摘要:

Experimental and theoretical results on the nonspecular reflection of finite acoustic beams incident at and near the Rayleigh angle onto a fluid‐solid interface loaded by an elastic layer are presented. Measurements with Gaussian‐shaped acoustic beams have been performed on the reflected field amplitude distribution and surface wave speed dispersion in copper‐loaded stainless‐steel specimens where the fluid medium is water. It is shown that existing theory can be utilized to explain the results by constructing and solving boundary condition equations for the Rayleigh wave pole appropriate for the lossless layered halfspace with the fluid. Excellent agreement with the exact treatment is observed in most aspects of the measurements. In the model calculation the imaginary part of the Rayleigh wave pole is found to display an unexpected maximum at a value of layer thickness over wavelength where the real part corresponds nearly to the transverse wave speed in the layer.

ISSN:0021-8979    

DOI:10.1063/1.329913

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Measurements of the surface wave attenuation of surface wave delay lines coated with nickel films in a magnetic field parallel to the film plane in the frequency range 500–665 MHz exhibit attenuation peaks which depend strongly on film thickness, temperature, and magnetic field. The effect is very strong for 200‐A˚ thick nickel film‐coated 618‐MHz quartz delay lines at room temperature. The maximum attenuation change is about 40 db/cm. The attenuation can be varied by over 25 db/cm by a magnetic field change of less than 10 Oe. A 665‐MHz LiNbO3delay line coated with a 200‐A˚ nickel film exhibits a maximum attenuation of about 10 db/cm and similar behavior to that seen on the quartz substrate in a magnetic field. Thicker films exhibit similar but smaller attenuation peaks. Attenuation measurements were carried out at various temperatures. In general, the maximum attenuation decreases at lower temperature and the peaks occur at higher field values. Attenuation measurements in a pulsed magnetic field demonstrated that the attenuation follows the magnetic field change in a few microseconds. A theoretical explanation of these experimental results based on the Stoner‐Wohlfarth model has been proposed. The low‐frequency susceptibility and the imaginary part of the high‐frequency susceptibility were calculated in this model and were found to be in qualitative agreement with the experimental curves for the low‐frequency susceptibility and the high‐frequency surface wave attenuation, respectively. The explanation of the attenuation in this model required an interaction mechanism between the magnetization and the surface wave. Three possible mechanisms are considered in this paper. One of them, the change of the anisotropy energy due to the strain of the wave, appears to give the right order of magnitude for the effect.

ISSN:0021-8979    

DOI:10.1063/1.331582

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A method for measuring the change of sound velocity from a reference state in a liquid is described. Two interdigitial transducers on a piezoelectric substrate are in contact with the liquid, and are used as an input and an output transducer for constructing a liquid delay device. The phase difference between an input and a delayed output signal is detected as a dc voltage via a phase comparating circuit. The temperature dependence of the sound velocity in water, measured by the present method, agrees well with the published data. The influence of hydrostatic pressure on the sound velocity in water is given.

ISSN:0021-8979    

DOI:10.1063/1.331583

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

We consider the problem of obtaining maximum work from an arbitrary two degree of freedom working fluid coupled to a periodic source of pumped thermal energyf (t). The working fluid is also coupled to a heat bath of temperatureTex(t) by a conductor of conductanceK. We assume thatf (t) andTex(t) are given functions of time which are piecewise continuous but are otherwise arbitrary. For periodicf (t) andTex(t) we find that the available power is given by the variance off+KTexover the period. Even withf0, the result is interesting. It reduces to the Curzon–Ahlborn7power for step functionTex(t). It also provides a measure of the power available from temperature fluctuations of the atmosphere.

ISSN:0021-8979    

DOI:10.1063/1.331584

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Ion currents produced by two‐photon ionization were investigated for several organic molecules. At low charge density, the ion current is proportional to the product of gas pressure and the square of laser intensity. At high gas pressure and high laser intensity, the ion current saturates. The saturation effect is well interpreted by a model including electron attachment and charge recombination processes. The two‐photon‐ionization coefficients were determined from the low ion current data. Among the molecules studied, trimethylamine has the highest coefficient of 3.6×10−26cm4/W at the KrF laser wavelength of 248 nm. Such a high photoionization coefficient indicates that trimethylamine could be used to produce a high density of initial electrons for laser‐controlled switches. The present results could also be used to develop new techniques for investigating the electron attachment and charge recombination processes at high gas pressures.

ISSN:0021-8979    

DOI:10.1063/1.331593

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The parametric decay instability of two high‐frequency surface plasma waves, which arises when an intense electromagnetic plane wave is incident on a semi‐infinite, unmagnetized plasma from a vacuum, is investigated on the basis of the hydrodynamic equations for warm electrons. It is shown that the instability occurs only if the incident wave is of the transverse magnetic type. The growth rate for the instability and the threshold amplitude of the incident wave are determined. In contrast to the usual ’’two‐plasmons decay instability’’ in an infinite, unmagnetized plasma, the present ’’surface‐decay instability’’ is possible even if the incident pump field is spatially homogeneous.

ISSN:0021-8979    

DOI:10.1063/1.331594

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

By using optical Fourier‐transform techniques, we have obtained photgraphs of plasma density striations in the focal region of a 350‐MW CO2laser beam focused by anf/7.5 lens into a plasma with density 0.1nc. The observed spatial frequency and the estimated density change imply that the striated regions are also the regions of self‐trapped radiation channels.

ISSN:0021-8979    

DOI:10.1063/1.331595

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The results of an experimental investigation of the infrared recombination emission from a dense sodium plasma produced by resonant optical pumping are reported. Possible explanations for the important experimental features of the infrared emission are discussed. The temporal behavior of the decay of the infrared emission indicates that the primary electron loss mechanism of the plasma is three‐body collisional recombination.

ISSN:0021-8979    

DOI:10.1063/1.331596

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The effect of radiation losses in determining scaling laws of electron temperature on laser flux &Fgr; is discussed for moderateZplasmas created by lasers in the flux range 1011–1013W/cm.2Temperature scaling slower and faster than &Fgr;4/9, observed by various authors, is explained by including radiation losses in a self‐regulatory model. Results of experiments on carbon plasma produced by 1‐GW, 5‐nsec Nd: glass laser show temperature scaling &Fgr;0.25and &Fgr;0.75, and are in good agreement with the predicted behavior.

ISSN:0021-8979    

DOI:10.1063/1.331597

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Magnetic field measurements, which are performed at a dynamicz‐pinch experiment, yield the ion temperature of the dense plasma column. The value of the ion temperature is consistent with results obtained from the speed of signal propagation, from the equilibrium of the pressure of the plasma boundary and from a consideration of the balance of energy. From an analysis of the speed of compressional Alfve´n waves in the region between the wall and the compressed plasma channel, the density outside the dense column is derived. The electron density in the corona is about three orders of magnitude smaller than in the core.

ISSN:0021-8979    

DOI:10.1063/1.331598

出版商:AIP    

年代:1982

数据来源: AIP