IOP PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY Plasma Sources Sci. Technol. 17 (2008) 025013 (13pp) doi:10.1088/0963-0252/17/2/025013 DC normal glow discharges in atmospheric pressure atomic and molecular gases David Staack, Bakhtier Farouk 1 , Alexander Gutsol and Alexander Fridman Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 19104, USA E-mail: bfarouk@coe.drexel.edu Received 2 September 2007, in final form 1 February 2008 Published 17 April 2008 Online at stacks.iop.org/PSST/17/025013 Abstract DC glow discharges were experimentally investigated in atmospheric pressure helium, argon, hydrogen, nitrogen and air. The discharges were characterized by visualization of the discharges and voltage and current measurements for current of up to several milliamperes. Significant differences are seen in the gas temperature; however all the discharges appear to operate as temperature and pressure scaled versions of low pressure discharges. In the normal glow discharges, features such as negative glow, Faraday dark space and positive column regions are clearly observable. In hydrogen and to a lesser degree in helium and argon standing striations of the positive column were visible in the normal glow regime. Normal glow characteristics such as normal current density at the cathode and constant electric field in the positive column are observed although there are some unexplained effects. The emission spectra for each of the discharges were studied. Also the rotational and vibrational temperature of the discharges were measured by adding trace amounts of N 2 to the discharge gas and comparing modeled optical emission spectra of the N 2 2nd positive system with spectroscopic measurements from the discharge. The gas temperatures for a 3.5 mA normal glow discharge were around 420 K, 680 K, 750 K, 890 K and 1320 K in helium, argon, hydrogen, nitrogen and air, respectively. Measured vibrational and excitation temperatures indicate non-thermal discharge operation. Mixtures of gases achieved intermediate temperatures. (Some figures in this article are in colour only in the electronic version) 1. Introduction Atmospheric pressure plasma discharges are of great interest because of the lower costs and simplified operation in comparison with low pressure plasma processing, and also because of the possibility of applications to non-vacuum compatible materials and processes [1]. A requirement for many of these applications is a non-thermal plasma. Non- thermal plasmas (also called non-equilibrium plasmas and cold plasmas) are characterized by a non-equilibrium distribution of energy between different degrees of freedom, different excited states and different particles. Usually the situation can be simplified a little assuming that energy distribution 1 Author to whom any correspondence should be addressed. can be described by several temperatures such as the electron temperature (T e ), electronic excitation temperature (T elex ), vibrational temperature (T vib ), rotational temperature (T rot ) and translational temperature (T trans ). In non-thermal plasmas created by externally applied electric fields typically T e > T elex > T vib > T rot = T trans . The non-equilibrium nature allows for the creation of active species without generating excessive heat which may damage substrates or cause excessive dissociation. Also the chemical processes which occur in the non-equilibrium plasma are beyond those which are accessible by the addition of only thermal energy. The most well known low pressure non-thermal discharge is the normal glow discharge [2, 3]. The normal glow discharge exists typically in the discharge gap between parallel electrodes at currents higher than the threshold for Townsend breakdown 0963-0252/08/025013+13$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK