焊缝结构中超声相控阵聚焦声场的数值模拟及分析*

针对焊缝内相控阵声场聚焦问题,建立焊缝结构中多高斯声束法的相控阵声场计算模型,分别用直接聚焦和底面反射聚焦两种方式对焊缝内部的相控阵声场进行数值模拟,分析不同方式下的相控阵聚焦声场特性,讨论不同区域适用的聚焦方式,分析了界面倾斜角度对聚焦效果的影响。结果表明,在相同焊缝结构条件下直接聚焦的方式更适合对厚壁焊缝的中下部进行聚焦扫描,在焊缝上表面附近,当声束偏转角大于80°时,直接聚焦方式无法有效聚焦;反射聚焦的方式更适合对焊缝上表面附近区域进行聚焦扫描,随着焦点深度增加,反射聚焦的声束偏转角增大,当声束偏转角大于55°时,反射聚焦无法形成明显焦点;两种聚焦扫描方式可形成有效的互补。当设置的焦点固定时,直接聚焦的实际焦点随界面倾斜角度增大向预设点远处偏移,而反射聚焦的效果基本不受倾斜角的影响,实际焦点始终在预设焦点附近。     

Optimum Conditions for Elliptical Focusing in Singly Resonant Optical Parametric Oscillators with Hemispherical Resonator

Optimum conditions for elliptical focusing are derived for singly resonant optical parametric oscillators with hemispherical resonator in dependence on double refraction. By elliptical focusing it is possible to focus more tightly in the non-walk-off plane. In a LiNbO₃ oscillator pumped with 1,06 μm radiation the threshold pump power can be lowered about a factor 6 by elliptical focusing in comparison with circular focusing.     

Focusing a compensated proton beam with a high compression coefficient

An analysis is made of the possibility of focusing 10–30 keV charge-compensated proton beams with a current of 1 kA and an area compression coefficient >1.5×10³. To obtain a high degree of compression a combination of ballistic focusing and magnetic compression is used. The parameters of the focusing channel are determined and an estimate is made of the possible effect of the compensating electron background on the effectiveness of focusing of the proton beam. Zh. Tekh. Fiz. 67, 73–80 (May 1997)     

Optimal ultrasonic array focusing in attenuative media

This paper presents a parametric study on the efficiency of ultrasound focusing in an attenuative medium, using phased arrays. Specifically, an analytical model of ultrasound wave focusing in a homogeneous, isotropic and attenuative fluid with point sources is presented. Calculations based on the model have shown that in an attenuative medium, an optimum frequency exists for the best focusing performance for a particular size of aperture and focal distance. The effect of different f numbers on the focusing performance in the attenuative medium is further investigated. The information obtained from the analytical model provides insights into the design and installation of a phased transducer array for energy efficient wave focusing.     

Theorie der optimalen Fokussierung für die direkte Erzeugung höherer optischer Harmonischer

Theory of Optimum Focusing for Direct Higher Optical Harmonics Generation Optimum focusing conditions for Gaussian beams for direct higher harmonics generation are derived neglecting the pump-wave depletion. The conditions for optimum focusing are obtained numerically and in the limits of weak and strong focusing analytically. The results show that at optimum conditions strong focusing is favourable. The power of the m-th harmonics for m > 3 is proportional to focusing parameter, L medium length, b confocal parameter of the pump wave). Numerical results are given for odd and even (up to the seventh) harmonics.     

Synchronization Properties of Random Piecewise Isometries

Defocusing mechanism provides a way to construct chaotic (hyperbolic) billiards with focusing components by separating all regular components of the boundary of a billiard table sufficiently far away from each focusing component. If all focusing components of the boundary of the billiard table are circular arcs, then the above separation requirement reduces to that all circles obtained by completion of focusing components are contained in the billiard table. In the present paper we demonstrate that a class of convex tables—asymmetric lemons, whose boundary consists of two circular arcs, generate hyperbolic billiards. This result is quite surprising because the focusing components of the asymmetric lemon table are extremely close to each other, and because these tables are perturbations of the first convex ergodic billiard constructed more than 40 years ago.     

低信噪比环境下的实时聚焦方法研究

低信噪比环境下,人们运用中值滤波与维纳滤波等作为图像预处理手段获得了一定的效果,但是图像预处理严重地影响了对焦速度。在低信噪比环境下所提出的避开图像预处理的最佳自动对焦方案为:首先,采用HSI分析法把RGB彩色图像转换为亮度信号;其次,利用能量梯度算子作为图像评价函数;最后,运用爬山算法控制策略进行自动对焦。结果表明,此方案的运算时间优势非常明显,对焦准确度最佳,可满足实时对焦系统的对焦速度与对焦准确度的要求。     

零电磁材料聚焦器

基于二维LC网络,设计了一种零电磁材料聚焦器,导出了该器件等效节点电压分布矩阵,研究了聚焦区域为三角形、四边形、六边形、八边形的电磁聚焦器,并在此基础上仿真了形状和尺寸对其聚焦特性的影响。结果表明：当聚焦区域小于两倍波长时,该器件的特性与聚焦区域的形状无关; 当聚焦区域逐渐增大时,焦点的能量也随之增强; 当聚焦区域大于两倍波长时,主焦点逐渐消失,同时将产生多个副焦点。     

Maximum “focusing” of surface phonons

“Focusing” of surface phonons is studied in detail. Three types of surface phonon “focusing” are found. The asymptotic dependence of the amplitude u of surface wave on the distance r from a point source is obtained for every “focusing” type. The type of maximum “focusing” where $\text{u((r)～ O(r}{}^{\mathrm{<mtext>?1</mtext><mtext>4</mtext>}}\text{)}$ holds is picked out. The “focusing” type is shown to be changed as elastic anisotropy is altered.     

Influence of focusing depth on the microfabrication of waveguides inside silica glass by femtosecond laser direct writing

We studied the influence of focusing depth on the index change threshold and damage threshold of silica glass irradiated by a focused 120 fs laser beam. Both thresholds increased with the focusing depth. The aspect ratio of the waveguide cross section can be selected by changing the focusing depth. A 5 mm long waveguide was written at the depth of 2100 μm, which was single mode at 632.8 nm and exhibited propagation loss of 0.56 dB/cm. The refractive index change was calculated to be ∼2.47×10^-3. The influence of the focusing depth should be considered in multi-layer devices as shown in the fabrication of a 3×3 waveguide array. PACS 42.62.-b; 42.82.Et; 81.05.Kf     

PMN-PT single crystal focusing transducer fabricated using a mechanical dimpling technique

A ∼5 MHz focusing PMN-PT single crystal ultrasound transducer has been fabricated utilizing a mechanical dimpling technique, where the dimpled crystal wafer was used as an active element of the focusing transducer. For the dimpled focusing transducer, the effective electromechanical coupling coefficient was enhanced significantly from 0.42 to 0.56. The dimpled transducer also yields a −6 dB bandwidth of 63.5% which is almost double the bandwidth of the plane transducer. An insertion loss of the dimpled transducer (−18.1 dB) is much lower than that of the plane transducer. Finite element simulation also reveals specific focused beam from concave crystal surface. These promising results show that the dimpling technique can be used to develop high-resolution focusing single crystal transducers.     

Using integrability to produce chaos: Billiards with positive entropy

A new open class of convex 2 dimensional planar billiards with positive Lyapunov exponent almost everywhere is constructed. We introduce the notion of a focusing arc and show that such arcs can be used to build billiard systems with positive Lyapunov exponents. We prove that under smallC ⁶ perturbations, focusing arcs remain focusing and thereby show that perturbations of the Bunimovich stadium billiard have positive Lyapunov exponents.Partially supported by NSF grant DMS 8806067     

激波聚焦反射的实验和数值研究

 采用高速摄影技术和数值模拟方法,对入射激波在两种不同形状的抛物形反射器表面聚焦和反射的过程进行了研究,得到了激波聚焦反射过程的波系结构的实验阴影照片和数值计算结果,两者符合得很好。对激波聚焦形成的气体动力学焦点的特性进行了分析,结果表明,入射激波在两种反射器反射后聚焦所形成的气体动力学焦点均是由三波点在轴心处的会聚所导致的,气体动力学焦点位于相应的反射器壁面的几何焦点附近。不同的反射器中,激波聚焦前后的波系结构也不同,对较浅的反射器,入射激波反射前在反射器壁面形成了弓形激波,反射之后需要相对较长的时间完成聚焦,形成聚焦反射激波之后弓形激波仍未相交;对较深的反射器,入射激波反射后在更短的时间内聚焦,聚焦时弓形激波已经相交,聚焦反射激波之后的流场波系结构更加复杂。     

Self‐focusing of a cosh‐Gaussian laser beam in magnetized plasma under relativistic‐ponderomotive regime

The combined effect of relativistic and ponderomotive nonlinearities on the self‐focusing of an intense cosh‐Gaussian laser beam (CGLB) in magnetized plasma have been investigated. Higher‐order paraxial‐ray approximation has been used to set up the self‐focusing equations, where higher‐order terms in the expansion of the dielectric function and the eikonal are taken into account. The effects of various lasers and plasma parameters viz. laser intensity (a₀), decentred parameter (b), and magnetic field (ω_c) on the self‐focusing of CGLB have been explored. The results are compared with the Gaussian profile of laser beams and relativistic nonlinearity. Self‐focusing can be enhanced by optimizing and selecting the appropriate laser‐plasma parameters. It is observed that the focusing of CGLB is fast in a nonparaxial region in comparison with that of a Gaussian laser beam and in a paraxial region in magnetized plasma. In addition, strong self‐focusing of CGLB is observed at higher values of a₀, b, and ω_c. Numerical results show that CGLB can produce ultrahigh laser irradiance over distances much greater than the Rayleigh length, which can be used for various applications.     

Peculiarities of filamentation of sharply focused ultrashort laser pulses in air

Peculiarities of the self-focusing and filamentation of high-power femtosecond laser pulses in air have been experimentally and theoretically studied under conditions of broad variation of the beam focusing parameter. The influence of the numerical aperture (NA) of the initial radiation focusing on the main characteristics of laser-induced plasma columns (characteristic transverse size, length, and concentration of free electrons) is considered. It is established that, for a rigid (NA > 0.05) initial laser beam focusing, the transverse size of the plasma channel ceases to decrease at a level of R _pl ≈ 2–4 μm as a result of strong refraction of radiation on the plasma formed at the focal waist, which prevents further contraction of the laser beam due to its focusing and self-focusing.     

Focusing of low-frequency sound fields in shallow water

A numerical experiment is carried out to study the focusing of a low-frequency (100–300 Hz) sound field in a shallow-water acoustic waveguide typical of an oceanic shelf. Focusing with the use of time reversal of broadband acoustic signals, which is called time reversal mirror (TRM) of waves, is considered along with focusing by phase conjugation (PC) of a monochromatic sound field. It is demonstrated that, in the case of focusing by the TRM method in the waveguide of interest, it is sufficient to have a single source-receiving element. The use of a vertical array improves the quality of focusing. The quality achieved in the latter case proves to be approximately the same as that achieved in the case of focusing by phase conjugation of a monochromatic field at a frequency identical to the carrier frequency of the broadband signals. It is also shown that, in a range-independent waveguide, intense surface waves considerably reduce the quality of focusing. This effect is most pronounced in the case of using phase conjugation.     

A stigmatic,second-order,double-focusing mass spectrometer

A double-focusing mass spectrometer of second order was constructed and the focusing property was examined. The ion optical system was determined by computer calculations in order to correct all second-order image aberrations. In the calculations the influences of the fringing field were taken into account. This ion optical system has velocity focusing, radial focusing and axial focusing properties.The instrument is composed of an electric toroidal sector (r_e = 212 mm, φ_e = 85.2°, c = 0.5, ρ′ = −106 mm), and a uniform magnetic sector of non-zero entrance and exit angles (r_m = 200 mm, φ_m = 90°, ϵ′ = 30°, ϵ″ = −10°). In experimental results, a maximum resolving power of 83 000 at 10% valley separation, and a total transmission of 43% were obtained. Axial focusing action and correction of the crescent shape resulting from second-order aberrations were observed from the shape of spectral lines taken on photographic plates.     

Phase‐space analysis and experimental results for secondary focusing at X‐ray beamlines

Micro‐focusing optical devices at synchrotron beamlines usually have a limited acceptance, but more flux can be intercepted if such optics are used to focus secondary sources created by the primary optics. Flux throughput can be maximized by placing the secondary focusing optics close to or exactly at the secondary source position. However, standard methods of beamline optics analysis, such as the lens equation or matching the mirror surface to an ellipse, work poorly when the source‐to‐optics distance is very short. In this paper the general characteristics of the focusing of beams with Gaussian profiles by a `thin lens' are analysed under the paraxial approximation in phase space, concluding that the focusing of a beam with a short source‐to‐optics distance is distinct from imaging the source; slope errors are successfully included in all the formulas so that they can be used to calculate beamline focusing with good accuracy. A method is also introduced to use the thin‐lens result to analyse the micro‐focusing produced by an elliptically bent trapezoid‐shaped Kirkpatrick–Baez mirror. The results of this analysis are in good agreement with ray‐tracing simulations and are confirmed by the experimental results of the secondary focusing at the 18‐ID Bio‐CAT beamline (at the APS). The result of secondary focusing carried out at 18‐ID using a single‐bounce capillary can also be explained using this phase‐space analysis. A discussion of the secondary focusing results is presented at the end of this paper.     

Focusing of a Flat Left-Handed Metamaterial Lens in a Heterogeneous and Lossy Medium

For applications such as near-field target detection and tumor hyperthermia with a fiat left-handed metamaterial （LHM） lens, a microwave will be focused in the heterogeneous and lossy medium. Different from the focusing of a fiat LHM lens in vacuum as reported in most previous studies, the medium loss and heterogeneity will affect the focusing performance of the LHM lens. Numerical simulations indicate that the medium loss will degrade the focusing resolution, while the heterogeneity of random variability within ±30% will affect the focusing resolution to a limited extent. Both the loss and heterogeneity of the medium will shift the focal point away from the image plane. When focusing in a medium with different permittivity values, an LHM lens will also have different focusing resolutions due to different electric thicknesses.