22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium Characterization of double dielectric barrier discharge and microwave plasma jets in argon at atmospheric pressure for biomedical applications F. Judée1,2, N. Merbahi1,2, G. Wattieaux1,2, J.-M. Plewa1,2 and M. Yousfi1,2 1 Université de Toulouse; LAPLACE-UMR5213, 31062 Toulouse, France 2 CNRS; LAPLACE-UMR5213, 31062 Toulouse, France Abstract: A microwave and a double DBD plasma jets are compared by using optical emission diagnostics to determine their rotational and excitation temperatures, electron density, UV-C irradiance and atomic O concentration. Keywords: plasma devices, optical spectroscopy, density measurement, UV-C irradiance 1. Introduction Low temperature plasmas ejected in open air are nowadays successfully used in many biomedical applications due to their ability to produce various active species at quasi-ambient plasma temperature. It is noteworthy that many plasma jet setups use helium carrier gas because it is easier to ignite helium plasma that requires less energy. However, in order to generate plasma jet with different active species, it is interesting to use other carrier gases such as argon which is renewable and less expensive than helium. This is why the present work deals with low temperature plasma jets using argon carrier gas. More precisely, two different setups will be considered. The first one is a double DBD induced plasma jet whereas the second one is a microwave (MW) induced plasma jet. Both jets are optically diagnosed in order to determined their plasma parameters, UV-C irradiance and atomic O concentration. 2. Experimental set-up a. Double DBD plasma jet The double DBD plasma jet is provided through a double cylindrical dielectric barrier configuration (see Fig. 1.a). This arrangement is useful to prevent arcing between the (inner) powered electrode and a plasma exposed sample which is particularly unwanted for biomedical applications. The device can operate at atmospheric pressure in argon. The (inner) high voltage (HV) electrode is made of a 2 mm diameter copper rod encapsulated in a quartz tube having an external diameter of 4 mm. This HV electrode is set in the axis of a quartz tube having 8 mm outer diameter and 1 mm thickness. There is a 1 mm gap between the dielectric of the HV electrode and the quartz tube. A piece of aluminum foil (17 mm wide) is ground connected and wrapped around the external quartz tube at about 6 mm from the tube outlet. Ar flow (4.5 purity) is injected into the tube through a mass flow controller at a rate of 1 l/min. The inner electrode is powered by a high voltage pulse generator delivering a 1 µs length voltage pulse at 9 kV with a repetition rate of 9.69 kHz. The power consumed by the P-III-10-11 double DBD device under these conditions is estimated at about 5.1W from current/voltage waveforms. b. Microwave Setup A Sairem Surfatron set-up is used to create the argon plasma jet (see fig. 1.b). It consists on injecting microwave energy (40 W, 2.45 GHz) in a cavity surrounding a cylindrical quartz tube with an internal diameter of 4 mm. Argon with a purity grade of 4.5 flows inside the tube. The flow is monitored by a mass flow controller at 1 l/min. The gas is partially ionized in the cavity leading to the formation of a plasma and to the propagation of surface microwaves at the interface between the Ar plasma and the quartz tube. These waves ionize the gas when propagating leading to the formation of a plasma plume or a plasma jet at the tube outlet. A second mass flow controller allows compressed air to flow along the external surface of the tube at 22 l/min in order to cool down the Surfatron and the plasma jet when mixes with it after the tube outlet. More information can be found elsewhere [12]. a) Ar flow (1 l/min) b) Surfatron quartz tube µwave generator (40W, 2.45 GHz) compressed air (22 l/min) plasma jet Fig. 1. Schematic of the experimental setups. 1 c. Optical emission Spectrum and UV-C irradiance diagnostics Optical emission spectroscopy is carried out by using a 0.75 m focal length imaging spectrophotometer (Acton Spectra SP 2750, in the Czerny Turner configuration) equipped with a 2400 grooves per meter grating for the UV region of the electromagnetic spectrum and a 1800 grooves per meter grating for the visible and near infrared region of the electromagnetic spectrum. The detecting device is a CCD camera (PIXIS - 100, 1340 x 100 imaging array of 20 µm x20 µm pixels). The plasma radiations are guided through an optical fiber (UV-silicon LG- 455-020-3) onto the entrance slit (200 µm wide) of the spectrometer. The spectral domain lies within 200 nm and 920 nm. Lateral radiations of the plasma jets are collected by a 2x magnification optical system offering a 1 mm spatial resolution. It is mounted on a sliding stand to perform a scan along the plasma jet axis (see fig.1.a). An optical high pass filter is used for the visible range analysis in order to prevent any perturbation of the visible range spectra due to the unwanted grating second order of diffraction of UV radiations. Perpendicular UV-C (190280 nm) irradiance (relatively to the axis of the jet) is determined by using a photodiode which is only sensitive to radiations between 225 and 280 nm (SG01S-C18, sglux GmbH). The photocurrent is determined by short circuiting the photodiode with a transimpedance amplifier circuit having a conversion factor of 1.82 V/nA. The active area of the photodiode is 0.06 mm2 and its quantum efficiency is about 0.1 A/W at 265 nm. 3. Results a. Optical emission and plasma temperatures The optical emissions spectra obtained in both plasma jet configurations are quite similar thus only the emission spectrum in the double DBD configuration is given in figure 2. The optical emission spectra of double DBD plasma jet is dominated by emission of OH (A2∑+ u →X2∏ g ), N 2 (C3∏ u →B3∏ g ), Ar, and O species. This is in accordance with the literature in the case of Ar plasma jets [12, 15]. NO bands in the UV-C range between 225 and 245 nm and the H α Balmer line at 656 nm are also present with a weaker intensity. The same spectrum is observed in the case of the Ar microwave plasma jet with the H β Balmer line peaking at 486.14 nm in addition. Rotational temperature (gas temperature) is estimated by fitting the simulated spectrum of OH(AX) on the experimental spectrum between 306 and 310 nm. The rotational temperature of the MW plasma jet is higher than the rotational temperature of the double DBD plasma jet (see tab.1). It is due to the fact that in the case of MW plasma random occurrence of filamentary structures are observed at the tube outlet. According to Motret et al [19] the OH rotational temperature is strongly correlated with the gas temperature in the streamer region. It is worth noting that the compressed air used to cool down the Surfatron plasma jet manages to keep the plasma jet 2 temperature at about 320 K that is to say in the same range of the double DBD plasma jet temperature. Excitation temperature Texc is obtained by using neutral argon emission lines (4d-4p, 6s-4p, 4p-4s) under Boltzmann approximation [12]. The Ar excitation temperature with the double DBD is slightly higher than the Ar excitation temperature obtained with the microwave plasma jet (see tab.1). b. Electron density The electron density was estimated from the Stark broadening of hydrogen lines. The estimation of the Stark broadening of the lines has been obtained by comparing the observed full width at high maximum (FWHM) of the lines with their theoretical FWHM which have been computed by the convolution of the instrument broadening, the Doppler broadening, the Stark broadening and the Van der Waals broadening assuming a gas temperature of 380 K and a pressure of 105 Pa. More information about the computation of these broadenings can be found in [12] and [21]. a) (SPS) b) Fig. 2. Optical emission spectra of the double DBD plasma jet between 200 and 440 nm (a) or 680 and 880 nm (b) in the case of Ar flow of 1 l/min, repetition rate of 9.69 kHz, pulse duration of 1 µs, voltage magnitude of 9 kV, and the viewing position of the spectrometer corresponding to the tube outlet. Table 1. Rotational and excitation temperatures in the axis of the Ar plasma jets measured in tube outlets. MW Plasma jet DBD plasma jet Trot (K) 800 ±100 380 ±30 Texc (K) 4200 ±200 4780 ±200 P-III-10-11 Table 2. Electron density in the axis of the plasma jets up at a distance of about 15 mm from the gas carrier tube outlet. Estimation from the Stark broadening of H α and H β lines. MW jet DBD jet Line H β (λ=486 nm) H α (λ=656 nm) H β (λ=486 nm) H α (λ=656 nm) Stark Broadening (nm) 0.05 0.006 N/A 0.04 n e (cm-3) 1.1 x1014 1.4 x1014 N/A 2.5 x1015 c. UV-C radiation measurement A thick aluminum foil was wrapped around the MW plasma jet to act as a Faraday shield in order not to disturb the UV-C detector circuit during the analysis. This arrangement prevented measurements at distance shorter than 30 mm from the gas carrier tube outlet of the MW plasma jet. A quartz plate protected the detector from potential arcing during the diagnostic of the double DBD plasma jet. The occurrence of random arcing between the body of the detector and the double DBD setup prevented measurements at distance shorter than 15 mm from the gas carrier tube outlet of the double DBD plasma jet. UV-C irradiance measurements are displayed in fig. 3. Production of UV-C irradiance by the MW plasma jet is about twice that produced by the double DBD plasma jet. In both cases, the UV-C irradiance level is high enough to obtain a significant bactericidal effect after a plasma exposure of bacteria during several minutes [24]. Fig. 4. Evolution of the argon UV-C irradiance in the axis of the plasma jets with the distance from the gas carrier tube outlet. d. Atomic oxygen concentration Oxygen atoms are well known to play an important role in plasma biological applications [13]. Th variation of the atomic oxygen concentration can be estimated by optical emission spectroscopy based on the comparison of the intensity of the argon line at 852 nm and the oxygen line at 844 nm [12, 27]. Comparative measurements have shown that the concentration of atomic oxygen determined by this spectroscopic method is in the same order of magnitude as what is given by TALIF measurement [29]. Figure 5 shows the evolution of the order of magnitude of atomic oxygen concentration along the plasma jets after the tube outlet. We observe that the atomic oxygen concentration in the double DBD plasma jet (770 ppm) is one order of magnitude higher than in the MW plasma jet (80 ppm) at the tube outlet and that this difference decreases when one moves away from the tube outlet of the jets. 1000 Atomic oxygen concentration (ppm) This diagnostic is usually performed with H β line (λ = 486 nm) but the intensity of this line was not observable in the case of the double DBD plasma jet. Consequently, the electron density in the DBD plasma jet was determined according to the Stark broadening of H α line (λ = 656 nm) which was easily observed. In the meantime, as both lines were easily observed in the MW plasma jet, we checked that the determination of the electron density of this jet provides similar values either from H α or H β lines (see tab. 2) and is in accordance with Hofmann's work on helium and argon RF plasma jets [22] and with Thomson scattering diagnostic [23]. We observed that the electron density in the double DBD plasma jet (2.5 x1015cm-3) is one order of magnitude higher than in the MW plasma jet (1.1 x1014cm-3). This finding explains why H β line does not appear on the spectrum of the double DBD plasma jet. In fact, the FWHM of this line is about 0.4 nm according to an electron density of 2.5 x1015cm-3. In this condition the spontaneous emission of H β line is spread over a large portion of the electromagnetic spectrum and the line does not clearly emerge from the spectrum baseline of the double DBD plasma jet. MW plasma jet DBD plasma jet 100 10 1 0 10 20 30 5 15 25 Distance from the gas carrier tube outlet (mm) Fig. 5. Evolution of the atomic oxygen concentration in the axis of the plasma jets. 4. Summaries In this work, cold argon plasma jets are generated with both double DBD and MW configurations at atmospheric pressure. Several interesting information on active species and thermodynamic plasma properties have been obtained. Table 4 summarizes these data. The main P-III-10-11 3 discrepancies between the two jets characteristics are the electron density and the atomic oxygen concentration which are higher by about one order of magnitude for the double DBD plasma jet configuration at the tube outlet in comparison to the MW plasma jet. This is probably due to the fact that high energetic electrons are more present in the double DBD plasma jet since an electron temperature higher than about 7 eV [27] is generally required to selftransport the ionization wave generated in a DBD setup whereas the electron temperature due to the action of the electromagnetic field does not overpass about 5eV inside the tube of the MW plasma jet and fall down to about 1eV after the tube outlet [7]. It is also interesting to underline the fact that the MW plasma jet produces twice as much as UV-C radiations than the double DBD plasma jet whereas this last contains about twice as much as O atoms for distances greater than 1 cm away from the tube outlet. Finally it is interesting to emphasize that the gas temperature measured by a thermocouple is much lower than the rotational temperature for both plasma jets and reaches about 320 K at a distance of 1.7 cm from the tube outlet. Consequently these two plasma jets are suitable to be used in biomedical applications without significant thermal effects. Malley. Technical report, U.S. Environmental Protection Agency, Office of Water (4601), 2006. [10] Y Hong, N Lu, J Pan, J Li, YWu, and K F Shang. Journal of Electrostatics, 71(2):93_ 101, 2013. [11] A F H van Gessel, S C van Grootel, and P J Bruggeman. Plasma Sources Sci. Technol., 22(5):055010, 2013. [12] M. D. Calzada, M. Saez, and M. C. Garcia. J. Appl. Phys., 88(1), 2000 Table 3. MW and double DBD plasma jets comparison. Measurement taken 1.7 cm after the tube outlet. Trot (K) Texc (K) Ne (x 1014 cm-3) [O] (ppm) UV-C (µW/cm2) MW jet 800 ± 100 4200 ± 200 1.1 12 48 DBD jet 380 ± 30 4780 ±200 25 38 23 5. References [1] G Wattieaux, M Yousfi, and N Merbahi. Spectrochi. Acta B, 89(0):66 - 76, 2013. [2] F Judée, G Wattieaux, N Merbahi, M Mansour, and M P Castanié-Cornet. J Phys. D. Appl. Phys., 47(40):405201, 2014. [3] X Lu, Z Jiang, Q Xiong, Z Tang, X Hu, and Y Pan. Appl. Phys. Lett., 92(8):-, 2008. [4] Q Xiong, A Y Nikiforov, X P Lu, and C Leys. J Phys. D. Appl. Phys., 43(41):415201, 2010. [5] O Motret, C Hibert, S Pellerin, and J M Pouvesle. J Phys. D. Appl. Phys., 33(12):1493, 2000. [6] M. Gigosos, M.González, and V. Cardeñoso. Spectrochi.Acta B, 58(8):1489 _ 1504, 2003. [7] S Hofmann, A F H van Gessel, T Verreycken, and P Bruggeman. Plasma Sources Sci.Technol., 20(6):065010, 2011. [8] A F H van Gessel, E A D Carbone, P J Bruggeman, and J J A M van der Mullen.Plasma Sources Sci.Technol., 21(1):015003, 2012. [9] Inc. Malcolm Pirnie, P.C. Carollo Engineers, Inc. The Cadmus Group, Dr. Karl G. Linden, and Jr. Dr. James P. 4 P-III-10-11