Mário Janda

Transient spark: a dc-driven repetitively pulsed discharge and its control by electric circuit parameters

The paper presents an analysis of electrical characteristics of streamer-to-spark transition discharge in air at atmospheric pressure named transient spark (TS). The TS is applicable for flue gas cleaning or bio-decontamination and has potential in plasma shielding, combustion and flow control applications. Despite the dc applied voltage, TS has a pulsed character with short (~10–100 ns) high current (>1 A) pulses, with repetitive frequencies 1–20 kHz. Estimation of the temporal evolution of electron density shows that ne ≈ 1016 cm−3 at maximum and ~1011 cm−3 on average are reached using relatively low power delivered to the plasma (0.2–3 W). Thanks to the high repetition frequency, ne between two current pulses does not fall below a critical value and therefore plasma exists during the whole time. A detailed analysis of the TS control by electrical circuit parameters is presented. With appropriate circuit components, the current pulse tail (>1 mA) can be extended and the electron density can be held above ~1013 cm−3 for several tens of μs.

Time-Resolved CRDS Measurements of the N2(A3Σu+) Density Produced by Nanosecond Discharges in Atmospheric Pressure Nitrogen and Air

Cavity ring-down spectroscopy (CRDS) is used to measure the number density of N2(A3Sigmau+) metastables produced by nanosecond repetitively pulsed discharges in nitrogen and air preheated at 1000 K and atmospheric pressure. The densities of N2(A) are inferred from the absorbance of the Q1(22) and Q3(16) lines of the (2 <-- 0) vibrational band of the first positive system (B3Pig - A3Sigmau+) of N2 at 769.945 nm. The procedure for determining the temporal evolution of the density of metastable from the measured ring down signals is presented. The maximum number densities are in the range of 10(14)-10(15) molecules cm-3 for air and nitrogen discharges, respectively. In nitrogen, the decay of the N2(A) density is shown to be a second-order process with a rate coefficient of 1.1 x 10(-9) cm3 s-1 at 1600 K with a factor of 2 uncertainty. In air, the decay is estimated to be 1 order of magnitude faster than that in nitrogen owing to quenching by atomic and molecular oxygen. Furthermore, the rotational temperature is determined by comparison of CRDS measurements and simulations of several rotational lines of the (2 <-- 0) band of the first positive system of N2 between 769.8 and 770.7 nm. The rotational and vibrational temperatures are also determined by comparison of optical emission measurements and simulations of the second positive system of N2 between 365 and 385 nm. In these CRDS measurements, we achieved a temporal resolution down to 50 ns.

Effects of air transient spark discharge and helium plasma jet on water, bacteria, cells, and biomolecules

Atmospheric pressure DC-driven self-pulsing transient spark (TS) discharge operated in air and pulse-driven dielectric barrier discharge plasma jet (PJ) operated in helium in contact with water solutions were used for inducing chemical effects in water solutions, and the treatment of bacteria (Escherichia coli), mammalian cells (Vero line normal cells, HeLa line cancerous cells), deoxyribonucleic acid (dsDNA), and protein (bovine serum albumin). Two different methods of water solution supply were used in the TS: water electrode system and water spray system. The effects of both TS systems and the PJ were compared, as well as a direct exposure of the solution to the discharge with an indirect exposure to the discharge activated gas flow. The chemical analysis of water solutions was performed by using colorimetric methods of UV-VIS absorption spectrophotometry. The bactericidal effects of the discharges on bacteria were evaluated by standard microbiological plate count method. Viability, apoptosis and cell cycle were assessed in normal and cancerous cells. Viability of cells was evaluated by trypan blue exclusion test, apoptosis by Annexin V-FITC/propidium iodide assay, and cell cycle progression by propidium iodide/RNase test. The effect of the discharges on deoxyribonucleic acid and protein were evaluated by fluorescence and UV absorption spectroscopy. The results of bacterial and mammalian cell viability, apoptosis, and cell cycle clearly show that cold plasma can inactivate bacteria and selectively target cancerous cells, which is very important for possible future development of new plasma therapeutic strategies in biomedicine. The authors found that all investigated bio-effects were stronger with the air TS discharge than with the He PJ, even in indirect exposure.

The streamer-to-spark transition in a transient spark: a dc-driven nanosecond-pulsed discharge in atmospheric air

We present a study of the streamer-to-spark transition in a self-pulsing dc-driven discharge called a transient spark (TS). The TS is a streamer-to-spark transition discharge with short spark duration (?10?100?ns), based on charging and discharging of the internal capacity of the electric circuit with repetition frequency 1?10?kHz. The TS can be maintained under relatively low energy conditions (0.1?1?mJ?pulse?1). It generates a very reactive non-equilibrium air plasma applicable for flue gas cleaning or bio-decontamination.Thanks to the short spark current pulse duration, the steady-state gas temperature, measured at the beginning of the streamers initiating the TS, increases from an initial value of ?300?K only up to ?550?K at 10?kHz. The streamer-to-spark transition is governed by the subsequent increase in the gas temperature in the plasma channel up to ?1000?K. This breakdown temperature does not change with increasing repetition frequency f. The heating after the streamer accelerates with increasing f, leading to a decrease in the average streamer-to-spark transition time from a few ?s to less than 100?ns.

Fast imaging of intermittent electrospraying of water with positive corona discharge

The effect of the electrospraying of water in combination with a positive direct current (dc) streamer corona discharge generated in air was investigated in this paper. We employed high-speed camera visualizations and oscilloscopic discharge current measurements in combination with an intensified charge-coupled device camera for fast time-resolved imaging. The repetitive process of Taylor cone formation and droplet formation from the mass fragments of water during the electrospray was visualized. Depending on the applied voltage, the following intermittent modes of electrospraying typical for water were observed: dripping mode, spindle mode, and oscillating-spindle mode. The observed electrospraying modes were repetitive with a frequency of a few hundreds of Hz, as measured from the fast image sequences. This frequency agreed well with the frequency of the measured streamer current pulses. The presence of filamentary streamer discharges at relatively low voltages probably prevented the establishment of a continuous electrospray in the cone?jet mode. After each streamer, a positive glow corona discharge was established on the water filament tip, and it propagated from the stressed electrode along with the water filament elongation. The results show a reciprocal character of intermittent electrospraying of water, and the presence of corona discharge, where both the electrospray and the discharge affect each other. The generation of a corona discharge from the water cone depended on the repetitive process of the cone formation. Also, the propagation and curvature of the water filament were influenced by the discharge and its resultant space charge. Furthermore, these phenomena were partially influenced by the water conductivity.

Measurement of the electron density in Transient Spark discharge

This paper presents our measurements of the electron density in a streamer-to-spark transition discharge, which is named transient spark (TS), in atmospheric pressure air. Despite the dc applied voltage, TS has a pulsed character with short (~10–100 ns) high current (>1 A) pulses, with a repetition frequency on the order of kHz. The electron density ne ~ 1017 cm−3 at maximum is reached in TS with repetition frequencies below ~3 kHz, using relatively low power delivered to the plasma (0.2–3 W).The temporal evolution of ne was estimated from the resistance of the plasma discharge, which was obtained by a detailed analysis of the electric circuit representing the TS and the discharge diameter measurements using a fast intensified charge-coupled device (iCCD) camera. This estimate was compared with ne calculated from the measured Stark broadening of several atomic lines: Hα, N at 746 nm, and O triplet at 777 nm. Good agreement was obtained, although the method based on the plasma resistance is sensitive to an accurate determination of the discharge diameter. We have found that this method is also limited for strongly ionized plasmas. On the other hand, a lower ne detection limit can be obtained by this method than from the Stark broadening of atomic lines.

Imaging of Transient Spark in Atmospheric Air by Fast iCCD Camera

A transient spark (TS) pulsed discharge of streamer-to-spark transition type is studied using fast iCCD camera and fast photomultiplier tube. Emission profiles and images of single TS pulse at different frequencies help to understand the influence of repetition frequency on the streamer-to-spark transition process.

Two Photon Absorption Laser Induced Fluorescence Study of Repetitively Pulsed Nanosecond Discharges in Atmospheric Pressure Air

Nanosecond repetitive discharges generated by high voltage pulses in a pin-to-pin electrode configuration in atmospheric pressure air are currently used to stabilize lean flames, for the purpose of reducing pollutant concentrations. The goal of this work is to gain an understanding of the plasma-flame stabilization mechanism. Two-photon absorption laser induced fluorescence was employed here for the measurement of atomic oxygen, that is considered to be the key species for stabilization mechanism. Time resolved measurements of the atomic oxygen number density during plasma formation and decay were performed. The hypothesis that the plasma creates reactive O via a two-step mechanism is partially demonstrated.

Study of transient spark discharge focused at NOx generation for biomedical applications

The paper is focused at nitrogen oxides generation by transient spark (TS) in atmospheric pressure air. The TS is a DC-driven self-pulsing discharge with short duration (~10-100 ns) high current pulses (>1A), with the repetition frequency 1-10 kHz. Thanks to the short spark duration, highly reactive non-equilibrium plasma is generated, producing ~300 ppm of NOx per input energy density 100 J.l-1. Further optimization of NO/NO2 production to improve the biomedical/antimicrobial effects is possible by modifying the electric circuit generating the TS.

Electrical and optical study of DC driven repetitively pulsed transient spark discharge in atmospheric air

We present a study of electrical properties and optical emission of transient spark (TS) - streamer-to-spark transition discharge in atmospheric pressure air. TS was succesfully applied for flue gas cleaning or biodecontamination and has a potential in plasma shielding, combustion, and flow control applications. Despite the DC applied voltage, TS has a pulsed character with short (∼10 –100 ns) high current (>1 A) pulses, with repetitive frequencies 1–20 kHz. Estimation of the temporal evolution of electron density ne using discharge diameter measured by time-resolved iCCD imaging, supported by preliminary spectroscopic ne measurements from Hα broadening, show that ne ≈ 10 16 -10 17 cm −3 at maximum and ∼10 11 cm −3 in average are reached using relatively low power delivered to the plasma (0.2–3 W). Thanks to the high repetition frequency, ne between two current pulses does not fall below a critical value and therefore plasma exists during the whole time. A detailed analysis of the TS control by electrical circuit parameters is presented. With appropriate circuit components, the current pulse tail (>1 mA) can be extended and the electron density can be held above ∼10 13 cm −3 for several tens of μs.