Investigation of pulsed corona discharges in water by Fast imaging diagnostics

Wladimir An, Kurt Baumung, and Hansjoachim Bluhm

Forschungszentrum Karlsruhe,

Institute for Pulsed Power and Microwave Technology,

P.O. Box 3640, 76021 Karlsruhe, Germany

Pulsed underwater corona discharges are considered to be an effective method for killing bacteria, yeasts, and other microorganisms. Nevertheless, many of the physical phenomena causal to the initiation and propagation of the streamers and to the production of oxidants are not well understood yet. This is why we are investigating various properties of positive streamers in point-plate geometry by fast imaging techniques. Two high-resolution intensified CCD-cameras with minimum gate times of 3 ns are used to record light emission, shadowgraph, and Schlieren pictures, and Mach-Zehnder interferograms. By imaging streamers using an intermediate 300 L/mm reflective grating we were able to take temporally and spatially resolved emission spectra covering the 280 nm to 800 nm wavelength range.

Limiting the voltage to just above the inception level delays the formation of secondary streamers (SS) and allows to observe the development of primary streamers (PS). These bush-like structures start from a single root-point and consecutively branch to ~100 individual tips spanning perfect hemispheres with diameters reaching up to 1mm. The propagation velocity is about 2-3 km/s. Compared with the ohmic current no clear current signal could be assigned to the PS. Light emission is weak compared with SS and limited to a few main branches. The continuum-like emission spectrum could be due to extreme broadening of hydrogen Lyman lines. Intense light emission by the SS starts at one of the tips at the periphery of the PS. A single luminous channel advances in the direction of the root-point whereas a bush-like structure of ~10 branches develops into the hemisphere towards the cathode at 30-40 km/s. Thereof, only 2-3 persist for several microseconds until the end of the discharge. Light emission lasts a few 10 ns per branch switching alternately between the branches at repetition rates of 5-100 MHz. The light bursts can be assigned to current peaks of 1-2 A. The diameter of the active branches increases for about 100 µs up to 0.5 mm. Then the channels collapse. The initial radial expansion rate is 300m/s suggesting driving pressures in the channel of ~5 kbar. Schlieren images reveal many faint, short-lived (100 ns) ~1mm-long twigs growing out of the branches. From interferograms, pressure pulse durations of 20-30 ns, peak pressures of a few kbar, and channel diameters <10µm could be derived for these small channels. Spectrally resolved images show extreme broadening of the hydrogen lines and distinct broadening of O^I-lines indicating electron densities n_e>10¹⁹cm^-3 at the propagating tips of the SS. As the channel expands, densities drop to ~10¹⁵cm^-3, the luminous channel remaining limited to a fairly narrow central zone. Hydroxyl emission in the 309 nm-band shows ~300ns after SS onset.

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