Theoretical study of threshold intensity dependence on gas pressure in the breakdown of helium by COlaser radiation: Influence of preionization mechanism

An electron cascade model adapted to provide a thorough understanding of the physical mechanisms involved in CO2 laser-induced helium plasma is presented. The model combines a time-dependent calculation of the electron energy distribution with rate equations, describing how the population of excited states changes [Y. E. E.-D. Gamal and G. Abdellatif, Appl. Phys. B 117(1), 103 (2014)]. It encountered the possible elastic and inelastic electron collisional processes that enhance the electrons' growth, leading to gas breakdown. The analysis explores the experimental threshold intensity dependency on gas pressure [J. J. Camacho et al., Spectrochim. Acta, Part B 66(1), 57 (2011)]. The measurements are carried out using 9.621 μm over pressure in the range from 12.0 to 87.0 kPa. Since multiphoton ionization is improbable, ionization proceeds via the inverse bremsstrahlung absorption. In this experiment, the ignition of this process is initiated by the experimentally assumed pre-breakdown approach. No experimental estimation was given for the initial electron density. The electron diffusion and the loss of electron energy through elastic collisions have no contribution to this experiment. The calculations of the threshold intensity are performed to determine the initial electron density. The model's validity is assured by the reasonable agreement between the calculated thresholds, and the measured ones are only achieved at a specific initial electron value for each gas pressure. Over pressure exceeding 30.0 kPa, the agreement was reasonable in the presence of recombination losses. The threshold intensity is controlled by the initial electron density for lower pressures. The analysis showed how the gain and loss of electrons control the breakdown threshold for helium concerning the determined initial electron density for the tested pressures.