Semiquantitative estimates of currents generated by monochromatic waves in toroidal plasmas

Landau damping and transit time damping are analyzed from the point of view of toroidal electric current generation. Steady current density and energy absorption are estimated. The importance of momentum transfer to non-resonant electrons via Coulomb collisions is pointed out. If the bounce period of trapped electrons is less than their collisional relaxation time, the most part of the toroidal current is generated due to collisional dragging of non-resonant electrons.The author gratefully acknowledges valuable discussions with K.Jungwirth, V.Kopecký, V. L.Sizonenko and K. N.Stepanov.     

Production of monochromatic radiation in an argon plasma by means of acoustic waves

It is shown that monochromatic radiation (λ₁=5888 Å, λ₂=5882 Å, and λ₃=4876 Å) can be obtained in a gas-discharge plasma in argon with the use of acoustic waves. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 355–357 (10 March 1999)     

Directivity pattern of parametrically generated acoustic waves

Summary The generation of down-converted frequency waves in the nonlinear propagation of acoustic waves is examined, in order to investigate the effects produced by the finiteness of the medium or of the media of interaction. Oscillations of the directivity patternsvs. the width of the media of interaction are set in evidence and related to resonance conditions of the medium cavity. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

High-frequency single-electron transport in a quasi-one-dimensional GaAs channel induced by surface acoustic waves

We report on an experimental investigation of the direct current induced by transmitting a surface acoustic wave (SAW) with frequency 2.7 GHz through a quasi-one-dimensional (1D) channel defined in a GaAs - AlGaAs heterostructure by a split gate, when the SAW wavelength was approximately equal to the channel length. At low SAW power levels the current reveals oscillatory behaviour as a function of the gate voltage with maxima between the plateaux of quantized 1D conductance. At high SAW power levels, an acoustoelectric current was observed at gate voltages beyond pinch-off. In this region the current displays a step-like behaviour as a function of the gate voltage (or of the SAW power) with the magnitude corresponding to the transfer of one electron per SAW cycle. We interpret this as due to trapping of electrons in the moving SAW-induced potential minima with the number of electrons in each minimum being controlled by the electron - electron interactions. As the number of electrons is reduced, the classical Coulomb charging energy becomes the Mott - Hubbard gap between two electrons and finally the system becomes a sliding Mott insulator with one electron in each well.     

Nondestructive determination of film thickness with laser-induced surface acoustic waves

The application of surface acoustic waves(SAWs) for thickness measurement is presented. By studying the impact of film thickness h on the dispersion phenomenon of surface acoustic waves, a method for thickness determination based on theoretical dispersion curve v( fh) and experimental dispersion curve v( f) is developed. The method provides a series of thickness values at different frequencies f, and the mean value is considered as the final result of the measurement. The thicknesses of six interconnect films are determined by SAWs, and the results are compared with the manufacturer's data.The relative differences are in the range from 0.4% to 2.18%, which indicates that the surface acoustic wave technique is reliable and accurate in the nondestructive thickness determination for films. This method can be generally used for fast and direct determination of film thickness.     

On acoustic waves generated in a rectangular basin by low-frequency gravity modes

Summary Sound waves generated by low-frequency gravity modes are further examined. In this consideration, the study is extended to a closed inland body of water. By the use of the appropriate Green's function and traditional shallow water approximations, the mean response is completely evaluated. The result suggests that this response function is inversely proportional to fourth power of the exciting wave number.

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Riassunto Si esaminano ulteriormente onde sonore generate da modi di gravità a bassa frequenza. In questa considerazione, lo studio è esteso ad un bacino d'acqua chiuso nell'entroterra. Mediante la funzione di Green appropriata e le approssimazioni tradizionali dell'acqua bassa, si valuta completamente la risposta media. Il risultato suggerisce che questa funzione di risposta è inversamente proporzionale alla quarta potenza del numero d'onda di eccitazione.

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Резюме Исследуются звуковые волны, образованные гравитационными модами низкой частоты. В этой работе рассматривается случай замкнутого прямоугольного бассейна. Используя соответствующую функцию Грина и традиционные приближения мелкой воды, полностью определяется средний отклик. Полученный результат предполагает, что функция отклика обратно пропорцоинальна четвертой степени возбуждающего волнового числа.

     

Modeling of atmospheric propagation of acoustic gravity waves generated by different surface sources

A model of acoustic gravity wave propagation is developed and a numerical algorithm for solving the corresponding hydrodynamics equations is elaborated. Behavior of the waves in the upper atmospheric layers is studied in relation to the parameters of the ground-based sources.     

A study of acoustic waves generated by the shock wave of an antihail gun

The processes of propagation of a shock wave generated by an antihail gun are analyzed. It is shown that a shock wave can reach an altitude of 100 km. The measurement results of the time dependence of variation in acoustic intensity and sound frequency for a vertically propagating shock wave are presented.     

声表面波检测铝板表面微缺陷深度实验研究

为了探究声表面波与不同深度微裂纹缺陷相互作用的关系,将脉冲激光作用于一系列不同缺陷的试件铝板上进行线光源激励,激发激光超声波。用超声传感器接收在铝板中传播的激光超声信号,通过数字荧光示波器采集激光超声在铝板中的传播数据。对采集到的反射波数据进行分离谱分离过程得到的铝板中激光超声的时域分布和透射波数据进行频域分析。实验发现:缺陷深度影响着反射回波两峰值特征点到达时间差,两者之间近似线性关系,也影响着透射波的截止频率且二者呈现递减关系。     

High-frequency acoustic emissions generated by a 20 kHz sonochemical horn processor detected using a novel broadband acoustic sensor: a preliminary study

This paper describes the application of a novel broadband acoustic sensor to evaluating the acoustic emissions from cavitation produced by a typical commercial 20 kHz sonochemical horn processor. Investigations of the reproducibility of the processor, and of the variation in cavitation emissions as a function of output setting and sensor location are described, and resulting trends discussed in terms of the broadband integrated power in the megahertz frequency range. Companion studies with a conventional membrane hydrophone have illustrated for the first time that cavitation emissions produced by a sonochemical horn processor can extend to frequencies beyond 20 MHz, and the sensor shows that significant nonlinearity can be seen in measured cavitation activity with increasing nominal output power.     

Nonlinear scattering of acoustic waves by natural and artificially generated subsurface bubble layers in sea

The paper describes nonlinear effects due to a biharmonic acoustic signal scattering from air bubbles in the sea. The results of field experiments in a shallow sea are presented. Two waves radiated at frequencies 30 and 31-37 kHz generated backscattered signals at sum and difference frequencies in a bubble layer. A motorboat propeller was used to generate bubbles with different concentrations at different times, up to the return to the natural subsurface layer. Theoretical consideration is given for these effects. The experimental data are in a reasonably good agreement with theoretical predictions.     

Influence of oxygen on the elastic properties of nanocrystalline diamond films studied by laser-induced surface acoustic waves

Laser-induced surface acoustic waves (SAWs) were used to study the influence of oxygen on the elastic properties of nanocrystalline diamond films. A series of samples was grown by chemical vapor deposition (CVD) on the (100) plane of p-type silicon for 3h under different O(2)/(CH(4)+H(2)) flow ratios from 0% to 8%. The elastic properties of these nanocrystalline diamond films were determined from the SAW dispersion curves. The maximum frequency realized was about 310 MHz in wideband SAW experiments and about 700 MHz using a narrowband mask method with several higher harmonics. It was found that the densities of all samples were surprisingly high, approaching the ideal value of diamond, whereas the Young's modulus increased from 700 to 950 GPa with the addition of oxygen. It is concluded that oxygen has a significant positive effect on the elastic properties of nanocrystalline diamond.     

Chaotic mixing generated by acoustic streaming

The acoustic mixer presented in this paper is a closed cylinder containing chemical solutions. The ultrasound was generated in a water bath outside of the cylinder. The mixing efficiency was measured by using a laser-photodiode system and by mixing iodine and sodium thiosulfate. Iodine solution has a light brown color which becomes transparent if the solution is mixed with sodium thiosulfate. The unmixed regions of the solution in the container remain dark. Starch was used to make the solution darker. Considering the relative position between the cylinder and the transducer, it was shown that displacements parallel to the axes of the streaming have little effect on the mixing speed. Indeed, the distance of the cylinder from the center of the streaming is more important to obtain ideal mixing performances. The frequency and size of the transducers has been investigated too and little effect was found. In contrast, the wave amplitude seems to be the biggest factor in obtaining rapid mixing.     

Air-borne sound generated by sea waves

This paper describes a semi-empiric model and measurements of air-borne sound generated by breaking sea waves. Measurements have been performed at the Baltic Sea. Shores with different slopes and sediment types have been investigated. Results showed that the sound pressure level increased from 60 dB at 0.4 m wave height to 78 dB at 2.0 m wave height. The 1/3 octave spectrum was dependent on the surf type. A scaling model based on the dissipated wave power and a surf similarity parameter is proposed and compared to measurements. The predictions show satisfactory agreement to the measurements.     

Pipe wall damage detection by electromagnetic acoustic transducer generated guided waves in absence of defect signals

Most investigators emphasize the importance of detecting the reflected signal from the defect to determine if the pipe wall has any damage and to predict the damage location. However, often the small signal from the defect is hidden behind the other arriving wave modes and signal noise. To overcome the difficulties associated with the identification of the small defect signal in the time history plots, in this paper the time history is analyzed well after the arrival of the first defect signal, and after different wave modes have propagated multiple times through the pipe. It is shown that the defective pipe can be clearly identified by analyzing these late arriving diffuse ultrasonic signals. Multiple reflections and scattering of the propagating wave modes by the defect and pipe ends do not hamper the defect detection capability; on the contrary, it apparently stabilizes the signal and makes it easier to distinguish the defective pipe from the defect-free pipe. This paper also highlights difficulties associated with the interpretation of the recorded time histories due to mode conversion by the defect. The design of electro-magnetic acoustic transducers used to generate and receive the guided waves in the pipe is briefly described in the paper.     

Identification of laser generated acoustic waves in the two-dimensional transient response of cylinders

The published model [Appl. Phys. Lett. 82, 4379-4381 (2003)] for the two-dimensional transient wave propagation in a cylinder is modified to avoid the inherited integration of the numerical inverse scheme. The Fourier series expansion is introduced for one spatial coordinate to resolve the transient response problem: theoretical radial displacements in either the ablation or the thermoelastic regime are obtained with little numerical noise and short computation time. The normal mode expansion method fails to deliver results with the same accuracy. Acoustic waves are fully identified by the ray trajectory analysis. These identified waves are further verified on the experimental results observed with the laser ultrasonic technique.     

Exciton transport by surface acoustic waves

Long-range acoustic transport of excitons in GaAs quantum wells (QWs) is demonstrated. The mobile strain field of a surface acoustic wave creates a dynamic lateral type I modulation of the conduction and valence bands in a double-quantum-well (DQW) structure. This mobile potential modulation transports long-living indirect excitons in the DQW over several hundreds of μm.     

Theoretical study on line source laser-induced surface acoustic waves in two-layer structure in ablative regime

The generation of ultrasound in film–substrate system by a laser line source is studied in the case of ablation mechanism, which can be realized by adding a liquid layer at the excitation point. The time domain displacement can be yielded by the numerical jointed inversed Laplace–Fourier transformation technique. The typical surface acoustic waves (SAW) of two layer structures, slow film on fast substrate and fast film on slow substrate, are obtained and the effect of the propagation distance and the thickness of the film on the SAW are given.