Development of a low-cost atmospheric non-thermal plasma ... Engineering/2441...Development of a low-cost atmospheric non-thermal plasma jet and its characteristics in air and nitrogen

Eur. Phys. J. Appl. Phys. (2016) 76: 10803 DOI: 10.1051/epjap/2016160235

Development of a low-cost atmospheric non-thermal plasma jetand its characteristics in air and nitrogen

Tarek M. Allam, Kamal M. Ahmed, Mohamed A. Abouelatta, Sayed A.Ward, Ahmed A. Lashin,and Hanaa M. Soliman

Eur. Phys. J. Appl. Phys. (2016) 76: 10803DOI: 10.1051/epjap/2016160235

THE EUROPEANPHYSICAL JOURNAL

APPLIED PHYSICS

Regular Article

Development of a low-cost atmospheric non-thermal plasma jetand its characteristics in air and nitrogen

Tarek M. Allam1, Kamal M. Ahmed1, Mohamed A. Abouelatta2, Sayed A. Ward2, Ahmed A. Lashin1,a,and Hanaa M. Soliman1

1 Plasma and Nuclear Fusion Department, Nuclear Research Center, Atomic Energy Authority, Egypt2 Electrical Engineering Department, Faculty of Engineering – Shoubra, Benha University, Egypt

Received: 14 June 2016 / Received in final form: 7 August 2016 / Accepted: 4 October 2016c© EDP Sciences 2016

Abstract. This paper deals with the development of a low-cost atmospheric non-thermal plasma jet (ANPJ)which was designed and operated previously in our laboratory. The purpose of the developed design witha small size less than 4% of the previous volume is to obtain a more portable device which holds promisefor various fields of applications. The discharge is operated separately with compressed air and nitrogengas with flow rates varied within the range of 3–18 L/min. The plasma plume length and thickness aremeasured as a function of the gas flow rate and input voltage Vinput within the range of 3–18 L/min and2–6 kV respectively. The results showed that for nitrogen gas, the maximum values of the plume lengthand thickness are 20 mm and 1.3 mm respectively at a flow rate of 12 L/min and Vinput = 6 kV. Results ofelectrical characterization at Vinput = 6 kV such as discharge voltage, discharge current, the mean consumedpower and energy showed that the maximum values of these parameters are obtained at a flow rate of12 L/min. The developed design is found to be saving up to 65.47% and 68.54% of the consumed powercompared to the previous design in the case of air and N2 respectively. The new proposed configurationfor the developed ANPJ offers more suitable characteristics than the earlier designs, especially for nitrogengas.

1 Introduction

Plasma is a special state of gaseous matter which is gener-ated by different techniques from the ionization processesof neutral gases. In particular, cold plasmas generated inatmospheric pressure have recently attracted much atten-tion. An atmospheric non-thermal plasma jet (ANPJ)generates plasmas in open air thus eliminating the dis-advantage of using vacuum chambers. ANPJs represent apromising source for various industrial applications, andoffer unique processing of surface modifications withoutwet chemistry and without any damage to the treatedsample [1–4].

ANPJs represent a rapidly developing technology ofgreat application promise. They represent ideal devicesbecause of their unique properties such as low gas temper-ature, i.e., ANPJ does not cause any thermal or electricshock upon contact, highly reactive chemical species andeasy plasma dynamics control [5–7].

It is well known that the length of the plasma plumerepresents an important factor to be considered. If the

a e-mail: [email protected]

ANPJ can be developed such that it can give out a longerplasma plume, its industrial applications may expand dra-matically. In this field, many experiments have beenperformed on the ANPJ to obtain a longer plasma plume.The ANPJ was developed by Takemura et al. [8]. in sucha way that the plasma jet has been elongated to about20 cm. Barankova and Bardos [9] reported a hybridhollow electrode-activated discharge (H-HEAD) which cangenerate a long plasma plume up to 15 cm long in theopen air. Lu et al. [10] offered a room temperatureatmospheric pressure plasma jet device which is capableof generating a plasma plume up to 11 cm long in theopen air. Another atmospheric cold plasma jet capable ofgenerating a plasma plume up to 4.4 cm was suggested byZhang et al. [11].

The present work aims at developing the ANPJdevice built in our laboratory with a different geome-try to the traditional designs. The proposed ANPJ offerssavings in time, money and power consumption comparingwith previous devices. The length, thickness and electricalparameters of the plasma plume at different flow rates ofair and nitrogen and at various input voltages are inves-tigated and compared with the previous design to show

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(a)

(b)

Fig. 1. (a) Photograph of the developed device, (b) schematicdiagram of the gas valve and the inner electrode (anode).

the influence of the proposed device. The results of theexperimental measurements demonstrate the effectivenessof the proposed device.

2 Experimental setup

The proposed device utilizes a developed compact elec-trode system which consists of a gas valve attached withtwo gas burner orifices, cathode and anode electrodesrespectively. These two electrodes are separated by a rec-tangular sheet of insulating material made of ceramic.The components of the desired device are as follows.

2.1 Electrode system

The 32 W ANPJ, previously prepared in the laboratory,used two parallel aluminum electrode discs with diametersof 21, 9 mm respectively with thickness of 3 mm separatedby a Teflon insulator [12,13]. The Teflon sheet has a diam-eter of 21 mm and 1.5 mm thickness. The two electrodesand the insulator were assembled together and drilled witha hole of 0.8 mm diameter, through which air is flow-ing. The whole electrode system has been modified with adeveloped compact device. The developed device is a gasvalve with gas burner orifices (Fig. 1a). The schematicdiagram of the valve and the inner electrode is shown inFigure 1b.

The present device consists of two electrodes made ofbrass. The inner electrode (anode) has a thickness of 3 mmand a diameter of 8 mm and its nozzle has a diameter

Fig. 2. Two electrodes with the insulator.

of 0.5 mm. The outer electrode (cathode) is cut and fittedin the laboratory into a circular shape with a thicknessof 2 mm and a diameter of 7.5 mm and the nozzle hasa diameter of 0.4 mm as shown in Figure 2. The anodeis connected to one terminal of the power supply throughthe body of the valve while the cathode is connected tothe other terminal of the power supply directly throughan isolated cable.

The developed device saves the time and effort wastedin the laboratory for preparing the electrodes such asmachining, sizing, cutting and drilling the electrodes.Also, the developed electrode system is more economicalthan the previous system as the cost of the whole systemdoes not exceed $1.

For the developed device, the Teflon insulator, usedin the previous design, was exchanged with a rectangularpiece of ceramic (1.5 × 2 cm, 2 mm thickness and 1 mmnozzle diameter) to minimize the erosion area [14]. Table 1illustrates the difference between the previous [12,13] andthe developed design of the ANPJ device.

2.2 Electric circuit

The employed power supply is a commercially availableGearBox Neon power supply, Model EL 1000-30, whichis commonly used for Neon lighting and Neon signs. Thispower supply has a sinusoidal AC output of 10 kV, 30 mAand 20 kHz. It is self-protected through overload, opencircuit, earth leakage and short circuit protections.To control the input voltage to the power supply, itsinput terminals are connected to a (220/250 V, 12 A)Variac (Model TDGC2-KVA contact voltage regulator).The applied voltage is increased step by step through theVariac until the onset value is reached at which the dis-charge starts. This onset voltage was found to be 6 kV.

The schematic diagram of the plasma jet’s electric cir-cuit is demonstrated in Figure 3. The output terminalsof the power supply are joined to the electrodes of theplasma jet via a 2 mm single copper isolated cable. Thepower supply is connected in series with the electrode sys-tem via a high voltage resistor of 25 Ω and a 5 nF ca-pacitor. This high voltage resistor is used to protect theelectric circuit from high short-circuit current and to mea-sure the discharge current id(t) by measuring the voltage

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Table 1. Comparison between the two designs.

Previous design Developed design Reduction (%)Material of electrodes Aluminum Brass –

Cathode diameter (mm) 21 8 61.9Cathode nozzle diameter (mm) 0.8 0.5 37.5

Anode diameter (mm) 9 7.5 16.7Anode nozzle diameter (mm) 0.8 0.4 50

Insulator Teflon Ceramic –Total volume (cm3) 156 6 96

The widest area (cm2) 3.85 0.37 90Electrode system cost ($) 17 1 94

Fig. 3. Electric circuit of the plasma jet device.

across this resistor and then dividing it by 25 Ω. The volt-age across the 5 nF capacitor Vc(t), which is required forQ-V Lissajous figure analysis [15], is measured via a 21:1resistor divider voltage probe. The discharge voltageacross the anode and the cathode, Vd(t), is measured via a1000:1 resistor-divider voltage probe. Then, the three volt-age signals (Vd(t), Vc(t), and the voltage across the 25 Ωresistor) are acquired using a TDS 1002 Tektronix digitaloscilloscope via a 50 Ω coaxial cable and BNC connectors.

The working gas in our experiment (compressed airand nitrogen gas) is fed to the plasma jet device underconsideration with different flow rates ranging from 3 to18 L/min and controlled through an analogue flow meter(Model Air Liquide DYNAREG) with its needle valve.

The gas velocity, v, (defined as the gas flow ratedivided by the cathode hole area; 0.196 mm2) is esti-mated for compressed air and N2 to be in the range from254.69 m/s to 1528.12 m/s respectively at the previousflow rates ranged from 3 to 18 L/min. The Reynoldsnumber; Re is also calculated for air at different flow rates.Re is obtained from the following equation [16]:

Re =ρυd

η, (1)

where ρ, d and η are the gas density, cathode hole diame-ter and the gas viscosity respectively. For air with a flowrate of 12 L/min and gas velocity mentioned above, thecorresponding Re is 2580 at d = 0.5 mm for the viscos-ity of air at a temperature of 1500 K (which is the esti-mated temperature at the cathode inlet [17]); η is equalto 4.638 × 10−5 kg/m s at an air density of 0.235 kg/m3.

The diagnostic tools used in this work are a voltageprobe, to step down the high values of the voltage to safe,easily measured values, and the thermocouple thermome-ter, to measure the temperature of the plasma plume andthe components of the device.

3 Experimental results

The experimental results were repeated three times andthen an average was taken of these three results for eachdischarge condition under consideration.

3.1 Electrical parameters of the plasma jet device

The electrical parameters of the plasma jet device such asthe discharge voltage, discharge current, consumed power,energy and power efficiency were investigated at differentair and nitrogen flow rates in the range from 3 to 18 L/minand at 6 kV input voltage.

From the results of id(t) and Vd(t), the mean con-sumed power P , was estimated by the formula (traditionalmethod) [18]:

P =1T

∫id(t)Vd(t)dt, (2)

where T is the period time of the discharge voltage.The discharge voltage Vd(t), discharge current id(t)

and the mean consumed power waveforms were measuredfor air and nitrogen flow rates from 3 to 18 L/min.Figures 4 and 5 show Vd(t), id(t) and p(t) waveforms for

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Fig. 4. Voltage, current and power waveforms at 12 L/min (air).

Fig. 5. Voltage, current and power waveforms at 12 L/min (nitrogen).

gas flow rate of 12 L/min. The period time of the dischargevoltage, T , is 50 μs. For compressed air, Vd(t), id(t) andp(t) values are 5.75 kVp-p, 31.22 mAp-p and 12.7 W respec-tively, while for nitrogen gas, these values are 4.24 kVp-p,37.2 mAp-p and 10.27 W respectively.

Also, the mean consumed power was measured usingLissajous figure analysis (V-Q figure) which requires thevalues of the discharge voltage Vd(t) and the charge Q(t)on the 5 nF capacitor which in turn can be estimated usingthe equation [15]:

Q(t) = CVc(t), (3)

where Vc(t) is the voltage measured across the 5 nFcapacitor, C.

The mean power consumed was determined byemploying a V-Q Lissajous figure and multiplying theareas of these figures by the frequency of the discharge

voltage. The Lissajous figures for both air and nitrogengas at the flow rate of 12 L/min are shown in Figure 6.

Figure 7 assesses the variation of the discharge volt-age and the discharge current with the flow rate for bothair and nitrogen gases. It was confirmed that a flow rateof 12 L/min for both of compressed air and nitrogen isthe maximum condition for the proposed device opera-tion. It is important to note that the “off” state plasmapower is increased from 0.17 to 0.55 W for air and from0.15 to 0.34 W for N2 when the flow rate is increasedfrom 3 to 12 L/min. However, their values decreased from0.44 to 0.37 W for air and from 0.22 to 0.21 W for N2

when the flow rate increased from 15 to 18 L/min. Ata flow rate of 12 L/min for compressed air and nitrogengas, the mean consumed power per each voltage periodusing the Lissajous figure is 11.05 and 9.36 W respec-tively, while the mean energy is 0.553 and 0.468 mJrespectively.

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(a) (b)

Fig. 6. Lissajous figure at 12 L/min of: (a) air and (b) N2.

Fig. 7. Discharge voltage and discharge current as function of the flow rate for air and N2.

Figure 8 describes the variation of mean consumedpower by using the Lissajous figure and energy withdifferent flow rates for air and nitrogen at 6 kV input volt-age. The figure clarifies that for compressed air, the meanconsumed power and energy are increased from 3.63 Wand 0.182 mJ to 11.05 W and 0.553 mJ as the flow rateis increased from 3 to 12 L/min and then they decreaseto 7.83 W and 0.392 mJ as the flow rate is increased to18 L/min. For nitrogen gas, the mean consumed powerand energy are increased from 4.04 W and 0.202 mJ to9.36 W and 0.468 mJ as the flow rate is increasedfrom 3 to 12 L/min and then they decreased to 5.86 Wand 0.292 mJ as the flow rate is increased to 18 L/min

i.e., the maximum value of the above parameters isdetected at a flow rate of 12 L/min for compressed air andnitrogen.

The comparison between the maximum value of theelectrical parameters, at 6 kV input voltage and 12 L/minflow rate, for the proposed developed design and those ofthe previous plasma jet device [12,13] is summarized inTable 2. This table concludes that the consumed power isdecreased from 32 W to 11.05 W in the case of air, andfrom 29.75 W to 9.36 W in the case of N2. In other words,the newly-developed plasma jet saves 65.47% and 68.54%of the consumed power compared to the previous designin the cases of air and N2 respectively.

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Fig. 8. Mean consumed power and energy as function of flow rate for air and nitrogen.

Table 2. Electrical parameters of the previous and the developed designs for the two operating gases.

Parameter Previous design Developed designAir Nitrogen Air Nitrogen

Discharge voltage (kVp-p) 6.8 6.8 5.75 4.24Discharge current (mAp-p) 62 56 31.22 37.2

Traditional mean 33.6 30.37 12.15 9.93consumed power (W)

Lissajous mean 32 29.75 11.05 9.36consumed power (W)Mean energy (mJ) 1.68 1.29 0.553 0.468

3.2 Plasma plume shape

The images of the plasma plume at different flow ratesof air and nitrogen are shown in Figure 9. The imageswere captured using a digital camera (Canon Model EOSKiss X7i). Its characteristics are an 18 mega-pixel CMOSsensor, a full-time ISO range of 100 to 12 800 and a DIGIC5 imaging processor.

ImageJ software was employed to measure thelength and the thickness of the plasma plume at each flowrate [19]. The length of the plasma plume was measuredfrom the cathode surface to the end of the plume lightwhile the width was measured as the maximum width ofthe plasma plume. ImageJ software was used to rescale im-ages from pixel into mm dimensions and to obtain precisemeasurements of the light length and thickness.

Figure 10 describes the variation of the length and thethickness of the plasma jet plume with the air and nitro-gen flow rate at input voltage of 6 kV. The plume lengthand thickness are increased with increasing gas flow ratebut at higher flow rates, an unstable plume is detected.This instability causes a decrease in the plume length

and thickness due to flow turbulence. The experimentalresults indicated that the maximum plume length of8.2 mm and the maximum plume thickness of 1.16 mm areobtained at an air flow rate of 12 L/min. Also, the max-imum plume length of 20 mm and the maximum plumethickness of 1.3 mm are obtained at a nitrogen flow rateof 12 L/min.

The plume length is increased with increasingflow rate until a critical number is reached. This number isrelated to the Reynolds number mentioned in equation (1).The critical Reynolds number is not universal and theprocess of the laminar to turbulent transition is influencedby the individual molecular properties of the gases [20].The transition from laminar to turbulent flow experimen-tally occurred at a flow rate greater than 12 L/min. Thistransition leads to the disturbance of the charged particlesand active species and hence the plume length is decreasedwith increasing gas flow rate.

By comparing the results of the plasma plume lengthfor the two operating gases, it is clearly seen that the max-imum plasma plume length is much longer for nitrogen gas(20 mm) than compressed air (8.2 mm).

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3 L/min 6 L/min 9 L/min 12 L/min 15 L/min 18 L/min

Fig. 9. Plasma plume images at different flow rates of N2 (left) and air (right).

Fig. 10. Plume length and thickness vs. air flow rates.

This is related to the presence of oxygen gas inthe compressed air injected to the plasma jet device.Previous work concluded that as the oxygen concentra-tion is increased in the working gas, the plume length isdecreased [8]. Oxygen gas is an electronegative gas whichhas a tendency to attract free electrons towards itself, sothe free electrons of the plasma plume are attracted to theoxygen molecules resulting in a shorter plasma plume incase of compressed air [1].

Plasma plume lengths for the previous and the devel-oped devices are listed in Table 3 for Vinput = 6 kV and12 L/min flow rate for both gases under consideration.

3.2.1 Plasma plume length and thickness vs. input voltage

The electron drift velocity is calculated according to therelation [21]:

vd = μeE, (4)

where vd, μe and E are the electron drift velocity, mobil-ity of electron and electric field respectively. In our exper-iment, the product (electron mobility with pressure; μep)

Table 3. Plume length for both of the two designs for the twogases.

Plume length Previous design Developed Increasingdesign ratio

Air 7 mm 8.2 mm 17.1%Nitrogen 14 mm 20 mm 42.9%

is 0.45 × 106 cm2 torr/V s. In the 12 L/min air case, thedischarge voltage is 5.75 kV, the pressure is 760 torr andthe gap distance between the anode and cathode is 2 mm,so the maximum electron drift velocity is estimated to be1.7 × 105 m/s.

Figure 11 expresses the plasma plume length and itsthickness as a function of the input voltage at flow rateof 12 L/min for compressed air and nitrogen. In general,the length and the thickness of the plasma plume aredecreased by decreasing the input voltage from the onsetvoltage; 6 kV to 2 kV, i.e., their maximum values aredetected at the onset voltage (6 kV).

This figure shows that the plume length is increasedby increasing the input voltage i.e., the electric field is

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Fig. 11. Plume length and thickness for the two gases vs. input voltage.

increased which leads to the drift velocity increasing.This gives rise to increases in the charge carrier and elec-tron density. This increase results in a rise in the plumelength. In other words, in non-thermal plasma, propaga-tion at higher electric fields can be provided simply byelectron diffusion in front of the discharge. The most spe-cific mechanism of discharge propagation is related to thepreliminary propagation of species as excited atoms andmolecules or products of chemical reactions providing aneffective electron detachment [22]. This mechanism givesa longer jet length.

This clarifies that both parameters (gas flow rate andinput voltage) have an effect on the plasma plume length.

4 Conclusion

The paper proposed a compact developed ANPJ devicewith lower cost, smaller volume and lower power con-sumption compared to the previous design. Its electricalparameters (discharge voltage, discharge current, meanconsumed power and mean energy) and plasma plumeshape (length and thickness) at different air and nitro-gen flow rates were examined. The mean power consumedwas computed using both the traditional method andLissajous figure analysis. This device offers power con-sumption of 34.53% and 31.46% for air and N2 respec-tively less than the previous design. The plasma plumelength is increased for both gases compared to the pre-vious design. All the experimental results confirmed thatthe proposed developed ANPJ has more suitable charac-teristics than the previous one, especially for nitrogen gas.The maximum values of the plume length, plume thick-ness and electrical parameters were detected at 12 L/minwith an input voltage of 6 kV. Thus, the performance ofthe device is found to be optimum at this condition. In thenear future, the developed ANPJ device will be used for

different applications such as surface treatment especiallyfor heat-sensitive materials, water treatment and biomed-ical applications.

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