The Effects of External Parallel Direct Current Magnetic Field on a Cold Atmospheric Pressure Argon Plasma Jet

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1 International Research Journal of Applied and Basic Sciences 014 Available online at ISSN X / Vol, 8 (7): Science Explorer Publications The Effects of External Parallel Direct Current Magnetic Field on a Cold Atospheric Pressure Argon Plasa Jet Reza Safari 1 1. Departent of Atoic and Molecular Physics, Faculty of Basic Sciences, University of Mazandaran, Babolsar, Iran Corresponding Author eail: reza8_safari@yahoo.co ABSTRACT: In this work, external direct current agnetic field effect on the atospheric pressure plasa jet was investigated, experientally. The agnetic field was produced using a Helholtz coil configuration. It was applied parallel to the jet flow. The strength of the agnetic field was Tesla between the two coils in parallel application. It was shown that the plasa gas flow plays the ain role in agneto-active collision-doinated plasa. The effect of plasa fluid velocity on the jet eission was discussed, qualitatively. It was observed that the external agnetic field has effect on plasa jet in parallel application. The easureents revealed that the plasa jet irradiance increases in parallel field. The forer was attributed to increasing plasa nuber density and in turn shrinks plasa jet nuber density. As a result, it was concluded that the plasa fluid velocity is responsible for such trends though the agneto-active plasa reains isotropic. Keywords: Collisional; Isotropic; Magneto Active Plasa; Plasa Fluid Velocity INTRODUCTION Recently, uch attention has been focused on the atospheric pressure plasa jet and its applications such as processing technology, engineering, sterilization and treatent such as deposition and etching. In these applications, it is iportant to identify and study the plasa jet characteristics under various conditions. Fro this point of view, uch experiental and nuerical investigation has been carried out to recognize non-theral plasa jet characteristics (Schutze et al., 1998; Lu et al., 005; Walsh et al., 010). Various types of atospheric pressure plasa jet with different configurations have been reported, where ost of the jets are working with noble gas ixed with a sall percentage of reactive gases, such as O. Plasa jets that operate with noble gases, can be classified into four categories, i.e. dielectric-free electrode (DFE) jets, dielectric barrier discharge (DBD) jets, DBD-like jets and single electrode (SE) jets that in this research, we applied DBD structure and pure argon gas. The DBD jet devices can be operated either by khz ac power or by pulsed dc power. The length of the plasa jet can easily reach several centieters and depends on gas type and operating voltage. There are several advantages of the non-theral plasa jets (Lu et al., 008). First of all, the plasa gas teperature is close to roo teperature and second, there is no arcing because of the use of the dielectric barrier. These two characteristics are very iportant for plasa edicine application, where safety is a strict requireent. The excitation frequency is iportant since it influences the behavior of the electrons and the ions. The atospheric plasa sources can be classified regarding their excitation ode. Three groups are then highlighted: the direct current (DC) and low frequency discharges; the plasas which are ignited by radio frequency waves and the icrowave discharges (Tendero et al., 006). In this work, the atospheric pressure plasa jet was driven by a sinusoidal alternating high voltage power supply 15 kv with very low frequency, 10 khz. In order to study the plasa characteristics under various conditions, soe researchers studied the external agnetic field effects on theral plasa jet in vacuu chaber because it can be controlled by a strong agnetic field (Koike et al., 004; Koike and Ono, 008; Ono et al., 006; Ono et al., 007). The results showed that the theral plasa jet was constricted by strong agnetic field and several paraeters such as nuber density, teperature and light intensity can be influenced. In our knowledge, the issue of external agnetic field effect, on non-theral atospheric pressure plasa jet has not been studied, yet. In this work, an experiental study was carried out to exaine the external direct current agnetic field effects on non-theral atospheric pressure plasa jet. Here after,

2 plasa jet in external agnetic field is naed agneto-active atospheric pressure plasa jet. Optical eission spectroscopy apparatus and iaging technique were utilized to quantify the plasa jet behavior under different conditions. In recent work, we deal with a collisional agneto-active atospheric pressure plasa, therefore, the behavior of the transport phenoena sees not to be the sae as a typical low pressure agneto-active gas discharge, although the strength of the direct current agnetic field is oderately high. For a while, look at obility, conductivity and diffusion tensors in the presence of external agnetic field. These tensors are given by (Krall and Trivelpiece, 1973) T 0 T 0 KT T 0, T 0, D q (1) Where,,, and,, are obility and conductivity tensor eleents, respectively. Diffusion T T tensor D is proportional to obility tensor. K is Boltzann s constant, T is electron teperature and q is the electron charge. q q T c c nq T c c c nq c q () nq (3) where, and c are electron collision frequency for oentu transfer and electron cyclotron angular frequency, respectively. n and are the electron nuber density and ass, respectively. Now, let us exaine the agnitude of collision frequency and cyclotron frequency for the electrons in argon plasa. Without loss of generality, assue the electron teperature in the following estiations to be T e = 1 ev for both atospheric and low pressure plasas. Refer to reference (Raizer, 1997), one can find that for an atospheric pressure plasa jet, s and c rad s of B=4000 G. While for a typical low pressure plasa at p=1torr, with external direct current agnetic field s with the sae agnetic field strength as before. It is seen that in our case pressure case, c and c c 7 10 rad. s , while in low This eans at atospheric pressure, when c 1, collisions ake the agneto-active collisional plasas to be isotropic for transport phenoena in which obility, conductivity and diffusion play roles. Although the applied agnetic field is oderately high, but, 0 T. This also holds for conductivity. As a result, the aforeentioned tensors lead to be a scalar coefficient for agneto-active atospheric pressure plasa jet in the presence of external agnetic field. This characterization indicates that the agneto-active atospheric pressure plasa jet description is not the sae as low pressure agneto-active plasas. As we treated the atospheric pressure plasa jet under external agnetic field, we presented analytical odels for agneto-active atospheric pressure plasa jet behavior using MHD fluid equation. This paper is organized as follows: In section, we present the experiental setup and ethods, in section 3, we discuss about the experiental results. Discussions and analytical odels will be given in section 4. Finally, conclusions will be given in section 5. EXPERIMENTAL SET UP AND METHODS Figure 1 shows the experiental setup and ethods in this study. The atospheric pressure plasa jet consists of a tube of Pyrex glass with a length of 35. Powered electrode was set inside the Pyrex tube and grounded ring electrode was attached to the surface of the Pyrex nozzle. The atospheric pressure plasa jet was driven by a sinusoidal alternating high voltage power supply 15 kv with frequency of 10 khz. 896

3 The plasa was generated at the gas gap between two copper electrodes and exited into the surrounding air outside the nozzle (Fig. 1a). The flow rate of working gas, argon, was 5 lit/in that controlled with ass flow controller. In addition, the jet length was about 0 which easured fro the nozzle orifice. The axiu width of the jet was 3. Typical V-I characteristics of plasa jet is shown in Fig.. Figure1. (a) Scheatic picture of the APPJ set up (b) agnetic field strength in parallel state (c) parallel application of the direct current external agnetic field on the APPJ. 897

4 Figure. V-I charestirstics of plasa jet The strength and configurations of the external agnetic field in parallel state were shown in Fig. 1b and Fig. 1c, respectively. The agnetic field was produced by Helholtz coil configuration and it was applied parallel and transverse to the jet flow. For deriving the Helholtz coil, we utilized two direct current power supplies (30 V, 0 A). In order to characterize the Helholtz coil configurations, nuerically, we first estiated the produced agnetic field by flowing current up to 18 A which was supplied by DC power supply(30 V, 0 A). In figure 3, the scheatic drawing of each coil is seen. Figure 3. Geoetrical feature of the coil carrying axiu DC current 0 A. Finally, to copute the agnetic field flux on the Helholtz coil axis, we obtained the following relation for each coil t NI t t t t t t t t Bz z Ln r r z z Lnr r z z Lnr1 r1 z z Lnr1 r1 z (4) To ake coil, two parallel wires were used, so I is input current of each turn (up to 9 A) while output current of power supply is 18 A. N is the nuber of turns in each row (40 turns). Nuber of coil turn is 1600 turns, t is the coil thickness and it was 60, 0 is pereability of free space. is the wire diaeter equals to r ) and outer ( r ) 1.5, z is the distance fro the coil center that the field strength is easured there. Inner ( 1 radius of the coil was 3 and 83, respectively. The resistance of each coil was 1.5 Oh. Separation between the two coils in parallel state was 6 c. The easureents showed that there is good agreeent between coputational and easured agnetic field strength. In order to increase parallel 898

5 agnetic field strength, we inserted ferrite cores into the coils. The diension of the ferrite core was Finally, the agnetic field strength applied to plasa jet, was at focused point for optical intensity easureent, about 0. Tesla, in the case of the axiu specified current of power supply, I=18 A. The focused point for intensity easureent by spectroscopy and photography ethods was shown in fig1a. In this point, the width of jet is about 3. To record the eission of the jet, we eployed UV-IR copact wide range n spectroeter (S-100, Solar-Laser syste). To capture the photograph of the jet and process the intensity profile, a digital caera (SONY PSC-HX Megapixel) was used. During optical eission spectroscopy, argon eission spectra was easured at vertical position to the jet direction and to the agnetic field direction. The relationship between the eission intensity, I, and the optical radiation intensity, I, is estiated by the following expression I I d (5) P where, is the wavelength. After running the jet, the teperature of plasa jet increases gradually. For exaple: it raises fro roo teperature to 50 C in 0 inutes. it leads to the plasa nuber density increase. The atospheric pressure plasa jet irradiance increases gradually due to gas teperature increase. Teperature increent takes place both with and without agnetic field. To decopose gradually increent of teperature effect fro that of the agnetic field, spectroscopy was taken in discrete and continuous fors. In discrete for, by spectroscopy, argon spectra eission was collected with optical probe, 60 seconds after ignition. Then the jet was turned off for several inutes for cooling. Then it was generated again and its eission was recorded in the presence of the agnetic field. This ethod guarantied the sae teperature for plasa easureents. In the continuous for, the plasa jet wasn t turned off during the easureents; instead, the external agnetic field was switched off in tie intervals. Plasa jet was generated and after 10 seconds, by spectroscopy, the jet eission was collected, then after 60 seconds, its eission was recorded again. In the next process, agnetic field was applied and after 60 seconds, the jet eission spectra was recorded. Then its eission was collected after 60 seconds in agnetic field-free and in final process, agnetic field was applied again and plasa jet eission was recorded. Both continuous and discrete fors were carried out by photography in conjunction with optical eission spectroscopy. EXPERIMENTAL RESULTS Application of the direct current external agnetic field has effect on the non-theral plasa jet radiation. In the discrete case, Figure 4 shows coparison between argon eission spectra in the presence and absence of the external agnetic field in the parallel state. As shown in Figure 4, the eission intensity increased at all wavelengths P Figure4. Coparison between argon eission spectra in the presence and absence of the parallel external agnetic field 899

6 The variation of the optical radiation intensity obtained fro iaging technique and spectroscopy ethod, in the discrete for, was shown in Figure 5. In the presences of the parallel external agnetic field, the optical radiation intensity of the jet increased. Figure5. The variation of the optical radiation intensity in the presence of the parallel agnetic fields, by (a) iaging technique and (b) Spectroscopy ethod As entioned, iage analysis software was utilized to obtain the optical radiation in transverse and axial directions with respect to the flow velocity. Figure 6 shows parallel agnetic field effect on the optical radiation intensity distribution along with the jet flow in the discrete for. Figure6. The variation of the optical radiation intensity in the presence of the parallel agnetic field, along the jet flow direction, captured by the caera In presence of parallel agnetic field, optical intensity in direction of the centerline axis, increased. These results indicate that the distribution of optical radiation intensity in this direction is in good agreeent with vertical direction. In the continuous for (without turning off the jet), the external agnetic field is applied in two tie intervals. Figure 7 shows the optical radiation intensity that was recorded by the spectroscopy ethod in the case of parallel agnetic field. As can be observed, by applying the external parallel agnetic field, the optical radiation intensity increased while in field-free case, it was increased due to gradually increent of the plasa jet teperature. 900

7 Figure7. The variation of the optical radiation intensity in the presence of the parallel agnetic field, in the continuous for by (a) Spectroscopy ethod and (b) Iaging technique DISCUSSIONS AND ANALYTICAL MODELS In this paper, the optical eission spectroscopy and the iaging technique were utilized to quantify the plasa jet behavior under the external agnetic field. The distribution of the optical radiation intensity obtained fro raw iage files, agree well with that of the spectral intensity in the continuous and discrete situations. Parallel field In the parallel state, the jet assebly inserted between the Helholtz coils. In this state, as shown in Fig. 1c, both agnetic field and the plasa jet axis are in Z-direction. Figure 8 depicts the flow velocity vectors of the plasa jet in three distinct zones. Zone 1 illustrates the space between the electrodes where the plasa initiates, zone depicts the nozzle orifice where it is contracted and zone 3 shows effluent plasa jet with negligible excitation electric field. One can use single fluid MHD equation to investigate the plasa jet. In zones 1 and 3, the flow velocity is parallel to the external agnetic field, therefore the ter VB0 in MHD Oh s law. In steady state, in zone, MHD Oh s law can be written as J E V B ( 6 ) Where E, is induced electrostatic electric field, V describes the plasa jet flow velocity, B is the external agnetic field, J is the induced current outside the electrodes and is the plasa conductivity. 901

8 Figure 8. Three distinct zones for the flowing plasa gas: zone 1, represents inter-electrode region having parallel velocity with B ; zone, represents nozzle orifice with radial velocity and zone 3, depicts effluent plasa jet having parallel velocity with B Regarding to the plasa jet assebly that illustrated in Fig. 1a, the nozzle was contracted led to radial flow velocity during exiting the nozzle. Then, the flow velocity has two coponents at the nozzle region (zone ), parallel and perpendicular to the external agnetic field V V V r z Where, V r is the radial flow and (7) V z is the flow velocity of the plasa jet along with the external agnetic field. It can be seen that the radial flow induces an aziuthally plasa current density J during exiting the nozzle orifice, J ( V B E ) (8) r z This process reduces plasa loss to the walls led to enhancing plasa nuber density and plasa jet irradiance as well. It is seen that the induced current density is proportional to the jet flow velocity and external agnetic field strength. Another reason for increasing plasa nuber density can be addressed to the previous works on the agneto-active plasas (Sohbatzadeh et al., 004; Sohbatzadeh and Latifi, 004). As illustrated in Figs. 1(a) and 8, the electric field between the electrodes has two coponents at zone 1, parallel and perpendicular to the external agnetic field that can be described by E t E t E t r z Our previous works showed that the transverse electric field (9) E r between the electrodes increases in a agneto-active plasa. This process can lead to further ionization and produces ore charged particles, i.e., increases plasa nuber density as well. CONCLUSIONS An experiental study was carried out to clarify non-theral atospheric pressure plasa jet behavior under the application of the external agnetic field, in the parallel for. It was concluded that by applying external agnetic field parallel to the jet axis, irradiance of the plasa jet can be affected and changed considerably. It was confired that in the presence of a agnetic field parallel to the centerline axis of the jet, the optical intensity increases. The increase of light intensity was attributed to plasa loss decrease by agnetic force in zone. The plasa considered here is collisional; therefore the obility and diffusion coefficient could not be affected by the external agnetic field, substantially. As a result, the related obility and conductivity tensors are to be scalar coefficients for agneto-active atospheric pressure plasa jet in the presence of external agnetic field. This investigation confired that the agneto-active atospheric pressure plasa description is not the sae as that of low pressure. It was concluded that the plasa fluid velocity is responsible for changes in agneto-active plasa jet irradiance via agnetic force V B, where V is the flow velocity vector. The results of this work can be used to control the plasa jet nuber density. REFERENCES Koike K, Ono N, Watanable Y, Musha K Measureent on the excitation teperature of argon plasa jet under strong agnetic field. Vacuu. 73: Koike K, Ono N Light intensity analysis of plasa jet constriction with applied agnetic field. Vacuu. 83: 5 8. Krall NA, Trivelpiece AW Principles of Plasa Physics. McGraw-Hill, New York. Lu X, Jiang Z, Xiong Q, Tang Z, Hu X, Pan YK An 11c long atospheric pressure cold plasa plue for applications of plasa edicine. Appl. Phys. Lett. Vol

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