Annular structures formed in a beam of ions during their collective acceleration in a system with dielectric anode

We have studied the collective acceleration of protons and deuterons in an electron beam emitted from plasma formed at the surface of a dielectric anode insert. The experiments were performed with a pulsed electron accelerator operating at an accelerating voltage up to 1 MV, current amplitude up to 40 kA, and pulse duration of 50 ns. Reduction of the accelerating voltage pulse front width and optimization of the diode unit and drift region ensured the formation of several annular structures in the electron beam. As a result, up to 50% of the radioactivity induced in a copper target was concentrated in a ring with 4.5-cm diameter and 0.2-cm width. The formation of high energy density in these circular traces and the appearance of an axial component of the self-generated magnetic field of the electron beam are related with the increasing efficiency of acceleration of the most intense group of ions.     

双金属复合管固-液铸轧复合工艺环形布流器设计及其流场模拟

为解决双金属复合管固-液铸轧复合（SLCRB）工艺覆层熔体金属的环形均匀稳定布流问题,提出并设计了一种带有三级环形阶梯分流和锥形缓冲的环形布流器,并且基于商用软件Fluent建立了稳态布流过程的流体动力学模型,分析了布流器结构和铸轧速度对系统流场的影响,给出了获得均匀、稳定出口流场的布流器内部结构和布流条件,并用水模实验进行验证。结果表明:布流器内通道高宽比不小于1有利于均匀分流,并且锥形缓冲区的均流缓冲作用至关重要。以1.2 m/min的铸轧速度成功制备了外径为38 mm、内径为26 mm的铅-铝双金属复合管,覆层金属厚度均匀、界面结合效果良好。      

Computation of resonant frequency of annular microstrip antenna loaded with multiple shorting posts

Shorting post loaded annular microstrip antenna is of considerable interest. Recent works have presented formulations for annular microstrip patch with a shorting post located at the centre of a circular disc, thereby converting the structure to an annular ring with a centre shot. A theoretical formulation for multiple post loading (up to ten posts) of an annular patch is presented. The posts are located away from the centre of the patch and are thin in diameter with respect to the diameter of the ring. The formulation accurately predicts resonant frequency. For an accurate formulation, the shorting posts have been considered as inductive impedances at the frequency of interest. It is shown that for a short loaded ring, TM₀₁ is the dominant mode. The simple tool presented can be suitably modified to incorporate switching diodes or varactor diodes.     

环形孔径高斯光束的远场特性

讨论环形孔径高斯光束的远场光斑能量分布，并给出拟合表达式。首先推导光束没有抖动时远场光斑的光能分布表达式，然后研究各种不同中心遮拦和不同光束截断比，在有以及没有激光束抖动的情况下，对高斯光束的远场环围能量分布变化的影响。     

Ultrasonic imaging using a computed point spread function

An explicit point spread function (PSF) evaluator in the frequency domain is described for an ultrasonic transducer operating in the pulse-echo mode. The PSF evaluator employs the patch element model for transducer field determination and scattered field assessment from a small but finite "point" reflector. The PSF for a planar transducer in a medium has been evaluated in the near and the far field. The computed PSFs were used to deconvolve and restore surface images, obtained experimentally, of a single hole and a five-hole cluster in an Al calibration block. A calibration plot is arrived at for estimating, without the need for deconvolution, the actual diameters of circular reflectors from apparent diameters obtained experimentally for a single-medium imaging configuration. The PSF, when the transducer and the point reflector are in two media separated by a planar interface, was evaluated in the near and far field. The computed PSFs were used to deconvolve and restore subsurface images, obtained experimentally, of flat bottom holes (FBHs) in an Al calibration block. We show that the PSF, in the presence of a planar interface, can be obtained from a single-medium PSF model using an effective single-medium path length concept. The PSFs and modulation transfer functions (MTFs) are evaluated for spherical focused and annular transducers and compared with those for the planar transducer. We identify imaging distances to get better-resolved images when using planar, spherical focused, and annular transducers.     

磁场中旋转圆环板的气磁弹性动力稳定性

研究磁场作用下导电旋转圆形环板受气动载荷的磁弹性行波动力学特性问题。应用哈密顿原理推导出磁场作用下旋转运动圆板的磁弹性振动控制方程,给出电磁力和气动载荷的表达式。根据边界条件设定行波特性振型函数,应用伽辽金积分推得行波动力学特征方程。通过算例,得到磁场中旋转运动圆环板在不同空气动力载荷作用下的前、后行波振动频率变化曲线和不同阶模态下的临界转速值,分析了气磁弹性条件对不同阶模态振动频率和阻尼的影响,分别讨论了基于振动频率和阻尼的临界转速变化规律及动力失稳特性。结果表明,磁场、空气动力载荷、转速等参数对旋转圆板的行波振动有显著影响。     

Ultrasonic nondiffracting transducer for medical imaging

The nondiffracting J(0) Bessel beam is evaluated, and its application to medical imaging is suggested. Computer simulations and experimental results for a ten-ring annular Bessel shaded transducer are described. Both continuous-wave (CW) and pulse-wave (PW) excitations are shown and compared to conventional Gaussian beams. The nondiffracting beam has about 1.27-nm radius main lobe with a 20-cm depth of field compared to the Gaussian transducer of the same size with a 1.27-mm radius main lobe at a focus of 12 cm and 2x4-cm depth of field. The side lobes of the nondiffracting beam are the same as the J(0) Bessel function. The effects of heterogeneity due to tissue on the nondiffracting beam and on the focused Gaussian beam are also reported.     

Reduction of cavitation using pseudorandom signals [therapeutic US]

It is known that the scattering of ultrasound by cavitation bubbles reduces the efficiency of treatment by high-intensity focused ultrasound. In the authors' experiments striving to reduce grating lobe levels of annular arrays they observed less microbubble formation at the focus of the transducer when pseudorandom phase-modulated CW signals were used rather than single-frequency CW signals. To confirm this unexpected result, the authors performed an experiment in a solution of luminol which is known to be a cavitation detector. A 5-cm diameter spherical transducer (1.1 MHz center frequency and 0.6 MHz bandwidth), focused at 197 mm was used. The ratio of the sonoluminescence intensity produced by a single-frequency signal to that produced by a pseudorandom phase-modulated signal increased with the intensity of the applied held and attained a value of nearly 50 for an intensity of 4.6 W cm^-2      

Results of field trials using the NPL simulated reactor neutron field facility

The NPL simulated reactor neutron field facility provides neutron spectra similar to those found in the environs of UK gas-cooled reactors. Neutrons are generated by irradiating a thick lithium-alloy target with monoenergetic protons between 2.5 and 3.5 MeV (depending on the desired spectrum), and then moderated by a 40-cm diameter sphere of heavy water. This represents an extremely soft workplace field, with a mean neutron energy of 25 keV and, more significantly, a mean fluence to ambient dose equivalent conversion coefficient of the order of 20 pSv cm(2), approximately 20 times lower than those of the ISO standard calibration sources (252)Cf and (241)Am-Be. Results of field trials are presented, including readings from neutron spectrometers, personal dosimeters (active and passive) and neutron area survey meters, and issues with beam monitoring are discussed.     

新型单模光纤模场直径测试系统

介绍了一种新型的单模光纤模场直径测试系统，它采用国际推荐的基准测试方法－远场扫描法，具有测量精度高、速度快、抗干扰能力强和操作简单的特点，既可用于实验室中，也适合在工厂环境下使用，能够替代同类进口产品。     

Mode-coupling analysis in axisymmetric birefringent waveguides

Axisymmetric periodic media such as annular layers have axi-symmetric birefringence. I report the theoretical analysis of mode coupling in axisymmetric birefringent waveguides. Any of the LP(11) modes in a perturbed system can couple with a corresponding LP(11) mode that is orthogonal to the original mode with respect to both the polarization direction and the node line of an electric field. The coupling beat length depends on the amount of birefringence, which can be as short as 5 mm for a practical structure.     

连续变孔径法单模光纤模场直径（MFD）测量的数据处理

分析了连续变孔径法单模光纤秆径测量的数据处理方法,误差源及其对测量结果的影响。计算结果表明,用分段多项式最小二乘法拟合计算ＭＦＤ,可以取得满意的计算精度。文中给出了测量系统总的误差估计。     

Inkjet printing using copper nanoparticles synthesized by electrolysis

A modified electrolysis method was employed to synthesize copper nanoparticles for mass production. Processing parameters including electrolyte temperature, pH and reduction agents were controlled in order to synthesize the particles continuously and directly from copper plates. As a result, well-dispersed copper nanoparticles with an average diameter of 15 nm were obtained and the oxidation of these particles was prevented by controlling the parameters. Using an amphiphilic surfactant, a water-based copper nano-colloid was prepared and printed onto a polyimide substrate using a conventional ink-jet printer. Copper patterns sintered at 230 °C showed a resistivity of 5.4 μΩ-cm (bulk resistivity; 1.67 μΩ-cm).     

The effect of electric field on the formation and fragmentation of condensate film on the walls of a capillary in a flow of steam

An experimental investigation is performed of the effect of electric field on the parameters of a condensate film forming on the walls of a capillary in which hot steam flows. The increase in the field intensity in the vicinity of the capillary exit section causes a variation of the two-phase mode of flow in the capillary from annular to slug; the shape of the film surface varies from smooth to wavy; and the formation of large droplets at the capillary exit section changes to pulsating fragmentation of the film. With continuing increase in the intensity, the slug flow changes back to annular, water forms steady-state projections at the capillary exit section, the fragmentation of the film becomes monotonic, and an unsteady corona discharge arises on the projections. The variation of the capillary orientation from horizontal to vertical results in an increase in the intensity required for the development of the slug mode of flow. A mechanism of monotonic fragmentation of film is suggested, within which the characteristics of resultant fragments are assessed.     

环形波导微波场分布的计算

提出了环形波导场分布的计算模型 ,计算了环形波导内的微波场分布。结果表明即使在环形腔直径大于微波波长时 ,微波能量也能传到真空室中心附近 ,这为实现大面积均匀等离子体提供了理论依据     

弹性圆柱薄壳在耦合场作用下的混沌运动分析

研究了机械场与电磁场耦合作用下两端简支弹性圆柱壳的混沌运动问题。给出了纵向稳恒磁场、环向电流和均布载荷共同作用下弹性圆柱壳的振动方程,并采用四阶Runge-Kutta法求解方程。通过变化环向电流使系统运动状态改变,分析了电磁参量对系统运动的影响,研究结果表明变化电磁场强度和通入环向电流的大小及方向,可以实施在机械载荷作用下的简支弹性圆柱薄壳的混沌与周期运动间的转化。     

Diameter estimation of cylinders by the rigorous diffraction model

The Fraunhofer diffraction formula is commonly used for estimating the diameter of thin cylinders by far field diffractometry. However, an experimental systematic overestimation of the value of the cylinder diameter by this diffraction model and other three-dimensional models has been reported when this estimation is compared with those obtained from interferometric techniques. In this work, a rigorous electromagnetic diffraction model is analyzed to determine the cylinder diameter by using the envelope minima of the far field diffraction pattern. The results of this rigorous model are compared with those from the Fraunhofer diffraction formula. The overestimation by the Fraunhofer model is predicted theoretically, presenting a dependence on the wavelength, the polarization state of the incident wave, and the cylinder diameter. The discrepancies are shown to be due to the three-dimensional geometry.     

A theoretical assessment of the relative performance of spherical phased arrays for ultrasound surgery

Computer modeling of spherical-section phased arrays for ultrasound surgery (tissue ablation) is described. The influence on performance of the number of circular elements (68 to 1024), their diameter (2.5 to 10 mm), frequency (1 to 2 MHz), and degree of sparseness in the array is investigated for elements distributed randomly or in square, annular, and hexagonal patterns on a spherical shell (radius of curvature, 120 mm). Criteria for evaluating the quality of the intensity distributions obtained when focusing the arrays both on and away from their center of curvature, and in both single focus and simultaneous multiple foci modes, are proposed. Of the arrays studied, the most favorable performance, for both modes, is predicted for 256 5-mm diameter, randomly distributed elements. For the single focus mode, this performed better than regular arrays of 255 to 1024 elements and, for the case of nine simultaneous foci produced on a coplanar 3x3 grid with 4-mm spacing, better than square, hexagonal, or annular distributed arrays with a comparable number of elements. Randomization improved performance by suppressing grating lobes significantly. For single focus mode, a several-fold decrease in the number of elements could be made without degrading the quality of the intensity distribution.     

Acoustic Field of a Medical Ultrasound Probe Operated in Continuous-Wave Mode Investigated by TV Holography

Time-averaged TV holography has been shown to be a useful technique for investigating acoustic fields in transparent media. The theory of time-averaged TV-holography measurements of ultrasonic fields in water is described. Projections of the phase and the amplitude of a 3.25-MHz ultrasonic field from an annular ultrasound probe operated in cw mode are presented. Quantitative measurements with a spatial resolution of better than 100 mum have been obtained. A set of such projections may be processed into a three-dimensional mapping of the phase and the amplitude of the acoustic field by tomographic techniques. This process is described, and an example of a tomographic reconstruction of the same ultrasonic field is presented.     

In-line type micropulse lidar with an annular beam: experiment

An in-line type compact micropulse lidar (MPL) with an annular beam was developed for low-altitude cloud measurement. An optical circulator and a couple of axicon prisms for an annular beam were installed on the lidar optics. The advantage of using the in-line MPL is its ability to obtain a near-range measurement with a narrow field of view of 0.1 mrad and to obtain a depolarization measurement of the orthogonally polarized echoes caused by ice crystals of a low-altitude cloud. The total insertion loss of the lidar optics was 3 dB. Detectors such as avalance photodiode detectors can be operated in an analog mode near the breakdown voltage because of the high isolation of the optical circulator. The ideal lidar echo variation from the nearest distance was verified by measuring the mountain echoes at various distances. The depolarization measurement of a low-altitude ice cloud was also demonstrated.