Investigation of the microsecond-pulse acoustic wave generated by a single nanosecond-pulse discharge

Abstract

A single nanosecond-pulse discharge can produce a high-intensity pulsed acoustic wave. The pulse width of the acoustic wave is much wider than that of the current, more than 20  μs at 30 cm from the source, which is the basis of synthesizing low-frequency sound by repetitively nanosecond-pulse discharges. The investigations of electroacoustic characteristics and the sound formation process of the single nanosecond-pulse discharge are vital to advance this technology. In this paper, an experimental platform for the single nanosecond-pulse discharge was built, and time-domain waveforms of the voltage, the current, and the sound pressure were measured. The effects of electrode shape, current limiting resistors, and current pulse width on the acoustic wave were discussed. To analyze the formation process of the acoustic wave, the gas densities near the electrodes at different moments after the discharge were diagnosed by laser Schlieren photography. The result shows that the formation of the acoustic wave is much slower than the discharge. A two-stage model was developed to qualitatively describe the formation process of the acoustic wave, and numerical calculations were carried out using thermodynamic and hydrodynamic equations. At the end of the discharge, a huge pressure difference is formed inside and outside the gas channel due to the Joule heating, which can be considered as a shock wave. During the outward propagation, the wave tail is elongated by the difference in sound velocity at each point, and the thickness of the shock wave increases due to the dissipation. This eventually leads to the half-duration of more than 20  μs.