PRELIMINARY INVESTIGATIONS OF A VERY LOW POWER ATMOSPHERIC PRESSURE HELIUM PLASMA

Transcription

1 PLASMA PHYSICS PRELIMINARY INVESTIGATIONS OF A VERY LOW POWER ATMOSPHERIC PRESSURE HELIUM PLASMA S.D. ANGHEL, A. SIMON Faculty of Physics, Babes-Bolyai University, M Kogalniceanu 1, RO , Cluj-Napoca, Romania anghels@phys.ubbcluj.ro, asimon@phys.ubbcluj.ro Received April, 009 An atmospheric pressure helium plasma needle type is studied. The plasma is generated at 714 khz (continuous wave) at very low power levels, ranging from 450 mw to 900 mw. It is in contact with a single electrode and is part of a resonant electric circuit. The emissive atomic and molecular plasma species were identified. Based on plasma optical emission, the electron number density and the excitation, vibrational, and rotational temperatures were determined. The gas temperature was measured with a thermocouple having the hot junction covered with a glass-cap. Key words: non-thermal plasma, optical emission, temperature, electron number density. 1. INTRODUCTION The scientific community who studies the plasma state has recently paid much attention to the development of atmospheric pressure plasma sources having the gas temperatures close to the room temperature and to use these plasmas for technologies applied up to now only under low-pressure conditions. The interest in this topic is dictated by a potential economic benefit from numerous non-thermal plasma technologies: plasma assisted chemical vapor deposition, etching, polymerization, protective coating deposition, toxic and harmful gas decomposition, sterilization and decontamination, electromagnetic wave shielding, polymer surface modifications, and so on. These new technologies can be now applied on the materials which can not be treated at low-pressure or can not support the temperature much higher than the room temperature. Such plasmas are placed in non-thermal plasmas category which were defined as having the gas temperature lower than combustion temperature (T gas < T combustion = 300 K) [1]. A relatively recent review paper presents the generation mechanisms, the appearance of the plasma states and physical characteristics of some atmospheric pressure discharges, which can have a non-thermal character and could be cold []. A general recipe for obtaining non-thermal plasmas at atmospheric pressure is reduction of the discharge size and/or its duration. This effect is known from plasma display technology and from corona research. Rom. Journ. Phys., Vol. 55, Nos. 1, P , Bucharest, 010

2 186 S.D. Anghel, A. Simon Starting from these considerations, different kind of atmospheric pressure non-thermal plasmas were generated, the most important being: plasma needle, plasma pencil, dielectric barrier discharge and plasma jet. Plasma needle was developed by Stoffels and was characterized having in view the electrical and optical characteristics [3], the action on bacterial cells [4] and in vivo treatments of biological tissues [5]. The studies have established that the positive effects of the cold non-thermal plasma treatment is not the effect of the thermal energy radiated by the plasma, but it is the effect of the action of the active species, mainly O and OH radicals, and nitric oxide. Puac extended the application area of the plasma needle studying the effect over the biological cells [6]. He also presents a voltampere characteristic of the plasma needle. Inactivation of bacteria and the interaction of the plasma with microbial cells were studied by Laroussi using plasma pencil and dielectric barrier discharge [7, 8]. Plasma pencil was for the first time studied by Janca [9], and plasma jet was used by Cheng for modification of superficial characteristics of polymeric fibers [10]. Recently, there were developed other kind of cold plasma sources (some of them being biologically compatible) at atmospheric pressure, the most representative being: hollow slot microplasmas tested for bacterial deactivation [11]; a surface discharge generated by combining continuous and alternative electric fields, plasma which generates hydrogen and is used for deactivation of airborne microbial-contaminants and for neutralization of indoor OH radicals, being harmless to the human body [1]; and the so named rf APGD torch, tested for treatment of cells and biological surfaces [13, 14]. Based on temperatures and electron number density determination, this work presents a characterization of a very simple laboratory-made atmospheric pressure non-thermal plasma source.. EXPERIMENTAL AND METHODOLOGY The experimental arrangement is shown in Fig. 1. It consists of four main parts: the waveform generator, the plasma torch, the feeding with plasma gas system and the spectrometer. The construction of the waveform generator and of the torch used for sustaining of very low power atmospheric pressure plasma and the calculation algorithm for estimating the plasma power were described in details elsewhere [15, 16]. The plasma gas (helium) was controlled by a Cole&Palmer flow rate regulator. The gas flow can be adjusted in the range of 0 6 l. min 1. The axial gas temperature was measured with a K-type thermocouple connected to a Mastech M345 multimeter. To avoid the hot junction of the thermocouple to become an extra-electrode of the discharge, it was covered with a Pyrex-glass cap. To establish occurring errors induced by the cap, comparative

3 3 Very low power atmospheric pressure helium plasma 187 measurements were performed at 73 K and 373 K with and without it, respectively. There are no significant errors in measured temperatures, only the response time increased from s to 7s. PLASMA TORCH PVC electrode tip axial viewing spectrometer BNC teflon PLASMA waveform generator 714kHz, sine 0-10 W plasma gas pressure and flow-rate control HELIUM Fig. 1 Experimental arrangement. Plasma optical emission was collected with two Ocean Optics High- Resolution Fibre Optic Spectrometers: HR4000 for wavelengths in the range of nm, and HR4000CG-UV-NIR for wavelengths in the range of nm. The spectrometers were controlled by the SpectraSuite software. All spectral plots are the result of 3 5 data acquisition, depending on the total emission intensity of the plasma. For the spectra displaying and labelling the Spectrum Analyzer 1.6 software was used [17]. It was also used to calculate the electron excitation temperature of the He atoms, T exche and the temperature of excitation of vibrational states of the N molecules, T vibrn by the Boltzmann plot method [18, 19]. For calculation of T exche, the neutral He atomic lines with the wavelengths of , , , and 78.3 nm were used. TvibrN was calculated using the heads of the molecular N bands from the nd positive system (C 3 Π u B 3 Π g ), with the bandheads at ( 4), (1 3) and (0 ) nm. The used spectral data were taken from ref. [0]. The temperatures of excitation of rotational states of the OH radicals, T rotoh and of the N ionic molecules, T were estimated by finding the best fit (chi-square rotn method) of the measured molecular spectra with the synthetic spectra generated by the LIBASE 1.5 spectral simulation software [1]. For OH radical was used the emission band (A Σ, ν = 0 X Π, ν = 0) with a prominent line at nm and for N molecule was used the emission band of the 1 st negative system with bandhead at nm (B Σ X Σ ). The electron number u g

4 188 S.D. Anghel, A. Simon 4 density was calculated based on the Stark broadening of hydrogen emission line H α, nm []. 3. RESULTS AND DISCUSSION Function of plasma power and helium flow rate, there are three different developing stages of the plasma (Fig. 1): point-like plasma, ball-shape plasma and jet-like plasma. 0.5 l min l min l min mw 580 mw 900 mw Fig. Plasma spatial evolution as function of gas flow rate and plasma power. The gas temperature depends both on the helium flow-rate and on the axial distance from the tip of the plasma sustaining electrode. Some aspects regarding the gas temperature are shown in Fig. 3 and Fig. 4. For a gas flow rate of 3 l. min 1 (Fig. 3, plot a) the plasma has a jet shape and its temperature is lower the 55 o C, although at the distances shorter than 10 mm the plasma is in contact with the glass-cap, and it flows slowly on its surface. For a gas flow rate of 1. l. min 1 (Fig. 3, plot b) the plasma fills the space between the plasma electrode tip and the outer PVC tube. For distances longer than 10 mm, the measured gas temperature is the same as the room temperature. The sudden increase of the temperature for distances shorter Gas temperature [ o C] (a) (b) Distance [mm] Fig. 3 Dependence of the plasma temperature on the axial distance between the thermocouple and the exit of the torch for gas flow rates: (a) 3 l. min -1 ; and (b) 1. l. min 1. The dc supply voltage of the waveform generator, E dc = 4.5 V. Gas temperature [ o C] (a) (b) Gas flow-rate [l.min -1 ] Fig. 4 Dependence of the plasma temperature on the gas flow rate for: (a) dc supply voltage of the waveform generator, E dc = 4.5 V and axial distance of 1 mm; and (b) dc supply voltage of the waveform generator, E dc = 3 V, and axial distance of 0.5 mm.

5 5 Very low power atmospheric pressure helium plasma 189 than 10 mm is determined by the developing of micro-arc discharges between the plasma electrode and the glass-cap. For higher gas flow rates, the gas flowing dynamics do not permit micro-arcs generation. This behaviour is also confirmed by the dependence of the gas temperature on the gas flow rate (Fig. 4, plot a). The graph presents a maximum for a gas flow rate of 0.75 l. min 1, when micro-arc discharges are very visible even if the discharge is developed only inside the torch. At gas flow rates lower than 0.5 l. min 1 the plasma has a pointed shape, its volume is very small and the temperature decreases. At gas flow rates higher than 1 l. min 1 the plasma takes a jet form, its volume expands outside the torch and the gas temperature decreases because of the convection and conduction cooling of the gas [15]. To fully clear up which are the conditions under that the unexpected microarc discharges are generated, the dependence of the gas temperature on the gas flow rate at an axial distance of 0.5 mm for two different dc supply voltages of the waveform generator, was studied (Fig. 4). For the lowest dc voltage (Fig. 4, plot b), the maximum gas temperature is about 44 o C, which means that the plasma electrode voltage is not sufficiently high for generating micro-arc discharges. Fig. 5 presents the dependences of the gas temperature and of the plasma power on gas flow rate. Both graphics present maximums at approximately the same gas flow rate ( l. min -1 ), which means that they can be correlated. The temperature is not linearly dependent on the plasma power, probably because of the plasma convection cooling at higher gas flow rate Fig. 5 (a) plasma power, and (b) plasma temperature, as function of the gas flow rate for dc supply voltage of the waveform generator, E dc = 3 V. Plasma power [mw] (b) (a) Gas temperature [ o C] Gas flow-rate [l.min -1 ] A typical emission spectrum of the helium plasma in the range of nm is shown in Fig. 6. Its composition presents the same atomic and molecular emission lines and bands to those previously reported [3, 13, 3], but their relative intensities differ. The differences rise from the different experimental conditions: operating frequency, plasma power, gas flow rate and plasma chamber construction (the volume of the back diffused air is different). Beside the above mentioned helium emission lines, atomic emission lines of oxygen ( and nm) and hydrogen (656.7 nm) and molecular bands of NO, OH, N and N are presented in the radiation spectrum. Excepting helium, the presence

6 190 S.D. Anghel, A. Simon N Fig. 6 Emission spectrum of plasma He flow rate, 0.9 l. min 1 ; plasma power, 500 mw. Intensity [a.u] OH He He NO N He He H He O O Wavelength [nm] in the plasma of the other atomic and molecular species is inevitable because of the back diffusion of the ambient air. Recently it was demonstrated that small level of impurities (particularly nitrogen from ambient air) could have an important influence in radiation of noble gas plasmas [4]. The presence of free radicals and ions in the plasma could be important in those applications based on chemically active species. In the wavelength range of nm there are few very weak NO molecular γ-bands due to the chemical conversion of N and O [5]. It follows the emission band of OH radical at nm. The OH radicals represent the result of the dissociation of H O molecules from the humid back diffused air caused by the collisions with accelerated electrons or with long life species presented in the plasma, especially * m with helium metastables, He [3, 13]. Beginning with the wavelength of 315 nm the UV spectrum is dominated by the emission of nitrogen molecules, the most representative being: the emissions of nd positive system of N (315.93, , and nm) and of 1 st negative system of N ( nm). It must be mentioned here that the N emission is attributed to Penning ionization of N with helium metastables [11, 3]. Otherwise, it is well known that nitrogen molecules are very effective at quenching the helium metastables [13]. The presence of the basic atomic line of hydrogen, H α, is due to the excitation of hydrogen atoms generated by dissociation of H O molecules under the action of the energetic electrons in the plasma [13, 5]. The characteristic temperatures and electron number densities for two different helium flow rates at the same dc supply voltage of the waveform generator (E dc = 3 V) are shown in Table 1. The two flow rates correspond to the first two spatial developing stages of the plasma, point-like and ball-shape, which could be used in those applications where very small volumes of plasma are required.

7 7 Very low power atmospheric pressure helium plasma 191 Q He [l. min 1 ] Plasma power [mw] Table 1 Characteristic temperatures and electron number densities T exche [K] T vibr N [K] T rotoh [K] T rotn [K] n e [cm 3 ] ± ± ± 51± ± ±96 470±4 540± It must be mentioned that the emission spectra used for plasma diagnostics were recorded in the axial viewing mode (see Fig. 1), so that the most important contribution to the emission is due to the superficial luminescence, which is a very luminous zone of the plasma located on the surface of the tip of the powered electrode (like negative glow in dc plasmas). This fact can explain why the rotational temperatures determined based on emission spectra are higher than the gas temperature measured via thermocouple. It can be seen that, excepting the helium excitation temperature, all the other parameters are increasing with the increase in helium flow rate from 0.5 l. min -1 to 1 l. min -1. An explanation of this behaviour could be the decrease of the weighting of the atomic processes against the molecular processes with the spatial developing of the plasma. This means that the plasma gas atoms are mainly excited in the superficial luminescence of the plasma. At a helium flow rate of 0.5 l. min -1 the plasma appears as a very small point (1 mm diameter, see Fig. ) located on the electrode. Like the cathode in a dc glow discharge, the processes responsible for generating and maintaining of the discharge take place in the proximity of the powered electrode. The fact that T rotn is higher than T rotoh confirms the assumption according to which, in the presence of nitrogen, the excitation energy of helium metastables is particularly transferred to nitrogen molecules to ionise and excite them. The electron number densities are those characteristic for glow discharges at atmospheric pressure. 4. CONCLUSION The plasma volume depends on the helium flow rate and on the plasma power, having three developing stages. The second developing stage, ball-shape plasma, is recommended for possible applications at temperatures close to the ambient temperature. In this stage, the plasma generates both electrons and helium ions (plasma gas) and other atomic and molecular species which can have a chemically active character: NO, OH, N, N, H and O. The plasma is far from thermal equilibrium, the characteristic temperatures being very different: T > vibr N T exche > T > T rotoh > T gas. At rotn distances longer than 1mm from the tip of the sustaining electrode the plasma is thermally non-aggressive, its temperature being lower than 55 o C.

8 19 S.D. Anghel, A. Simon 8 Acknowledgments. This study was supported by National University Research Council, Ministry of Education, Research and Innovation, Romania, Grant IDEI, code 70/009. REFERENCES 1. J.S. Chang, Physics and chemistry of plasma pollution control technology, Topical invited, 8th ICPIG, Prague A. Fridman, A. Chirokov, A. Gutsol, Non-thermal atmospheric pressure discharges, J. Phys. D: Appl. Phys. 38, R1-R4, I.E. Kieft, E.P. van der Laan, E. Stoffels, Electrical and optical characterization of the plasma needle, New Journ. Phys. 6, , R.E.J. Sladek, E. Stoffels, Deactivation of Escherichia coli by the plasma needle, J. Phys. D: Appl. Phys. 38, , E. Stoffels, I.E. Kieft, R.E.J. Sladek, L.J.M. van der Bedem, E.P. van der Laan, M. Steinbuch, Plasma needle for in vivo medical treatment: recent developments and perspectives, Plasma Sources Sci. Technol. 15, S169-S180, N. Puac, Z.L. Petrovic, G. Malovic, A. Dordevic, S. Zivkovic, Z. Giba, D. Grubisic, Measurements of voltage-current characteristics of a plasma needle and its effect on plant cells, J. Phys. D: Appl. Phys. 39, , M. Laroussi, C. Tendero, X. Lu, S. Alla, W.L. Hynes, Inactivation of bacteria by the plasma pencil, Plasma Process. Polym. 3, , M. Laroussi, D.A. Mendis, M. Rosenberg, Plasma interaction with microbes, New Journ. Phys. 5, , J. Yanka, M. Klima, P. Slavicek, L. Zajickova, HF plasma pencil new source for plasma surface processing, Surf. Coat. Technol , , C. Cheng, Z. Liye, R.J. Zhan, Surface modification of polymer fiber by new atmospheric pressure cold plasma jet, Surf. Coat. Technol. 00, , A. Rahul, O. Stan, A. Rahman, E. Littlefield, K. Hoshimiya, A.P. Yalin, A. Sharma, A. Pruden, C.A. Moore, Z. Yu, G.J. Collins, Optical and rf electrical characteristics of atmospheric pressure open-air hollow slot microplasmas and application to bacterial inactivation, J. Phys. D: Appl. Phys. 38, , H. Nojima, R.E. Park, J.H. Kwon, I. Suh, J. Jeon, E. Ha, H.K. On, H.R. Kim, K. Choi, K.H. Lee, B.L. Seong, H. Jung, S.J. Kang, S. Namba, K. Takiyama, Novel atmospheric pressure plasma device releasing atomic hydrogen: reduction of microbial contaminants and OH radicals in the air, J. Phys. D: Appl. Phys. 40, , V. Leveille, S. Coulombe, Design an preliminary characterisation of a miniature pulsed rf APGD torch with downstream injection of the source of reactive species, Plasma Sources Sci. Technol. 14, , S. Yonson, S. Coulombe, V. Leveille, R.L. Leask, Cell treatment and surface functionalisation using a miniature atmospheric presure glow discharge plasma torch, J. Phys. D: Appl. Phys. 39, , S.D. Anghel, A. Simon, An alternative source for generating atmospheric pressure non-thermal plasmas, Plasma Sources Sci. Technol. 16, B1-B4, S.D. Anghel, A. Simon, Measurement of electrical characteristics of atmospheric pressure nonthermal He plasma, Meas. Sci. Technol. 18, , Z. Navratil, D. Trunec, R. Smid, A software for optical emission spectroscopy problem formulation and application to plasma diagnostics, Czech. J. Phys. 56, B944-B951, 006.

9 9 Very low power atmospheric pressure helium plasma A. Montaser, D.W. Golightli, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, nd edn, New York: VCH, 199, G. Herzberg, Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules, Princeton NJ: Van Nostrand, 1950, NIST Atomic Spectra Database 1. H. Luque, D.R. Crosley, Stanford Research Institute (SRI) International Report MP , P. Kepple, H.R. Griem, Improved Stark profile calculations for the hydrogen lines Hα,Hβ,Hγ, and Hδ, Phys. Rev , V. Poenariu, M. R. Wertheimer, R. Bartnikas, Spectroscopic diagnostics of atmospheric pressure helium dielectric barrier discharge in divergent fields, Plasma Process. Polym. 3, 17 9, T. Martens, A. Bogaerts, W.J.M. Brok, J.V. Dijk, The dominant role of impurities in the composition of high pressure noble gas plasmas, Appl. Phys. Lett. 9, , R. Brandenburg, J. Ehlbeck, M. Stieber, T v Woedtke, J. Zeymer, O. Schluter, K.-D. Weltmann, Antimicrobial treatment of heat sensitive materials by means of atmospheric pressure rf-driven plasma jet, Contrib. Plasma Phys. 47, 7 79, 007.

10 194 S.D. Anghel, A. Simon 10