Life Science Journal 2012;9(4) http://www.lifesciencesite.com 4815 Theoretical investigation of argon plasma formation induced by laser radiation Kholoud A. Hamam , Haifaa M. AL-Ghamdi and Yosr E E-D Gamal + Physics Department, Faculty of Science , King Abdul Aziza University , Jeddah , Saudi Arabia. + National Institute of Laser Enhanced Science, Cairo University, Giza, Egypt haifaa65@hotmail.com Abstract: A previously developed electron cascade model is modified and applied to investigate the breakdown threshold for plasma formation and propagation in the focal volume . The study is devoted to investigate the measurement that carried out on the breakdown of argon over a pressure range 0.013-1.00 atm (10 -760 torr) induced by 532 nm of Nd:YAG laser with pulse length 8 ns and maximum energy 500 mJ. The model solves numerically the time dependent Boltzmann equation and set of rate equations that describe the change of the excited states population .The result showed good agreement between the calculated threshold intensities/ or laser input energy and the measured ones over the tested pressure range, this in turn validate the applied numerical model. More over the calculation of the EEDF and its parameters showed the correlation between gas pressure and physical processes responsible for the gas breakdown and plasma formation . Taken into consideration the spatial and temporal variation of the laser intensity in the focal volume it was possible also to present in this work the study of the effect of laser input energy on plasma propagation along the axial distance of the spatially varying focal volume. The result of this study illustrated the increase rate of plasma propagation by increasing the input energy, where it is found that at input energy equals three and half time its threshold energy value, the plasma propagates to cover the whole Rayleigh range in the backward direction. [Haifaa M. AL-Ghamdi, Kholoud A. Hamam and Yosr E E-D Gamal. Theoretical investigation of argon plasma formation induced by laser radiation. Life Sci J 2012;9(4):4815-4822] (ISSN:1097-8135). http://www.lifesciencesite.com . 724 Keywords: argon, plasma pressure, plasma production by laser, electric breakdown, optical focusing, plasma diagnostics, laser –produced plasma, argon breakdown . 1. Introduction The phenomenon of laser induced breakdown and plasma generation in gases have been studied extensively both experimentally and theoretically during the last five decades. Recently ,this phenomenon found a great importance for various applications, which include micro industries in electronics , environmental application for the measurement of pollution, surface cleaning, besides its application in medicine and biology. These studies showed that such applications are mainly depend on the characteristics of the plasma formed in the breakdown region. One of the main features of the formed plasma is its propagation in the backward direction of the focal volume as the input laser energy exceeds the threshold energy for breakdown. Moreover as the gas pressure increases the rate of propagation increases and more absorption of the input energy occurs in the plasma causing less transmission in the forward direction. Therefore more interest is devoted to study the physical processes responsible for this propagation ( Yamada et al., 1985;Yamada et al., 1994;Tsuda et al., 1997;Mlejnek et al., 1999 ;Bindhu et al., 2003 ). In these studies it was found that these physical processes depend on the parameters of the laser source as well as the nature of the irradiated gas. Accordingly , A theoretical study of the phenomenon of laser induced breakdown and plasma generation in argon under the experimental conditions given by Bindhu et al. ( 2003) is presented . In this experiment the breakdown is obtained using a focused high intensity laser beam of wavelength 532 nm , pulse duration 8 ns and maximum energy 500 mJ , to irradiate argon gas over a pressure range varies between 10-760 torr, which is equivalent to 0.013-1.0 atm .This gas has been chosen since it has examined experimentally by various authors see for example: Yamada et al., 1985;Yamada et al., 1994; Tsuda et al., 1996; Mlejnek et al., 1999 ;Bindhu et al., 2003). Moreover ,it showed a minimum Ramsuer, in the relation between the momentum transfer collision cross-section and the electron energy. This minimum might has a noticeable effect on rate of energy gain by electrons from the laser field during the Inverse Bremsstrahlung absorption process which plays an important role in the breakdown of argon (Gamal et al., 1986). The investigation is based on a modified electron cascade model ( Gamal and Azzouz 1987; Gamal et al., 1999) which depends on the numerical solution of the time dependent Boltzmann equation simultaneously with a set of rate equations that describe the rate of change of the formed excited