Low group velocity traveling wave tube amplifiers

We report experimental and theoretical results from research into high power X-band traveling wave tube amplifiers designed to eliminate sidebands caused by reflections from the output of such structures. These amplifiers have a low energy velocity, such that the time it takes a wave to be reflected from the output to the input is of the order of, or greater than, the electron beam pulse duration. The elimination of sidebands and the effects of reflections is achieved by this transit time isolation, The bandwidth of the output spectrum is limited by the low pass-band of the periodic structures. Such amplifiers have been operated at power levels of up to 160 MW at 9 GHz for 50 ns pulse durations     

Acoustic particle velocity horns

The paper considers receiving acoustic horns designed for particle velocity amplification and suitable for use in vector sensing applications. Unlike conventional horns, designed for acoustic pressure amplification, acoustic velocity horns (AVHs) deliver significant velocity amplification even when the overall size of the horn is much less than an acoustic wavelength. An AVH requires an open-ended configuration, as compared to pressure horns which are terminated at the throat. The appropriate formulation, based on Webster's one-dimensional horn equation, is derived and analyzed for single conical and exponential horns as well as for double-horn configurations. Predicted horn amplification factors (ratio of mouth-to-throat radii) were verified using numerical modeling. It is shown that three independent geometrical parameters principally control a horn's performance: length l, throat radius R(1), and flare rate. Below a predicted resonance region, velocity amplification is practically independent of frequency. Acoustic velocity horns are naturally directional, providing maximum velocity amplification along the boresight.     

浅海声场垂直振速与水平振速相关特性及应用

基于矢量水听器记录的质点振速信号,研究了浅海声场垂直振速与水平振速的相关特性,理论与仿真结果表明记录的垂直振速与水平振速的相关系数是矢量水听器俯仰角度的函数,矢量水听器不发生俯仰时二者的相关系数约为零。基于上述特点,提出了一种利用矢量水听器记录信号自适应校正矢量水听器俯仰姿态的方法,实验结果表明,该方法能有效校正矢量水听器俯仰姿态。     

A particle timing laser velocity meter

A fluid velocity meter is described; this operates by measuring the time taken for individual dust particles carried in the flow to travel between two points in space. A laser is used to produce two light beams parallel to one another and approximately normal to the flow direction. These beams are focused to two point or line foci in the region of interest: these foci are separated by a distance of the order of 1 mm in the flow direction. A particle crossing both foci in succession gives two scattered light pulses, and the time interval between these is measured by an oscilloscope or digital chronometer. The particles are natural dust particles occurring in the laboratory air or tap water. As in Doppler and fringe anemometers, the advantage of the laser is its spatial coherence: it enables extreme intensity to be obtained in small, well-defined regions. The instrument might be described as a fringe anemometer in which all the light is concentrated into the two end fringes, the others being eliminated.     

用于空气声测量的质点振速传感器

矢量传声器的核心组成器件是质点振速传感器,该传感器具有与频率无关的‘8'字指向性。本文在分析空气声质点振速传感器工作原理基础上,采用数值分析方法系统研究了质点振速传感器中敏感头桥式结构尺寸、热扩散和材料比热容等参数对频率响应的影响,着重分析了质点振速传感器最优灵敏度和低通滤波器效应。根据优化设计结果,采用MEMS工艺技术加工出敏感头桥式结构,并测试了0℃条件下Pt的电阻值R和热阻温度系数θ,以及传感器的频响曲线和‘8'指向性。实验结果表明:空气声质点振速传感器具有良好的声学性能。     

Simultaneous measurement of moving particle size and velocity

An optical method that makes it possible to measure the size and velocity of particles in flows and mixtures independently and simultaneously is considered. Measurements are taken by analyzing a moving diffraction pattern using a multielement photodetector and a special signal processing algorithm. The shape of photocurrent pulses arising when the particle crosses a light beam is determined. Examples of applying this method to particles with internal optical inhomogeneities are given in comparison with other methods, such as the method of low-angle scattering.     

用于空气声测量的质点振速传感器

矢量传声器的核心组成器件是质点振速传感器,该传感器具有与频率无关的‘8'字指向性。本文在分析空气声质点振速传感器工作原理基础上,采用数值分析方法系统研究了质点振速传感器中敏感头桥式结构尺寸、热扩散和材料比热容等参数对频率响应的影响,着重分析了质点振速传感器最优灵敏度和低通滤波器效应。根据优化设计结果,采用MEMS工艺技术加工出敏感头桥式结构,并测试了0℃条件下Pt的电阻值R和热阻温度系数θ,以及传感器的频响曲线和‘8'指向性。实验结果表明:空气声质点振速传感器具有良好的声学性能。     

Influence of particle packing structure on sound velocity

The anisotropy in the particle systems of different packing structures affects the sound velocity. The acoustic propagation process in four kinds of packing structures(denoted as S45, H60, S90, and D) of two-dimensional granular system is simulated by the discrete element method. The velocity v_(tof) obtained by the time of flight method and the velocity vc obtained from the stiffness tensor of the system are compared. Different sound velocities reflect various packing structures and force distributions within the system. The compression wave velocities of H60 and S90 are nearly the same, and transmit faster than that of D packing structure, while the sound velocity of S45 is the smallest. The shear wave velocities of S45 and H60 are nearly the same, and transmit faster than that of D packing structure. The compression wave velocity is sensitive to the volume fraction of the structure, however, the shear wave velocity is more sensitive to the geometrical structure itself. As the normal stress p is larger than 1 MPa, v_(tof) and vc are almost equal, and the stiffness tensors of various structures explain the difference of sound velocities. When the normal stress is less than 1 MPa, with the coordination number unchanged, the law v_(tof) ∝ p~(1/4) still exists. This demonstrates that apart from different power laws between force and deformation as well as the change of the coordination number under different stresses, there are other complicated causes of v_(tof)∝ p~(1/4), and an explanation of the deviation from v_(tof) ∝ p~(1/6) is given from the perspective of dissipation.     

Computing particle velocity and intensity in diffracting systems

In a previous paper the authors have shown how it is possible to compute the acoustic pressure due to an axi-symmetric radiator with different boundary conditions. The technique involves the conversion of a double integral to a single integral by means of a geometric transform, and may be applied with the Green's function appropriate to each boundary condition.The present work shows how the form of the single integrals makes it possible to derive and compute both the particle velocity and the wave intensity. It has become apparent that the particle velocity must be described as an elliptic vector. A new concept, the ‘diffraction front’, has been defined as a surface in which there is no net energy transfer, and this has been applied to the evaluation of the magnitude and direction of energy flow. Plots have been made of flow lines and diffraction fronts, and also three-dimensional representations of energy intensity have been made for those acoustic fields in which pressure has already been evaluated.     

Noninvariant one-way velocity of light and particle collisions

Recently published space-time transformations between inertial systems (different from the Lorentz transformations) are reviewed. Energy and momentum are defined consistently with these new transformation laws. Formally they equal the usual relativistic expressions only in the privileged frame, but numerically they do so in all inertial frames. All the precise experimental data concerning thresholds for inelastic processes, particle masses, and so on, can thus be explained also within this new theoretical framework.     

Shooting algorithm and particle swarm optimization based Raman fiber amplifiers gain spectra design

An initial value determination method with a contraction factor for the counter-pumped Raman coupled equations is proposed. This method is used in conjunction with initial guess correction mechanism of Newton's method to construct a new efficient shooting algorithm for the solution of counter-pumped Raman coupled equations. The particle'swarm optimization is used to find the optimal wavelengths and powers for the pumps. By combining the new shooting algorithm and particle swarm optimization a powerful approach to the design of gain spectra for Raman fiber amplifiers is developed. Using this approach a counter-pumped broadband Raman fiber amplifier in C + L-band is designed and optimized. An average on-off gain of 9.3 dB for a bandwidth of 95 nm is obtained using only 4 pumps, with an in-band ripple level of ± 0.7 dB.     

质点振速传感器直线阵超指向性波束形成*

声矢量传感器及其组阵方式的研究已成为水声领域的热门课题。该文研究了当单自由度矢量传感器（质点振速传感器）的观测方向沿直线阵轴线方向时的空间均匀噪声场中两阵元的噪声相关系数,然后采用Gram-Schmidt模态波束分解与综合方法推导了高阶超指向性的最优解,得到了直线阵端射方向的指向性指数和稳健性水平。仿真研究表明,当d / λ < 0.5 (d为阵元间距,λ为波长)时,质点振速传感器直线阵（简称质点振速阵）采用超指向性方法在端射方向的指向性指数略高于声压传感器直线阵（简称声压阵）;而当d / λ > 0.5时,质点振速阵的指向性指数远远高于声压阵。该文的研究内容对水下声纳系统中质点振速传感器的有效组阵和实际应用具有一定的参考价值。     

Dielectric Cherenkov masers as powerful amplifiers with superwidebandwidth

Here, we explore an important feature of dielectric Cherenkov masers (DCM's) that distinguishes them from other relativistic microwave devices. Most previous DCM experiments were carried out with rather thin dielectric liners or films. In this paper we show, however, that when the coefficient of waveguide filling with dielectric is not small, the beam-wave coupling doesn't depend on frequency, and, hence, superwide bandwidth is possible. The value of bandwidth is also determined by the current and velocity of the electron beam. DCM dispersion relations are examined both analytically, in the simplest model of total waveguide filling, and numerically, for a hollow beam in a waveguide with a dielectric liner. We show that a maximum value of bandwidth is achieved at a certain optimal value of the beam current for fixed velocity (and vice versa). Numerical results demonstrate a -3 db bandwidth of 40-50% at a peak gain of 40 dB for electron energies of 300-600 keV and beam currents of 1-10 kA     

Experimental investigation of particle velocity distributions in windblown sand movement

With the PDPA (Phase Doppler Particle Analyzer) measurement technology, the probability distributions of particle impact and lift-off velocities on bed surface and the particle velocity distributions at different heights are detected in a wind tunnel. The results show that the probability distribution of impact and lift-off velocities of sand grains can be expressed by a log-normal function, and that of impact and lift-off angles complies with an exponential function. The mean impact angle is between 28° and 39°, and the mean lift-off angle ranges from 30° to 44°. The mean lift-off velocity is 0.81–0.9 times the mean impact velocity. The proportion of backward-impacting particles is 0.05–0.11, and that of backward-entrained particles ranges from 0.04 to 0.13. The probability distribution of particle horizontal velocity at 4 mm height is positive skew, the horizontal velocity of particles at 20 mm height varies widely, and the variation of the particle horizontal velocity at 80 mm height is less than that at 20 mm height. The probability distribution of particle vertical velocity at different heights can be described as a normal function. Supported by the National Natural Science Foundation of China (Grant No. 10532030) and the National Basic Research Program of China (Grant No. G2000048702)     

Delay and Doppler spreads in underwater acoustic particle velocity channels

Signal processing and communication in acoustic particle velocity channels using vector sensors are of interest in the underwater medium. Due to the presence of multiple propagation paths, a mobile receiver collects the signal with different delays and Doppler shifts. This introduces certain delay and Doppler spreads in particle velocity channels. In this paper, these channel spreads are characterized using the zero-crossing rates of channel responses in frequency and time domain. Useful expressions for delay and Doppler spreads are derived in terms of the key channel parameters mean angle of arrival and angle spread. These results are needed for design and performance prediction of systems that utilize underwater acoustic particle velocity and pressure channels.     

Dense-phase pneumatically conveyed coal particle velocity measurement using electrostatic probes

The fundamental measurement theory of an electrostatic probe and cross-correlation velocity measurement method are introduced in this paper. The effects of the probe's geometric parameters, including the length of the probe; the thickness, length and relative permittivity of the dielectric pipe; the radius of the screen on the dimensionless calibration coefficient (k), and the statistical error of the transit time (τ_m) of the correlation velocity measurement system using electrostatic probes were investigated theoretically. Finally the measurement system was applied in a 10 mm bore horizontal section of a dense-phase pneumatic conveying system under high pressure circulating pulverized coal over a superficial air velocity range of 0.5–7 m/s for a particle concentration 0.052–0.141 m³/m³. The experimental results that were obtained demonstrate that the system is capable of providing solid particle velocity measurements with repeatability better than 10% under the given experimental conditions.     

基于双面质点振速测量的声场分离技术

现有的声场分离技术或是基于双面声压测量或是基于单面声压和质点振速测量。本文以二维空间傅里叶变换法作为声场分离算法,提出一种基于双面质点振速测量的声场分离技术。论文首先推导出了该方法的理论模型,随后通过仿真研究验证了该方法的有效性,并通过与基于双面声压测量的声场分离方法的比较说明了其优越性。结果表明:采用基于双面质点振速测量的声场分离方法可以有效实现声场分离;基于双面质点振速测量的声场分离方法可以获得更高的质点振速分离精度,而基于双面声压测量的声场分离方法可以获得更高的声压分离精度。     

匹配垂直质点振速的浅海地声参数反演

海底声学参数对海洋波导中的声场特性研究和相关应用具有重要意义。针对一次夏季黄海声传播实验,分析了浅海负跃层环境下垂直质点振速的传播特性和简正波结构,说明当声源和接收器均位于负跃层下时,除海底附近外的大部分深度上垂直质点振速能量较高,且与声压相比,号数高的简正波对垂直质点振速的贡献更大,利用垂直质点振速进行匹配场反演能获得更高的海底参数敏感性。分析了海底吸收系数对垂直质点振速匹配场反演的影响,结果表明只有当进行匹配场反演时设置的海底吸收系数接近真实值时,才能获得准确的海底声速、密度和海深反演结果。利用实验中矢量水听器获取的垂直质点振速信号进行匹配场反演,将海底吸收系数在变化范围内取不同值对海底声速、密度和平均海深进行全局搜索,根据代价函数值最大确定了海底声速、密度及平均海深的反演结果,并利用不同距离上的声压传播损失反演出不同频率下的海底吸收系数。根据反演得到的海底声学参数计算声压传播损失,与实验中声压水听器测量结果符合较好。     

多程片状放大器用作电光开关检偏器研究

 用布儒斯特角放置的多张放大器片作大口径电光开关的检偏器，不仅有利光束的传输，而且可以有效降低装置的工程造价和难度。本文计算了不同的片放张数、放大器增益和普克尔盒效率与系统隔离效率之间的关系。在多程系统上进行了实验测试，结果为在相同输入光强时，普克尔盒工作与不工作时输出光束强度相差8倍。