Pulse positive streamer corona discharges in water solution with a different conductivity have been investigated in reactors with the needle-plate and coaxial electrode geometry. A special composite anode was used in the coaxial geometry. With such an anode hundreds of streamers were generated at each voltage pulse. Production of H, O and OH radicals by the discharge was proved by emission spectroscopy and formation of H 2 O 2 and degradation of phenol was demonstrated by chemical methods. Assuming that the broadening of the Hα line profile was caused by the dynamic Stark effect, plasma with an electron density over 10 18 cm −3 was generated during the initial phase of voltage pulse in the both reactors in spite of the very different electrode geometry and wave-forms of voltage pulses. Production of OH radicals was most effective at solution conductivity below 100 µS cm −1 .

Production of ozone generated by the gas phase pulse positive corona discharge above water was investigated for different discharge gap spacing (2.5-10 mm), applied input power (2-45 W), and gas composition (oxygen mixtures with argon or nitrogen). Ozone concentration increased with increasing power input and with increasing discharge gap. The production of ozone was significantly affected by the presence of water vapor formed through the vaporization of water at the gas-liquid interface by the action of the gas phase discharge. The highest energy efficiency for ozone production was obtained using high voltage pulses of approximately 150 ns duration in Ar/O 2 mixtures with the maximum efficiency of 23 g/kWh for 40% argon content.

Quantitative analysis of ultraviolet radiation from the pulsed corona discharge in water with needle-plate electrode geometry (∼1-3 J pulse −1 ) was performed using the potassium ferrioxalate actinometry. Photon flux J 190-280 and radiant energy Q 190-280 of the UV light emitted from the discharge at spectral region 190-280 nm was determined in dependence on the applied voltage (17-29 kV, positive polarity) and the solution conductivity (100-500 µS cm −1 ). The intensity of the UV radiation strongly increased with increasing water conductivity and applied voltage. Depending on the applied voltage the determined photon flux varied by more than two orders of magnitude within the range of solution conductivities 100-500 µS cm −1 . It was found that photon flux from the discharge may be directly related to the discharge pulse mean power P p as J 190-280 = 44.33 P 2.11 p (quanta pulse −1 ). A significant role of UV radiation in the production of hydrogen peroxide and bacterial inactivation by the corona discharge in water has been identified. As the solution conductivity increased the yield of H 2 O 2 produced by the discharge decreased due to increasing photolysis of H 2 O 2 accounting for up to 14% of the total decomposition rate of H 2 O 2 . As regards bactericidal effects, it was estimated that the UV radiation contributes about 30% to the overall inactivation of Escherichia coli.

We employed the techniques of time-resolved intensified charge-coupled device (ICCD) microscopy and spectroscopy to register basic morphologic and emission fingerprints of microdischarges produced in deionized water. Fast rise-time positive high-voltage pulses (full width at half maximum of ∼7 ns and amplitude of ∼100 kV) in a point-to-plane electrode geometry produced micro-discharges, either periodically or in a single-pulse regime with the energy of ∼0.1 J dissipated during a single discharge event. Time resolved ICCD images evidence typical streamer-like branched filamentary morphology. Luminous discharge filaments show very fast and approximately linear initial expansion of the length with propagation velocity of ∼2×10 5 m s −1 . When the HV pulse reaches its maximum value, the length of the primary luminous filaments reaches ∼1.3 mm. After initial expansion, the length of luminous filaments collapses and can be characterised by velocity of ∼1.9×10 4 m s −1 . The first collapse is followed by a second slightly slower expansion, which is driven by the arrival of a reflected HV pulse, and which can be roughly approximated by propagation velocity of ∼1.5×10 5 m s −1 . The second collapse (occurring after second expansion) proceeds at a nearly identical velocity compared with the first one. By combining two ICCD based techniques, we have been able to associate, for the first time ever, characteristic emission spectra with the most important phases of the micro-discharge development. The UV-vis-NIR emission spectra show a broad-band continuum evolving during the first expansion and collapse, followed by the well-known H I /O I atomic lines occurring together with continuum emission during the second expansion and collapse. We conclude that bound-free and free-free radiative transitions are basic emission characteristics of the nanosecond discharge initiation mechanism in liquid water which does not involve the formation of vapour bubbles.

Time resolved multichannel emission spectroscopy has been applied to study emission in the 200-500 nm spectral range produced by a pulsed positive corona discharge. The discharge was driven in coaxial geometry by an HV power supply (100 kV/1 kA) at atmospheric pressure in nitrogen, and in and mixtures. Emission of NO- (-) and 2.PG (-) bands has been studied in order to trace the development of and electronic states during both discharge and post-discharge periods. Analysis of spectroscopic data indicates an evolution of the electron mean energy during the discharge pulse in the range 20-1 eV and post-discharge kinetics of the NO and electronic states which is predominantly controlled by , v = 0,1) metastable species.