Underwater streamer propagation analyzed from detailed measurements of pressure release

Abstract

In this paper we describe experimental observations connected with the propagation of primary and secondary streamers in water. Using a Mach-Zehnder interferometer we determined the pressure field surrounding the streamer channel at a given instant in time with high temporal and spatial resolution. This pressure field contains information on the time evolution of the pressure pulse inside the discharge channel. The pressure history in the channel has been reconstructed by comparing the experimentally obtained fringe shifts in the interferograms with those derived from one-dimensional hydrodynamic calculations in cylindrical geometry. Assuming different trial pressure pulses, it has been possible to establish the channel pressure iteratively. A reproduction of the experimental data from secondary streamers requires short (2–3ns) pressure pulses with amplitudes of 2–3GPa. These findings are inconsistent with the assumption of bubble-initiated propagation of secondary streamers. It has also been inferred from estimates of the channel diameter that self-propagation of secondary streamers occurs at field strengths at the streamer tip of more than 2GV∕m. We can therefore conclude that field induced dissociation and ionization of molecules in the bulk liquid are the most likely mechanism for secondary streamer propagation. Rather high electrical conductivity (>0.2S∕m) is achieved at fields of 2GV∕m and an ionization wave is launched from the streamer tip into the liquid. To advance the streamer the electric field must be expelled from the newly generated section. This occurs with the Maxwellian relaxation time of a few nanoseconds. During this time the region of high conductivity is transformed into a plasma channel of lower density and a pressure wave is launched into the liquid. A different mechanism is suggested for primary streamer formation. Because of the low conductivity in the channels it is more likely that gas bubbles or phase instabilities are involved in this case.

Temporally resolved imaging on quenching and re-ignition of nanosecond underwater discharge

“…The discharge observed here is different from the discharge produced by microsecond or sub-microsecond duration pulses from aforementioned publications. [5][6][7] In our case, the discharge demonstrated a more spherical shape with a few major branches, while the microsecond or submicrosecond discharges usually have a complicated bush-like structure. The propagation of the nanosecond discharge reached about 1000 km/s, which was two orders of magnitude higher than the velocity observed in low density plasma filaments.…”

“…[5][6][7] For example, An et al 7 demonstrated that with 40-ns rise time and 10-μs duration time, the development velocity of the discharge is a Author to whom correspondence should be addressed. To answer these questions, a nanosecond pulsed power system (with pulse duration about 10 ns) synchronized with a picosecond ICCD camera (with minimum gate time 50 ps) was used to study the time-resolved evolution of nanosecond discharge in water.…”

Positive streamers in air and nitrogen of varying density: experiments on similarity laws

“…Obviously, pressure, temperature and gas composition vary from one phenomenon to the next. In electrical engineering, the higher densities of liquids are desirable for fast switching, but pre-existing microbubbles largely influence streamer properties in fluids [13,14]. For this reason, the operation of spark gaps filled with supercritical fluids is now under investigation [15].…”

Initiation process and propagation mechanism of positive streamer discharge in water

“…12,17,19 Underwater streamer discharges at positive polarity also have two propagation modes: a primary streamer and a secondary streamer. [20][21][22][23][24][25][26][27] The primary streamer appears at lower applied voltage with a propagation velocity of 2-3 km/s, having a semi-spherical brush-like structure ($500 lm) composed of many filaments. 22,26 All but a few filaments disappear within 400 ns, resulting in a tree-like structure.…”

“…[20][21][22][23][24][25][26][27] The primary streamer appears at lower applied voltage with a propagation velocity of 2-3 km/s, having a semi-spherical brush-like structure ($500 lm) composed of many filaments. 22,26 All but a few filaments disappear within 400 ns, resulting in a tree-like structure. 26 The primary streamer propagates during the flow of repetitive pulsed currents.…”