Abstract
Streamer discharges in water, which are a pre-breakdown phenomenon caused by underwater discharge, have gained attention because of the development of plasma-based technologies for water purification and medical treatment. In this study, a single-shot pulsed negative high voltage of –18 to –21 kV with a rise time of 100 ns was applied to a needle electrode in ultrapure water, and streamers were continuously imaged using an optical system combining a high-speed video camera, a streak camera, and an ultra-high-speed camera with a microscope lens. These observations were synchronized with the applied voltage and the discharge current to investigate the effect of the discharge on streamer propagation. Negative streamers propagated with a velocity of 820 ± 50 m/s in the presence of pulsed currents, and 100 ± 20 m/s when pulsed currents were absent. The streak camera detected light emission when the pulsed currents appeared, and it was also observed that 1480 m/s pressure waves were generated during streamer propagation. Furthermore, we developed a simple new method of imaging weak density changes similar to those detected using the Schlieren method or Mach–Zehnder interferometer. This method simply involves inserting a pair of polarizing plates on the optical axis, so that the pressure waves can also be imaged in two-dimensional photographs. Our results indicated that the pressure waves were generated from the propagating streamer head when the pulsed currents appeared in the waveform. Analysis of temporal resolution with nano-second order clarified that the branching phenomenon occurred at different times resulting in the branching streamer propagation with different directions.
Highlights
Extensive research has been conducted in the previous few decades regarding streamers in water6 and dielectric liquids such as transformer oil,7 liquid hydrocarbon,8 normal hexane,9,10 and cyclohexane.11 Streamers are known to contain bush-like, tree-like, and filament-like structures
∼1.5 μs Appearance pulsed current: propagation velocity 820 ± 50 m/s Without pulsed current: propagation velocity 100 ± 20 m/s Light emission at appearance of pulsed current Generation of pressure wave at tip of streamer at appearance of light emission Intermittent generation of pressure wave which corresponds to direction change of streamer discharge propagation: average interval time ∼35 ns Formation of branching streamer with time difference of 35 ns > tdelay > 10 ns
We succeeded in imaging the propagation process of negative streamers in water, synchronized with the discharge current, using three types of high-speed cameras
Summary
Extensive research has been conducted in the previous few decades regarding streamers in water and dielectric liquids such as transformer oil, liquid hydrocarbon, normal hexane, and cyclohexane. Streamers are known to contain bush-like, tree-like, and filament-like structures. Streamers are known to contain bush-like, tree-like, and filament-like structures. The morphology of the propagation processes of streamers differs depending on the polarity of the applied voltage. The propagation velocities of bush-like negative and positive streamers are several hundred meters per second. The propagation velocities of tree-like and filament-like negative streamers are several kilometers per second, whereas the positive streamers are more than ten times faster than negative streamers. This is owing to the fact that the electric charge moves to a low electric field region and loses the driving force in the case of negative discharge.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
