Continuous Visualization of Process of Positive Streamer Initiation, Propagation, Spark, and Bubble Expansion in Water

Significance of the topic

Understanding how positive streamers initiate and develop in water is critical for advancing applications in water treatment, dielectric insulation, and biomedical technologies.
Direct visualization at microsecond and micrometer scales uncovers fundamental mechanisms of plasma–liquid interactions, guiding optimized system design.

Objectives and study overview

This study aims to capture the entire sequence of events—from bubble nucleation and cluster formation through primary streamer initiation, propagation, spark transition, and bubble channel expansion—within a single high-voltage discharge in water.
The continuous imaging approach addresses inconsistencies inherent in multi-shot frame assembly methods.

Methodology and instrument

An experimental cell (10×10×45 mm quartz) filled with ultrapure water houses a needle high-voltage electrode and a wire ground electrode spaced by 2 mm.
Discharge is triggered by applying +14 kV to the needle electrode.
Visualization employs the Shimadzu HPV-X2 high-speed camera with microscope optics and laser backlighting.
Key recording parameters: 10 million fps frame rate, 50 ns exposure time, and 256 consecutive frames per discharge.

• Ultrapure water in a quartz cell
• Needle (HV) and wire (ground) electrodes, 2 mm gap
• High-voltage supply: +14 kV
• HPV-X2 camera: 10 Mfps, 50 ns exposure

Main results and discussion

Within 100 ns, microbubbles (~10 µm) appear at the needle tip; by ~1 µs, multiple bubbles coalesce into a cluster.
Localized protrusions on the cluster surface mark sites for primary streamer initiation.
The semi-spherical streamer propagates at ~2 km/s, pauses, then re-accelerates to reach the counter electrode at ~7.8 µs.
Spark discharge begins at ~7.9 µs, producing two channels: a finer channel with brief light emission and a thicker channel persisting until ~9.4 µs.
Subsequent heating expands the bubble channel to three times its original diameter by ~25 µs.

Benefits and practical applications

Single-shot visualization eliminates variability from repeated discharges, providing complete temporal evolution of streamer phenomena.
Data support design improvements in pulsed plasma reactors for water purification, advanced transformer insulation, and precision surgical plasma tools.
High temporal resolution enriches validation of computational models for streamer dynamics.

Future trends and possibilities

Combining ultrafast multi-angle imaging and spectral diagnostics will elucidate reactive species generation during discharge events.
Exploring higher repetition-rate pulsed discharges may reveal cumulative effects in continuous water treatment processes.
Integration of synchronized electrical measurements with imaging could clarify field enhancement roles in streamer branching and stability.

Conclusion

This work demonstrates the first continuous, single-discharge capture of positive streamer inception, propagation, spark transition, and bubble expansion in water at 10 Mfps.
The approach overcomes multi-shot limitations and provides comprehensive insight into rapid plasma–liquid interaction dynamics.

References

• H. Fujita et al., J. Appl. Phys., 113 (2013), 113304
• H. Fujita et al., EPL, 105 (2014), 15003
• H. Fujita et al., J. Appl. Phys., 116 (2014), 213301
• H. Fujita et al., IEEE Trans. Plasma Sci., 42 (2014), 2398–2399
• H. Fujita et al., J. Inst. Electrostat. Jpn, 39 (2015), 21–26