Direct electrodynamic electromagnetic-acoustic transformation in a normal polarizing field: II. Acoustic field of the transition radiation zone

The behaviors of the amplitude of shear displacements and the angle of injection of elastic vibrations have been studied as functions of the frequency and geometry of electromagnetic-field transmitters via a numerical analysis based on the direct electromagnetic-acoustic transformation (EMAT) phenomenon described earlier. The presented data are analyzed with an approximate expression for the difference scheme of sound emission. It is shown that, as the frequency increases, the displacement amplitude reaches a maximum and then decreases. As the frequency increases, the angle of vibration injection abruptly decreases until the frequency of stabilization of the radiation-maximum position is reached and then becomes virtually frequency-independent when the values approximately corresponding to the radiation far-field zone are reached. Expressions are derived for the depth of the end of the radiation near-field zone and the conventional beginning of the radiation far-field zone. It is shown that, in the case of the difference radiation scheme corresponding to two-phase transmitters, the boundaries of both zones during EMAT are shifted from the surface to a distance corresponding to the ratio of the average diameter of the transmitter to the width of the emitting element.