IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 1, FEBRUARY 2006 115 Gas Temperature Determination of an AC Arc Discharge at Atmospheric Pressure in Air Using a Mach–Zehnder Interferometer Javier A. López, Diego Echeverry, Gustavo Zambrano, Luis Fernando Castro, and Pedro Prieto Abstract—To determine the gas temperature of an alternating current (ac) (50 kHz) arc discharge at atmospheric pressure in air, a Mach–Zehnder type interferometer was built, utilizing an He-Ne laser source with vertical polarization and a power of 5 mW. Upon introducing the arc discharge into one of the inter- ferometer arms, a displacement of interference fringes takes place with respect to its position without the discharge. Keeping in mind that displacement of the fringes is related to the difference of the optical path with the variation of the refraction index and with tem- perature change in the discharge zone; the latter was determined from the displacements of the interference fringes. At the center of the discharge channel, a temperature near 3000 K was calculated, diminishing gradually to room temperature toward the discharge borders. This temperature value at the center of the channel agrees with results previously reported in similar discharges. This is an al- ternate method for the diagnosis of plasma parameters used in the production and treatment of materials. Index Terms—Arc discharge, optical interference, plasma diagnostics. I. INTRODUCTION A TWO-TEMPERATURE chemical kinetic model was de- veloped to understand the mechanisms governing ioniza- tion and electron recombination in discharges produced by an applied electric field. The temperatures are the electron temper- ature and the gas temperature [1]. In local thermodynam- ical equilibrium (LTE) plasmas, all internal energy modes can be associated to a temperature (vibrational rotational, , and electronic ). Frequently, to determine these temperatures, the optical emission spectroscopy (OES) method is used. The LTE and Boltzmann temperatures are based on the absolute and relative intensities, respectively, of various atomic lines. Usually, it is assumed that the rotational temperature is close to the gas Manuscript received May 26, 2005; revised August 16, 2005. This work was supported by COLCIENCIAS under Research Project 1106-05-11457, by the Excellence Center of Novel Materials, and by the Fundación para la Promoción de la Investigación y la Tecnologia del Banco de la República under Research Project 1441. J. A. López, G. Zambrano, L. F. Castro, and P. Prieto are with the Lab- oratorio de Peliculas Delgadas at Departamento de Fisica, Universidad del Valle, A.A. 25360 Cali, Colombia (e-mail: javierlo@calima.univalle. edu.co; gzambra@calima.univalle.edu.co; lfcastro@calima.univalle.edu.co; pprieto@calima.univalle.edu.co). D. Echeverry is with the Grupo de Investigación en Alta Tensión at Escuela de Ingeniería Eléctrica y Electrónica, Universidad del Valle, A.A. 25360 Cali, Colombia (e-mail: dieche@hotmail.com). Digital Object Identifier 10.1109/TPS.2005.863123 Fig. 1. Experimental arrangement of Mach–Zehnder interferometer laser source, Beam expander lens, focuser lens, Beam splitters and Reflector mirrors. temperature because rotational relaxation is fast at atmospheric pressure [2]. An advantage of this technique is that it does not require absolute or relative intensity calibration because the response of usual detection systems is nearly constant over the small spectral range of interest. However, one should be aware of potential difficulties associated with self absorption, non-Boltzmann rotational population distribution, and interferences with other species present in the plasma. In this work, we present an alternative method for determina- tion of gas temperature of an ac arc discharge at atmospheric pressure in air from the displacement of the interference fringes by using a Mach–Zehnder interferometer. A Mach–Zehnder interferometer is a beam-division device, consisting of two beam splitters and two reflector mirrors, as shown in Fig. 1. Its main advantage is that it allows for the interposition of elements in one of the beams without affecting the other, thus altering the optical path difference between them, and, thereby, changing the interference pattern [3]. For example, an arc discharge with a refraction index different to the refraction index of the air can be introduced into one of its arms, resulting in the appearance of an interference pattern with fringes displaced with respect to their position without the arc discharge. This interferometer can be used to study the variations that can occur when a beam of light passes through a medium, other than air, placed in one of its arms. This medium can distort the wave-front phase since its refractive incidence is not homogenous. The prior occurrence is due to temperature variations and concentration variations in such given medium [4]. 0093-3813/$20.00 © 2006 IEEE