Generation of Powerful Ultrasound in the Megahertz Range

The authors set themselves the goal of obtaining the most powerful possible ultrasonic beam, 1–2 cm in diameter, for various biological and physical investigations.

The source of the electrical voltage was a tube generator with a power of about 200 W, assembled according to the usual circuit (Fig. 1). The capacitance of the variable capacitor (with oil filling) made it possible to vary smoothly the frequencies of the generated oscillations in the range of 0.5–3 Mc/s.

The radiators used were quartz crystals 25.4 mm in diameter and 1–3 mm thick. The aluminum electrodes were applied by sputtering in vacuum onto a previously cleaned crystal. To eliminate the possibility of edge breakdown, the electrodes on both sides of the quartz did not extend to the edge of the plate. Special attention was paid to the quality of the transformer oil and to its purification. The authors point out that an arc once struck in the oil greatly lowers its

...breakdown voltage. The fastening of the quartz is shown in Fig. 2. All parts of the fastening were very carefully cleaned before immersion in oil. It was shown that the air cushion adjacent to the lower side—

Fig. 1.

—of the quartz can be replaced by a layer of oil whose thickness amounts to an integer number of quarter-waves.

The authors tested the so-called transition layers proposed by Ernst³ for increasing the power radiated by the crystal. The authors’ experiments did not confirm Ernst’s assumptions; the greatest—

Fig. 2.

—power radiated by the quartz through the transition layer proved to be equal to the power of the quartz radiated by it directly in oil.

The authors also tested lenses made of plastics, and it turned out that although the losses in the lenses are not large, they cause appreciable heating of the lenses, and at places of inhomogeneity strong local overheating may occur, leading even to destruction of the lens.

Concave quartzes and mirrors, in the authors’ opinion, give too nonuniform a distribution of intensity in the focal plane. The authors prefer not to use focusing devices, but to obtain the greatest possible power from a plane quartz radiator, thus obtaining an ultrasonic beam with a relatively uniform distribution of intensity over its cross-section.

The power radiated by the quartz was measured by means of a calorimeter. With the crystal indicated above, 1.43 cm² in electrode surface area, in the frequency range 0.95–1.1 Mc/s, a radiation intensity of 41 W/cm² was obtained.

At a frequency of 1.2 Mc/s it was possible to obtain 55 W/cm², which is of the same order of magnitude as the limiting value for quartz indicated in the work of Epstein et al.² (43 W/cm²). In this case a fountain 15 cm high was obtained, and individual splashes reached 80 cm. The authors believe that the principal factor limiting the power in their work was the electrical strength of the oil.

L. D. Rozenberg

CITED LITERATURE

1. G. G. Selman and M. H. F. Wilkins, J. Sci. Instr. 26, 229 (1949).
2. L. F. Epstein, M. A. Andersen and L. K. Harden, J. ASA 19, 248 (1947).
3. P. I. Ernst, J. Sci. Instr. 22, 238 (1945).