TRANSIENT SPARK – DC DRIVEN NANOSECOND PULSED DISCHARGE IN ATMOSPHERIC AIR Mário Janda 1 , Adriana Niklová 1 , Viktor Martišovitš 1 , Zdenko Machala 1 1 Division of Environmental Physics Faculty of Mathematics, Physics and Informatics Comenius University, Mlynska dolina F2 84248 Bratislava, Slovakia E-mail: janda@fmph.uniba.sk We introduce a DC-driven pulsed discharge named transient spark (TS): a repetitive streamer-to- spark transition discharge with short spark duration (~10-100 ns), based on charging and discharging of internal capacity C of the reactor with repetition frequency f ≈ 1-10 kHz. TS generates very reactive non-equilibrium air plasma and is applicable for flue gas cleaning, bio- decontamination or other applications, since it can be maintained at relatively low energy conditions (0.1-1 mJ/pulse). Streamer-to-spark transition is governed by the increase of the gas temperature Tg in the plasma channel. Initial Tg at the beginning of the streamer is ~300 K, though it increases with frequency up to ~450 K at 10 kHz. The transition to spark occurs at ~1000 K. This heating accelerates with increasing f, leading to a decrease of the average streamer-to-spark transition time from a few μs to less than 100 ns. 1. Introduction Atmospheric pressure plasmas in air generated by electrical discharges present considerable interest for a wide range of environmental, bio-medical and industrial applications, such as air pollution control, waste water cleaning, bio-decontamination and sterilization, or material and surface treatment [1-5]. New types of discharges are therefore still being developed and studied, with a focus on efficiency, power requirements, stability, reliability and simplicity [6]. A novel type of transition discharge in air at atmospheric pressure named transient spark (TS) is presented here. TS is a filamentary streamer-to-spark transition discharge initiated by a streamer, which transforms to a short (~10-100 ns) high current (~1-10 A) spark pulse due to the discharging of the internal capacity C of the reactor. TS is based on charging and discharging of C and a repetition frequency of this process from 1 to 20 kHz can be achieved [7]. We observed significant differences between two modes of TS with small and high repetition frequencies [8], studied by time-integrated optical emission spectroscopy. In order to understand the fundamental phenomena related to the evolution of TS and its changes due to increasing repetition frequency, we employed in this study a photomultiplier tube with fast 2.2 ns rise time and appropriate narrow band optical filters, as well as a 2-m monochromator coupled with ICCD camera with 2 ns gate, in order to monitor time evolution of the emission of excited species and of the temperature. 2. Experimental set-up Experiments were carried out at room temperature in atmospheric pressure air with a radial flow of about 20 cm/s. The distance between stainless steel needle electrode and planar copper electrode (point-to-plane configuration) was 4 mm. A DC High Voltage (HV) power supply connected via a series resistor (R = 6.56-9.84 MΩ) limiting the total current was used to generate a positive TS discharge. The discharge voltage was measured by a high voltage probe Tektronix P6015A and the discharge current was measured on a 50 Ω or 1 Ω resistor shunt. The 1 Ω resistor shunt was used when we focused on TS current pulse itself, whereas the 50 Ω resistor shunt was used to measure current from the streamer. Both voltage and current signals where recorded by a 200 MHz digitizing oscilloscope Tektronix TDS2024. The emission spectra were obtained using a 2-m monochromator Carl Zeiss Jena PGS2 covering UV and VIS (200-800 nm) and providing spectral resolution of 0.05 nm, cuopled with an intensified CCD camera (Andor Istar). The iCCD camera was triggered by a home-made generator of 5 V rectangular pulses with rise time less than 5 ns. This generator was triggered directly by the current signal, causing