IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 42 (2009) 142003 (5pp) doi:10.1088/0022-3727/42/14/142003 FAST TRACK COMMUNICATION Measurement of the electron density in atmospheric-pressure low-temperature argon discharges by line-ratio method of optical emission spectroscopy X M Zhu 1 , Y K Pu 1 , N Balcon 2,3 and R Boswell 2 1 Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China 2 Research School of Physical Sciences and Engineering, Australian National University, Canberra 0200, Australia 3 CPAT, Universit´ e Paul Sabatier, Toulouse 31000, France E-mail: puyikang@tsinghua.edu.cn Received 20 May 2009, in final form 9 June 2009 Published 26 June 2009 Online at stacks.iop.org/JPhysD/42/142003 Abstract A new collisional–radiative model for atmospheric-pressure low-temperature argon discharges is proposed, which illustrates the significant effect of electron density on the excited atom population distribution. This makes it possible to determine the electron density from the intensity ratio of emission lines of excited atoms. Results of this new method in several types of atmospheric-pressure discharges are found to be in agreement with those of the Stark broadening method and the electric model over a wide electron density range 10 11 –10 16 cm -3 . (Some figures in this article are in colour only in the electronic version) The electron density is one of the most fundamental parameters in gas discharges and plays a very important role in understanding the discharge physics and optimization of the operation of plasmas [1, 2]. Typical methods to measure the electron density include the use of a Langmuir probe [3], microwave interferometery [4], laser Thomson scattering (LTS) [5] and optical emission spectroscopy (OES) [6]. However, in atmospheric-pressure low-temperature discharges, both the probe and the microwave-based methods are difficult to use due to the small plasma dimensions and strong collisional processes. LTS has been proved to be effective but challenging because of the low signal and excessive stray light, as well as the complicated experimental setup [7]. However, the OES-based technique has the advantages of being non-intrusive, inexpensive and convenient. There are three major OES techniques to determine the electron density in low-temperature plasmas. One OES method involves the investigation of the Stark broadening of emission line profiles, such as that of hydrogen Balmer lines [8]. At electron densities lower than ∼10 13 cm -3 and at atmospheric pressure, this method is inappropriate since the van der Waals broadening or Doppler broadening becomes the dominant broadening process. Another method is to analyse the continuum radiation if it is observed, such as in some recombining plasmas [9]. The last one is the line- ratio method, in which the intensity ratio of emission lines is related to the electron density by a collisional–radiative model (CRM) [10]. This method has been successfully used in low- and medium-pressure discharges [10–12]. In this work, with a CRM of atmospheric-pressure discharges, a line-ratio method to determine the electron density from the mostly observed optical emission of argon 2p–1s transitions (in Paschen’s notation) is introduced. To obtain the relationship between the electron density and the line-ratios of optical emissions, the effect of electron density on the excited atom kinetics should be analysed. In 0022-3727/09/142003+05$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK