The Influence of Discharge Capillary Size, Distance, and Gas Composition on the Non-Equilibrium State of Microplasma Asif Majeed, Xiaoxia Zhong,* Shaofeng Xu, Xinhui Wu, Uros Cvelbar, Zhengming Sheng The non-equilibrium state of microplasma from the mixture of He and N 2 was studied by exploring the rotational and vibrational temperatures of molecular nitrogen under various discharge conditions. At a varying gap distance of 1–3 mm (with a step of 1 mm) from the exit of the capillary to the water surface, the average rotational temperature of N 2 was found to increase from 983 to 1250 K while the corresponding vibrational temperature of N 2 was reduced from 4875 to 3099 K. Consequently, the average vibrational to rotational temperature ratio of N 2 was decreased from 4.96 to 2.48. By widening the capillary inner diameter from 100 to 200 mm, the average rotational temperature of N 2 was elevated from 891 to 1090 K whereas the average vibrational temperature of N 2 was dropped from 4662 to 3646 K and hence, the ratio of vibrational to rotational temperature of N 2 was lessened from 5.23 to 3.34. Moreover, with the addition of more nitrogen into the flow, i.e., by increasing the flow rate of N 2 from 0 to 15 sccm (with an interval of 5 sccm), the average rotational temperature of N 2 was intensified from about 942 to 1404 K, whereas the corresponding vibrational temperature of N 2 was reduced from 5011 to 3254 K. Therefore, the corresponding ratios of vibrational to rotational temperature of N 2 were declined from 5.3 to 2.3. The results demonstrate that the surface area to volume ratio of microplasma and gas conductivity have a significant effect on the non- equilibrium nature of He-N 2 atmospheric pressure microplasma. 1. Introduction In recent years, the growing interest in the field of microplasmas in various gases at atmospheric pressure was perceived on account of their reduced dimensions (from mm to several mm), stable operation at atmospheric pressure, non-thermal and non-equilibrium characteristics, high electron densities, and non-Maxwellian electron energy distributions. [1–6] These atmospheric pressure microplasmas have manifold advantages over low pressure plasmas because of their low investment and operational costs, low power consumption, portability, and easy-to-use X. X. Zhong, A. Majeed, X. H. Wu, S. F. Xu, Z. M. Sheng Department of Physics and Astronomy, Key Laboratory for Laser Plasmas (Ministry of Education) and State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China E-mail: xxzhong@sjtu.edu.cn A. Majeed Department of Physics, University of Azad Jammu and Kashmir, Muzaffarabad (A.K), Pakistan U. Cvelbar Jozef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia Full Paper Plasma Process. Polym. 2016, DOI: 10.1002/ppap.201500199 ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 DOI: 10.1002/ppap.201500199 wileyonlinelibrary.com Early View Publication; these are NOT the final page numbers, use DOI for citation !! R