Pressure Field Around Underwater Negative Streamers

In this paper, experimental observations of propagation of negative streamers in distilled water (conductivity of 1.7 μS·cm^-1) in a needle-plane electrode geometry (gap of 6 mm) are described. The spatially resolved pressure field surrounding the tip of streamer channel in the given moment was determined by Mach-Zehnder interferometer with second harmonic of Nd:YAG laser as a source. The high-voltage needle electrode was brought on the potential of 23 kV and that remained practically unchanged during streamers propagation, while a significant noise with amplitude of several amperes was visible on the waveform of discharge current. Analysis of captured interferograms shows that propagation of negative streamers is not uniform: an active (propagating) streamer produces continuous moving pressure field (similar to that of moving object) with maximum amplitude greater than 40 MPa on the streamer tips and an inactive streamer nearly stops propagating being surrounded by a nearly spherical pressure wave moving away from the streamer´s tip. The inactive state can last even longer than 200 ns.