Newsletter 2015.2 Index
Theme : "The Conference of Fluid Engineering Division"
Underwater streamers are pre-breakdown phenomena observed as developing luminous filaments from a needle electrode to an opposite ground electrode. When a single-shot positive pulsed voltage with a duration of 10 μs was applied to the needle electrode with a tip radius of 40 μm, two propagation modes were observed: a primary streamer and a secondary streamer. The streamer propagation was visualized at nanosecond temporal and micrometer spatial resolution by using an ultra high-speed camera with a microscope lens. In addition, visualization results were synchronized with the discharge current with an accuracy of 1 ns. Figure 1 shows the successive images of secondary streamer propagation taken as luminescence and the synchronized current waveform. The secondary streamer formed a filamentary structure and propagated continuously with a velocity of around 30 km/s when a continuous component appeared on the current waveform. Figure 2 shows the successive images of primary streamer propagation and the synchronized current waveform. The primary streamer was observed as gas channels by the use of light source since the luminescence was too weak to detect at nanosecond temporal resolution. The primary streamer formed a semi-spherical brush-like structure and was characterized by the appearance of repetitive pulsed currents on the current waveform. However, the propagation of gas channels seemed to be continuous.
In this study, primary streamer luminescence was visualized by a streak camera (Hamamatsu, C10910) to investigate the contradiction of continuous propagation in spite of the detection of pulsed currents. The streak camera enables a temporally successive observation along a streak slit, as shown in Fig. 3. Figure 4 shows the shadowgraph streak image of primary streamer propagation with a streak time of 100 ns. Primary streamer propagation seemed to be continuous in spite of pulsed currents. However, Fig. 5 (a) shows the intermittent luminescence of a primary streamer. In addition, the intermittent luminescence pattern was synchronized with the pattern of the pulsed currents, as shown in Fig. 5 (b). These results suggest the intermittent propagation of a primary streamer, synchronized with the pulsed currents.
underwater discharge, streamer, streak camera
Fig. 1 Successive images of secondary streamer propagation taken with a gate time of 20 ns at 40 Mfps at 21.0 kV and the synchronized current waveform.
Fig. 2 Successive images of primary streamer propagation taken with a gate time of 5 ns at 100 Mfps at 18.5 kV and the synchronized current waveform.
Fig. 3 Schematic of the visualization results of primary streamer propagation by means of (a) an ultra high-speed camera and (b) a streak camera.
Fig. 4 Streak image taken with the backlight and the synchronized current waveform. (Applied voltage: 22.0 kV, streak time: 100 ns, space: 431 μm)
Fig. 5 Streak images without the backlight and the synchronized current waveforms. (a) (Applied voltage: 22.0 kV, streak time: 100 ns, space: 431 μm) (b) (Applied voltage: 22.0 kV, streak time: 47.5 ns, space: 221 μm)