Physics Letters A 375 (2011) 1733–1736 Contents lists available at ScienceDirect Physics Letters A www.elsevier.com/locate/pla Coaxial rf atmospheric pressure dielectric barrier air–helium plasma characteristics A. Gulec a,b , L. Oksuz a,b,∗ , N. Hershkowitz a a Department of Engineering Physics, University of Wisconsin-Madison, Madison, WI 53706, USA b Department of Physics, Suleyman Demirel University, 32200 Isparta, Turkey article info abstract Article history: Received 2 December 2010 Received in revised form 26 February 2011 Accepted 28 February 2011 Available online 21 March 2011 Communicated by F. Porcelli Keywords: Atmospheric pressure rf glow discharge Coaxial rf discharge Capacitive discharge The electrical and optical characteristics of a coaxial rf atmospheric pressure dielectric barrier discharge (DBD) with one electrode covered with glass and with helium in air were investigated. Optical measurements with an intensified charge coupled device (ICCD) camera combined with the voltage and power vs. current data provided identification of the α to γ mode transitions. The coaxial design allowed operation with very low power density plasmas (0.18 W/cm 3 ) and a much larger plasma size (13 mm) than the parallel plate designs.  2011 Elsevier B.V. All rights reserved. Radio frequency (rf) glow discharges operated under vacuum conditions [1] are widely used in the materials processing indus- try. There is a growing interest in atmospheric pressure glow dis- charges (APGD) because they have many similarities [2] to classical capacitively coupled low pressure plasmas but do not require ex- pensive and complicated vacuum systems. Atmospheric pressure rf discharges have already found many applications in thin film depo- sition [3], etching [4], sterilization [5] and polymerization [6] and nanocomposite [7] deposition but atmospheric pressure discharge physics is not yet fully understood. Atmospheric pressure rf plasma systems consist of different varieties of electrode forms: naked electrodes [8] and electrodes covered by dielectrics [9,10] and can have different geometries. Parallel plate and concentric cylinders [11] are two simple examples. In previous investigations the elec- trode separations were small, typically a few mm between the electrodes. Two operating modes have been identified [12]. In the α mode, trapped electrons provide volumetric ionization through- out the bulk plasma. In the γ mode, energetic electrons heated in the sheath provide local ionization at a much higher rate. Opti- cal studies [13–16] carried out with parallel plate electrodes, with representative separations the order of 2 mm, found the emission intensities were highest in the bulk plasma for the α mode consis- tent with electron trapping and highest near the electrodes in the γ mode consistent with the sheath electron heating. These stud- ies found the discharge intensities were periodic at the rf driving * Corresponding author at: Suleyman Demirel Universitesi, Fen Edebiyat Fakultesi, Fizik Bolumu, Isparta, Turkey. Tel.: +90 246 2111796; fax: +90 246 2371106. E-mail addresses: oksuz@fef.sdu.edu.tr, loksuz@wisc.edu (L. Oksuz). frequency with the brightest images at T /4 and 3T /4 taken at the discharge current peaks and also showed that the bright emitting layers get thinner and closer to the electrodes when the frequency increased. In a recent publication, Shang et al. [17] have modeled atmospheric pressure discharges in helium for a coaxial electrode configuration with both electrodes covered with dielectric barriers. In addition to the α and γ modes corresponding to low and high rf current respectively, they found the discharge was asymmetric with a higher ionization near the central electrode. Addition of ar- gon or helium [18] flow was very important for uniform and stable glow like plasma generation because of the Penning ionization by metastable argon or helium atoms. In this Letter, optical images resolved in time and space and optical emission spectra of atmo- spheric pressure dielectric barrier coaxial capacitively coupled rf discharges are investigated with helium flow in air and compared with previously modeled results. A schematic diagram of the experimental setup is given in Fig. 1. A grounded 2 mm diameter stainless steel electrode was placed on the axis of a glass cylinder, 3.0 cm in diameter and 22.5 cm in length. Rf power at 13.56 MHz was applied to the outer cylindrical rf copper electrode (11.4 cm length). Helium was in- troduced with controlled flow rate through the tube on the top while the ends of the cylinder were left open to air. The evolution of the discharge light emission in one rf cycle was recorded us- ing a Princeton PI-MAX Intensified Charge Coupled Device (ICCD) camera. The gate width for each photo was 5 ns. Plasma visible optical emission was collected with a Princeton Instrument Ac- ton SP 2500i (0.500 meter focal length) monochromator. A double probe consisting of two gold plated nickel wires was also inserted 1.5 mm away from the grounded central electrode. A Pearson cur- 0375-9601/$ – see front matter  2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2011.02.070