1. Introduction

The study of electric spark discharge in atmospheric air dates back to the late nineteenth and beginning of the twentieth centuries [1]. Even so, this kind of plasma discharge continued to attract researchers for over a century. This object was and still driven by the wide many applications related to this type of discharge. Some of the early applications include prevention of undesired explosions of ammunitions, and combustible dusts and vapor atmospheres [2,3], and engines spark plugs [4,5,6,7]. One of the main experimental tools used in particle physics research is the wire spark chamber [8]. Atmospheric micro discharges have also been used in medical and biological sterilization processes [9,10,11] With the last three decades advances in microchip and nanotechnology, atmospheric micro discharge studies have acquired ever-increased interest [12-16].

The electrodes configurations used to generate micro discharges vary widely depending on the particular application. These include the simple parallel plate [17,18,19], multi-needle to cylinder, [20], wire to plate, and wire to cylinder [21,22] configurations are very common.

In line with other plasma physics studies, micro discharges are produced using different types of bias voltage supplies. In addition to the simple DC biasing, pulsed [23,24, 25], 50 Hz [26], radio frequencies [27,28] and microwave frequencies have also attracted interests [29,30]. Many aspects related to works on micro discharged are covered in a recent review paper presented by Brandenburg et al (2023) [31].

The types of research sited in literature are mostly of the experimental type. Even so, some modelling and simulation works have been carried out [32,33,34]. However, and in spite of the large volume of experimental works on spark gaps in general, there seems to be no general theory, and only few experimental works related to the evolution of micro discharges in the sub millimetre gaps at atmospheric pressure [35]. This may be due to the lack of interest by industry in this range of discharge gaps compared to the more attractive microns, and several millimetre ranges on the two sides. Under the circumstances, the case for further experimental works attempting to throw more light on the subject can be regarded as a justified one.

One further point is worth stating. From observation of all above listed literature works and others, one may notice that a large percentage of these works are concerned with studying optical properties and spatial distributions of micro-discharges, and some macro electrical properties like voltage_ current characteristics rather that the electrical nature of the pulses produced. It is the purpose of this work to present experimental data on the later aspect of this subject.

2. Experimental setup

The experimental setup is shown in Figure (1-a). It consists of two circular well surface polished Aluminium electrodes 5.5 cm radius and 1 cm thick. One of the electrodes is fixed. The upper electrode is vertically movable using a micrometre screw moving mechanism. The inter-electrode distance between the electrodes is fixed using the required number of stacked circular plastic rings with inner and outer radii of 5 and 6 cm respectively as shown in Figure (1-b).

The thickness of each ring is 0.1 mm. Discharge measurements were carried out using between 1 and 10 such

rings allowing variation of inter-electrode spacing between 0.1-1 mm. Each inter-electrode spacing is further checked using the micrometre reading in addition to the number of stacked 0.1 mm dielectric rings. The two electrodes are connected to 10 kV dc power supply via the 200-kilo ohm, one-watt power rating ballast resistor R1. Due to the very nature of electric discharge in air at atmospheric pressure, electric discharge current cannot sustain itself for long periods. This will result in ON and OFF discharge current pulses when the applied voltage is sufficient enough. This results in discharge current fast pulses rather than continuous discharge current as it is the case in glow discharge. These current pulses are sampled from the 10 ohms sampling resistor R2. The value of R2 was set such that the maximum voltage pulse does not exceed the maximum one-volt input rating of the computer sound card used as the data acquisition device.

This corresponds to about 100 mA of pulse electric current, which is much higher than current pulsed observed during the experiment. Consequently, and in spite of the high DC voltages present within the circuit, voltages across R2 never exceed few tenth of one volt. The use of the sound card in conjunction with MATLAB software as our data acquisition system allows for 44000 Hz sampling rate that represent a time resolution of about sec. The voltage is increased between zero to several kV in steps of 0.1 kV, and discharge current data are acquired over a one second period each time. The acquired data are saved on hard disk for further analysis. The electrodes are cleaned after each run with a particular discharge cap.

The experiments were performed in a dry atmosphere with almost zero humidity. Typical data obtained just at, and after the full evolution of the discharge are shown in Figures (2-a) and (2-b) respectively. Figure (2-c) shows a magnified micro discharge current pulse.

Figure 1: Experimental setup (a) General setup. (b) Electrodes and plastic rings configuration

Figure 2: typical current pulses obtained for discharge cap of 0.5 mm (a) at voltage of 2.9 kV, which is slightly above the start of breakdown. (b) At 5 kV. (c) One typical pulse magnified.

3. Results and discussion

3.1 Breakdown Voltage

It is reasonable at first to present the results related to the breakdown voltage to the electrodes separation distance (d). The criterion used here to define the breakdown voltage is the minimum voltage at which we registered one current pulse at least over a period of one second. This is measured by repeating the pulse data acquisition process several times at each voltage value below the breakdown voltage in 0.1 kV steps. This puts an experimental error on the breakdown voltage of 100 Volts. The results are shown in Figure (3). The data tend to show an almost linear relationship between breakdown () voltage in kV and spark gap length in mm. Furthermore, this linear relationship extrapolates well to describe the experimental data for post one-millimetre spark gap published by Fujita et al. [36]. This linearity relation can be well described by the fitted equation

(1)

Figure 3: Micro discharge breakdown voltage versus discharge gap

3.2 I-V Characteristics

The relation between applied voltage and mean sparking discharge current is studied for all discharge gaps. For clarity reason, and space considerations, results for only four discharge gaps are presented in Figure (4). In this figure, the mean discharge current is defined as the total charge transfer (Q) during the one second of data acquisition. If the pulse height across the 10-ohm current sampling resistor is (H) volts, pulse width at half maximum is () and number of pulses registered during one second is N, then the current is given by;

(2)

The results for all discharge gaps indicate that after the discharge threshold voltage, the discharge current tend to follow almost linear relationship to the applied voltage. This is in contrast to the case with ordinary self-sustained glow discharge where a fast rise in current is usually observed.

Figure 4: Current – Voltage characteristics of micro spark discharge for inter-electrode distances 0f 0.3, 0.5, 0.7, and 0.9 mm

Assuming almost constant pulse widths (τ), the value of discharge current is the result of the combined effect of changes in the number of pulses per second and the pulse height. In order to investigate which of the two effects is predominant, these two properties are studied separately for all discharge gaps. Figure (5) shows the effect of increasing applied voltage on the number of pulses per second for the four discharge gap values presented in Figure (4). Results for all other discharge gaps are much similar. Results indicate that after the initial setup of the discharge, the pulse repetition rate tends to show an almost linear increase with applied voltage

Figure 5: Total number of pulses per second for the discharge gaps in figure (4)

Figure 6 shows the results of the effect of applied voltage upon the mean pulse height. It is clear that the mean pulse height shows only a slight increase with applied voltage compared to that of the pulse rate dependence. This leads one to conclude that the increase in the overall discharge current in mainly due to the increase in the number of pulses rather that the individual pulses

amplitudes. More detailed pulse height analysis is presented in the following section.

Figure 6: Mean pulse height dependence upon applied voltages for the discharge gaps presented in figure (4).

3.3 Pulse Height Analysis

In order to gain more insight about the properties of pulses, the pulse height distributions for all spark gaps and all voltages are studied using histogram plots. for all voltages and electrodes spacing. However, the smaller number of pulses at lower voltage values does not allow performing a full assessment of the voltage dependence. Even so, there does not seem to be a significant voltage dependence at higher voltage values. For this reason, the data for all voltages for every discharge gap are pooled and plotted. Typical examples of results for four discharge gaps are presented in Figure (7). It is clear from these and other similar plots that the pulse height seems to have an almost Gaussian distribution centred around 0.07 amperes. The discharge gap does not seem to have any significant effect on pulse heights.

(a)

(b)

(c)

(d)

Figure 7: Pulse heights distribution for discharge gaps (a) 0.1, (b) 0.6, (c) 0.8 and (d) 1 mm

COnclusions

From the above analysis, it can be concluded that the pulse height of micro spark discharges across sub-millimetre spark gaps is independent of the spark gap separation. The increase in overall discharge current after reaching the threshold discharge voltage is mainly due to the increase in the number of pulses (pulse repetition rate). It is also demonstrated that the threshold micro spark discharge voltage is linearly related to the discharge cap.

References

Williams E. H., “The Nature of Spark Discharge at Very Small Distances.” PHYSICAL REVIEW, Vol. XXXI., No.3,September, (1910)

BRITTON L. G.,” Avoiding Static Ignition Hazards in Chemical Operations” CENTER FOR CHEMICAL PROCESS SAFETY of the American Institute of Chemical Engineers (1999)

Thomas H. Pratt T. H., “Electrostatic Ignitions of Fires and Explosions” American Institute of Chemical Engineers (2000)

Arun1 J., Kumar S. P., Venkatesh M., Giridharan A. S. “Reliability Study on Spark Plugs Using Process Failure Mode and Effect Analysis”International Journal of Engineering Research and Development Vol.9, Iss. 2 ( 2013), PP. 13-21

WANG Q., ZHENG Y., YU J., JIA J. “Circuit model and parasitic parameter extraction of the spark plug in the ignition system” Turk J Elec Eng & Comp Sci, Vol.20, No.5, (2012)

Peters M. F., Blackburn G. F., Hannen P. T., “Electrical Character of the Spark Discharge of Automotive Ignition Systems” Journal of Research of the National Bureau of Standards, Vol. 19,( October 1937)

Idota Y. “ Measurement of the Equivalent Ratio in Spark Plug Gap for Low Emission Combustor” R&D Review of Toyota CRDL Vol. 38 No. 4 (2003)

Krige J. “ History of CERN III “ Google Books Elsevier (1996)

Lindsey F. Gaunt, Clive B. Beggs, and George E. Georghiou, “Bactericidal Action of the Reactive Species Produced by Gas-Discharge Nonthermal Plasma at Atmospheric Pressure: A Review “IEEE TRANSACTIONS ON PLASMA SCIENCE, Vol. 34, No. 4, (AUGUST 2006)

Shimizu T., Zimmermann J. L. and Morfill G. E., “Inactivation effect using surface microdischarge plasma “22nd International Symposium on Plasma Chemistry July 5-10; Antwerp, Belgium (2015)

Zaaba S. K., Hirayama-Katayama K., Akitsu T., Shimizu N., and Imanishi Y., “Study on the Antifungal Effect Atmospheric-Pressure Microplasma Excited by High-Repetition-Rate Inductive Energy Storage “International Journal of Plasma Environmental Science and Technology Vol.3, No.2, (SEPTEMBER 2009)

Shimizu, Y.; Koga, K.; Sasaki, T.; Mariotti, D.; Terashima, K.;

Koshizaki,N., "Localized deposition ofmetallicmolybdenum particles In ambient air using atmospheric-pressure microplasma," Microprocesses and Nanotechnology, 2007 Digest of papers , vol., no., pp.174-175, 5-8 (Nov. 2007)

Benedikt J., Raballand V., Yanguas-Gil A., Focke K. and von

Keudell A., “Thin film deposition by means of atmospheric pressuremicroplasma jet “Plasma Phys. Control. Fusion 49 (2007) B419-B427

Raballand V, Benedikt J., Hoffmann S., Zimmermann M., and

von Keudell A., “Deposition of silicon dioxide films using an atmospheric pressure microplasma jet “JOURNAL OF APPLIED PHYSICS 105, 083304 (2009)

McKenna J., Schmidt M., Maguire P., Mariotti D., “ Synthesis of ilicon Carbide Nanoparticles using an Atmospheric Pressure Microplasma Reactor” 30th ICPIG, August 28th– September 2nd Belfast, Northern Ireland, UK (2011)

Galaly A. R., “Nano-Coating Process for Si [1 0 0] Wafer Using

Atmospheric Pressure Plasma Jet (APPJ)” Journal of Modern Physics, (2012), 3, 1031-1039, Stefanovic I., “Light emission profiles of a parallel plate, dc micro discharge in different discharge modeExperimental Physics II , University of Bochum, Germany Application Note (June 2011)

Kothnur P. S., Yuan X. , and , Raja L. L., “Structure of direct-current microdischarge plasmas in helium “App. Phys. Let. Vol 82 No. 4 27 (Jan 2003)

Machala Z., Marode E., Laux C. O., and Kruger C. H., “ DC Glow Discharges in Atmospheric Pressure Air “J. Adv. Oxid. Technol. Vol. 7, No. 2, (2004)

Wang X., Yang Q., Yao C., Zhang X. and Sun C., “Dielectric

Barrier Discharge Characteristics of Multineedle-to-Cylinder Configuration “Energies, 4, 2133-2150 (2011)

Eden J. G., Gao C. J., W. J., Ostrom N. P., and Park S. J. “Microdischarge array-assisted ignition of a high-pressure discharge:Application to arc lamps” App. Phys. Lett. Vol. 79, No. 26 24 (Dec 2001)

Nayak G., Du Y., Brandenburg R., and Bruggeman P. J. “Effect of air flow on the micro-discharge dynamics in an array of integrated coaxial microhollow dielectric barrier discharges” 2017 Plasma Sources Sci. Technol. 26 035001DOI 10.1088/1361-6595/aa56a4

Tay W. H., Kausik S. S., Yap S. L., and Wong C. S., “Role of

secondary emission on discharge dynamics in aatmospheric

pressure dielectric barrier discharge “ Physics of Plasmas 21, 044502 (2014)

Rousseau A., and Aubert X., “Self-pulsing microplasma at mediumpressure range in argon “J. Phys. D: Appl. Phys. 39 (2006)

Nguyen-Smith R. T., Böddecker A. , Schücke L., Bibinov N., Korolov I. , Zhang Q. Z. , Mussenbrock T. , Awakowicz P., and Schulz J. “μs and ns twin surface dielectric barrier discharges operated in air: from electrode erosion to plasma characteristics” Plasma Sources Sci. Technol. 31 (2022) 035008 (16pp) https://doi.org/10.1088/1361-6595/ac5452

Naz M. Y., Ghaffar A. , Rehman N. U. , Shukrullah S., and Ali M. A., “OPTICAL CHARACTERIZATION OF 50 HZ ATMOSPHERIC PRESSURE SINGLE DIELECTRIC BARRIER DISCHARGE PLASMA” Progress In Electromagnetics Research M, Vol. 24, 193–207, 2012

Guo Y. B. and Honga F. C. N. “Radio-frequency microdischarge

arrays for large-area cold atmospheric plasma generation” App. Phys. Lett. Vol 82, No. 3 20 (Jan 2003)

Shah A. K., Shrestha R., Sah R. L., Nakarmi J. J., and Mishra L. N. “EXPERIMENTAL STUDY OF DIELECTRIC BARRIER DISCHARGE IN AN ATMOSPHERIC AIR PRESSURE AND ITS ELECTRICAL CHARACTERIZATION” JP Journal of Heat and Mass Transfer Volume 30, 2022, Pages 135-150 http://dx.doi.org/10.17654/0973576322060

JASIŃSKI M., KROPLEWSKI L., ZAKRZEWSKI Z, and MIZERACZYK J., “ATMOSPHERIC PRESSURE MICROWAVE MICROPLASMA SOURCES “Chem. Listy 102, s1322−s1326 (2008)

MIZERACZYK J. , HRYCAK B., JASIŃSKI M., DORS M.,

“ Low-temperature microwave microplbiodecontamination “ PRZEGLĄD ELEKTROTECHNICZNY R. 88 NR 9b(2012)

Brandenburg R·, Becker K. H., and Weltmann K. D.” Barrier Discharges in Science and Technology Since 2003: ATribute and Update” Plasma Chemistry and Plasma Processing(2023)43:1303133https://doi.org/10.1007/s11090-023-10364-5

Tong L., “Simulation of an Atmospheric Pressure Direct Current

Microplasma Discharge in He/N2 “ Proceedings of the COMSOL conference Boston US (2011)

Gunther Steinle G., Neundorf D., Hiller W. and Pietralla M., “Two-dimensional simulation of filaments in barrier discharges “J. Phys. D: Appl. Phys. 32 (1999)

Georghiou G. E., Papadakis A. P., Morrow R., and Metaxas A. C. Numerical modelling of atmospheric pressure gadischarges leading To plasma production” J. Phys. D: Appl. Phys. 38 (2005)

Bondarenko P. N., Emelyanov O. A., Shemet M. V., “ Investigation of Single Dilelectric Barrier Discharge in Submillimeter Air Gap: Uniform Field “ Technical Physics Vol. 59 No. 6 (2014)

Fujita H., Kouno T., Noguchi Y., and Ueguri S., “Breakdown

voltages of gaseous N2 and air from normal to cryogenic

temperatures” CRYOGENICS . (APRIL 1978) 195-200.