Role of Loss Mechanisms in the Breakdown of Argon Irradiated with IR Laser Radiation

Abstract

Threshold intensity measurements of the breakdown of argon are numerically analyzed. The breakdown is induced by a Nd: YAG laser radiation at wavelength of 1064 nm and pulse duration of 8.5 ns.The investigation considered argon over pressure range extended from 25 torr- 760 torr.The measured threshold intensity corresponding to this pressure range is found to vary between 5.4xl011 W/cm2 and 1.1xl011 W/cm2. These are the experimental conditions given by Davis et al. [1]. The analysis considers an electron cascade model given previously by Evans and Gamal (1980) [2] and modified by Gamal and Shafik [3]. The model solves the time dependent energy equation numerically in addition to a set of equations describing the rate of change of the formed excited states population. For realistic results, most of the physical processes that might take place during the interaction of laser radiation with the gas are included in the modified model. The calculated threshold intensity showed reasonable agreement with the measured values over the tested pressure range. The obtained values of the electron energy distribution function as well as the time variation of the electrons density for selected gas pressures, revealed that photo-ionization of the formed excited states plays a minor role for electron generation and plasma formation over the studied gas pressure range. Collisional ionization is the main mechanism responsible for the breakdown process. The loss process, due to electron diffusion, is found to have significant contribution only at pressures ≤ 40 torr. On the other hand, the recombination process is acting in a pronounced manner for the higher pressure regime.