Quasi-shear Waves Research Articles

On acoustic wave refraction at the interface of trigonal crystals

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Lower-hybrid wave absorption in the presence of simultaneous density and magnetic field gradients

It is shown that for the lower-hybrid wave (the slower of the two cold-plasma modes propagating at an angle to the static magnetic field), the WKB requirements are satisfied in almost the entire region from the antenna up to and including the ion-cyclotron-harmonic resonance. Expressions for the electric field and the energy density in the WKB approximation are derived. Both the collisionless and collisional damping become significant near the resonance, and the wave attenuation due to the two processes is estimated. From the differential equation for the electric field valid near the resonance, it is found that the wave energy would be completely assimilated even in the absence of a damping mechanism. The effect of impurities, although negligible near the plasma edge, becomes important following wave conversions and a significant fraction of the wave energy may end up heating the impurity ions in the plasma interior. The implications of using the ’’local’’ dielectric tensor approximation in the above context are examined. An alternative scheme for heating at the ion-cyclotron frequency using the lower-hybrid (quasi-shear) wave instead of the usual compressional wave is pointed out.

On diffraction and focusing in anisotropic crystals

This paper discusses various aspects of diffraction and focusing for waves in anisotropic crystals, and the concepts are applicable to electromagnetic, ultrasonic bulk, or ultrasonic surface waves in homogeneous but arbitrarily anisotropic crystals. A numerical technique based on an angular spectrum of plane waves representation is shown to produce theoretical intensity profiles in good agreement with acoustooptic probe measurements of the diffraction field of a slit-type ultrasonic transducer launching quasi-longitudinal and quasi-shear waves in quartz along on axis of extreme anisotropy where the wave normal and the direction of energy flow are separated by angles greater than 20°. A theoretical study of two-dimensional focusing structures for use in anisotropic crystals reveals that a launching transducer fabricated in the shape of the relevant group-velocity surface can be far superior in producing a well-defined focus to a structure having the shape of a circular arc. The computation techniques can be applied to obtain the diffraction field of any arbitrary aperture function and the examples described, theoretical and experimental, concern acoustic waves in crystals demonstrating extreme elastic anisotropies, in fact much more extreme than would be encountered in analogous cases of optical diffraction. Because of current relevance to device research, emphasis in the latter sections of the paper is placed on ultrasonic surface waves.