Fast propagation of an underwater secondary streamer by the appearance of a continuous component in the discharge current

Abstract

An underwater secondary streamer for a single-shot pulsed positive voltage was found to propagate with an average velocity of 18 km/s during the appearance of a continuous component on the discharge current waveform. The continuous component was the first discharge current with a direct-current–like component detected after a displacement current. However, the propagation velocity decreased to less than 2 km/s during the appearance of pulsed currents, which followed the continuous component. These results were analyzed by highly spatiotemporal visualization synchronized with the discharge current.

Initiation process and propagation mechanism of positive streamer discharge in water

“…The secondary streamer propagated during the appearance of a continuous discharge current, as previously reported. 27 This continuous current was followed by spike currents, which showed re-illuminations. With the backlight turned on, both luminescence and gas channels can be visualized simultaneously since the light emission intensity of a secondary streamer is strong enough to distinguish the luminescence from the backlight.…”

“…Details of the experimental setup can be found in our previous reports. 26,27 Highly spatiotemporal visualization was conducted using microscope lenses attached to an ultra high-speed camera (NAC Image Technology, ULTRA Neo), a set consisting of an image intensifier unit (I.I.) (Hamamatsu, C9546-04), a digital camera (Nikon, D4), and a streak camera (Hamamatsu, C10910) as shown in Fig.…”

“…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.…”

“…23,24,26 Above a certain threshold voltage, the secondary streamer propagates with a velocity of greater than 10 km/s, for example, 25 reports, forming a filamentary structure of a few millimeters, dependent on gap distance. [20][21][22][23][24][25]27 In the case of low conductive water, secondary streamer propagation is followed by re-illuminations, which are restarts of the discharge in the formed gas channel, and show a dielectric-barrier discharge (DBD) like behavior with a stepwise propagation. 21,23,24 The secondary streamer and the re-illuminations are characterized by a continuous current with a duration of around 100 ns and subsequent spike currents.…”

“…21,23,24 The secondary streamer and the re-illuminations are characterized by a continuous current with a duration of around 100 ns and subsequent spike currents. 21,23,24,27 The pressure of shock waves launched by secondary streamer propagation reaches 3 GPa. 22,28 Such a high pressure leads to a local electric field above 20 MV/cm, suggesting field induced dissociation and ionization of molecules in liquid for the secondary streamer inception: the direct ionization theory.…”