fluid influenced by an external magnetic field for different swept rate precisely. With increasing magnetic field intensity, the changes of the ultrasonic wave velocity are more pronounced. Sedimentation effect takes place in certain time for different swept rate due to magnetic particle size and it follows linear relationship in log scale. Significant differences of the ultrasonic wave velocity are established between the case when the field is swept at a constant rate and the case when it is stepped up.     

Method of Designing a Corrugated Optical Waveguide Filter

The method of designing a corrugated optical waveguide filter is proposed to yield an arbitrary spectral profile of the reflection coefficient. It gives the functional dependence of the corrugation depth on distance along the wave propagation direction. The basis of the theory of the method is a combination of the effective index approach and the Fourier transformation technique. It represents the spectral profile of the reflection coefficient approximately to Fourier transformation of the spatial profile of the effective refractive index. Examples are presented to illustrate how to apply the technique.     

固/固型二维正方晶格声子晶体缺陷态研究*

固/固型声子晶体在抑制噪音和隔离振动等工程领域有着潜在的应用。利用有限元方法对存在缺陷的二维正方晶格金/环氧树脂声子晶体进行了研究,数值计算结果表明弹性波在点缺陷处局域,带隙中出现多条缺陷模,点缺陷数目的增加对声子晶体局域特性产生显著影响;弹性波沿着线缺陷传播形成波导,改变线缺陷结构可以改变弹性波传播方向和实现信号的分离。对声子晶体缺陷特性的研究可为声学滤波器和波导等器件的设计提供参考。     

An Extension of the Effective Index Method to Analyze Leakage Losses in Rib Type Waveguides

An extension of the effective index method is proposed as a tool to investigate leakage losses in two-dimensional waveguides. The above method, referred to as the Extended Effective Index Method (EEIM), utilizes the Transfer Matrix Technique (TMT) for the calculation of the complex propagation constants. The results show that the EEIM can be applied to conventional rib leaky waveguides as well as to rib ARROW leaky waveguides. The results for rib ARROW waveguides show excellent agreement when compared to those obtained with Finite Element Method.     

Guidance of Surface Waves with Dielectric Waveguides Having Finite or Infinite Periodic Corrugations

A planar open dielectric waveguide with periodic rectangular corrugations is investigated in the case that surface wave is guided and propagates normally to the corrugation. Our approximate analysis with the propagation characteristics is to consider a corresponding bounded waveguide problem in which perfect electric or magnetic walls are introduced, and the periodic corrugation is regarded as consisting of step discontinuities connected by a length of uniform slab waveguide. By properly taking into account of both surface modes and only a few non-surface-modes, and using conservation of complex power technique (CCPT) as well as solution selection rule (SSR), we can readily derive propagation characteristics in the Bragg interaction region. The calculated results show an excellent agreement with previously published ones.     

Stimulated Cherenkov radiation of a relativistic electron beam moving over a periodically corrugated surface (quasi-optical theory)

The propagation of waves over periodically corrugated surfaces and their excitation by relativistic electron beams are investigated within the framework of a quasi-optical approach. The dispersion equation is derived for normal waves under the assumption of a small (in the scale of the period and wavelength) corrugation depth, based on which two limiting cases are identified. In the first limiting case, the wave frequency is far from the Bragg resonance, and the propagation of waves can be described in terms of the impedance approximation, in which the fundamental spatial harmonic slows down. In the second limiting case realized at frequencies close to the Bragg resonance, the field is represented as two counterpropagating quasi-optical wave beams coupled on a corrugated surface and forming a normal surface wave. When interacting with an electron beam, convective instability, which can be used to realize amplifier regimes, corresponds to the first case, and absolute one, which is applied in surface-wave oscillators, corresponds to the second case. The developed theory is used to determine basic characteristics of amplifier and oscillator schemes: the growth rates, the energy exchange efficiency, and the formation of a self-consistent spatial structure of the radiated field. The practical realization of relativistic submillimeter amplifiers and surface-wave oscillators is shown to hold promise.     

Dispersion Characteristics of Plasma Mode in Corrugated Plasma Waveguide

The dispersion equation of electromagnetic wave is derived for corrugated waveguide filled with plasma immersed in magnetic field under assumption of small corrugation, the dispersion characteristics of low frequency plasma mode are calculated and analyzed, the dense spectrum of plasma mode is demonstrated.