Y E E D Gamal

Laser-induced breakdown of helium

A theoretical investigation is made into the ionisation of helium, at pressures of 1800-17000 Torr (0.24-2.27 MPa) by ruby laser radiation of pulse-length 40 ns. It is shown that, when the variation of electron energy gain in elastic collisions is taken into account, the agreement between computed and experimental breakdown threshold intensities is improved by a factor of 10 compared with calculations in which only the mean energy gain is computed. The agreement is also improved significantly by incorporating the collisional and multiphoton ionisation of excited helium atoms.

Influence of the collision frequency on the electron distribution function in laser-induced discharges in argon

The influence of the collision frequency on the electron energy distribution function in laser-induced discharges in argon is studied. In this investigation three different cases of the collision frequency are considered. In the first two cases the authors used two constant values of the collision frequency namely 3.9*109 S-1 Torr-1 and 9.5*109 S-1 Torr-1. In the third case the exact correlation between the collision frequency and the electron energy (i.e. the collision frequency is allowed to vary with the electron energy) is taken into consideration. Useful information is obtained on the role the collision frequency plays in the electron energy distribution function and consequently its related parameters.

A numerical investigation of the dependence of the threshold irradiance on the wavelength in laser-induced breakdown in

Studies of laser-induced breakdown in molecular nitrogen were carried out to investigate the dependence of the threshold irradiance on the wavelength at various pressures. The analysis was based on the numerical solution of the time-dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations describing the rate of change of the excited states population. The rate coefficients and cross-sections as functions of the electron energy were introduced into this analysis in order to probe the exact contribution of each physical process to the breakdown phenomenon. The calculations were performed under the experimental conditions of Davis et al. In this experiment the breakdown of nitrogen was measured at wavelengths of 1064, 532, 355 and 266 nm, over gas pressures in the range 25-760 Torr, with laser irradiances in the range to . The computed thresholds were found to be in good agreement with the measured ones at all wavelengths. The calculated EEDF and its parameters showed that, at nm, vibrational losses are dominant. Collisional ionization of ground and excited state molecules was found to make a minor contribution to the breakdown phenomenon at 532, 355 and 266 nm. However, the contribution to this process at 1064 nm was more effective. Therefore, the breakdown phenomenon proceeds via an electron-cascade process that converts the molecules only into the excited states, whence multiphoton ionization plays its role.

An investigation of the wavelength-dependence of threshold intensity of laser-induced breakdown of molecular hydrogen

A previously developed electron cascade model is extended and used to examine the laser wavelength-dependence on the threshold intensity in laser-induced electrical breakdown of molecular gases. The analysis is based on numerical solution of the time-dependent Boltzmann equation and a set of rate equations describing the population of the involved excited states. The calculations are restricted to molecular hydrogen at atmospheric pressure irradiated with tunable laser radiation ( lambda =720-800 nm) with a pulse duration (FWHM) of 18 ns. The computed threshold intensities are compared with those experimentally measured. The calculated electron energy distribution function and its parameters provide an important correlation between the laser wavelength and the relevant physical processes involved during the interaction.

Study of recombination processes during the expansion phase of titanium-plasma produced by picosecond Nd:glass laser radiation

Abstract An investigation of the plasma dynamics during its expansion phase is presented. The analysis is based on a numerical model, which considers the adiabatic plasma expansion as well as a three-body recombination process as the main source for plasma cooling. The result shows that a fast energy transfer between electrons and ions is observed during the early stage of plasma expansion. Three-body recombination process exhibits a pronounced effect leading to a “freezing” feature in the average charge states of heavy particles. This process seems to control the fractional population of ionic species at the end of the expansion phase.

Theoretical analysis of the breakdown of neon induced by 248 nm laser radiation

This paper presents a numerical analysis on the electrical breakdown of neon over a pressure range 100-3000 Torr irradiated with 248 nm laser radiation of pulse length 18 ns operated at threshold irradiance varying between {4×1011} and 2×1013 W cm-2. The investigations are based on a previously developed electron cascade model which solves numerically the time dependent Boltzmann equation simultaneously with a set of rate equations describing the rate of change of the excited states population. The result of computations showed a good agreement between the calculated threshold irradiance and the measured ones over the whole range of gas pressure examined experimentally. A study of the effect of loss processes on the breakdown threshold revealed that diffusion losses are pronounced over pressures <1000 Torr. No evidence for recombination losses is observed at pressures up to 3000 Torr. Analysis of the calculated electron energy distribution function and its parameters reflected the importance of photoionization of the excited states as a physical mechanism responsible for the breakdown of neon under the experimental condition used in this work.

On the study of the electron kinetic processes in the breakdown of argon by 0.53 µm and 0.248 µm laser radiation

Based on a previously developed electron cascade model, a study is performed to investigate the electron kinetics in the breakdown of argon under two sets of experimental conditions in which argon over a pressure range 10-3 ? 103 Torr is irradiated with focused beams of laser radiation of wavelengths 0.53 ?m and 0.248 ?m and pulse duration 15 ns and 18 ns, respectively. The model takes into account all the possible electron, atom and photon interactions. The calculated breakdown threshold intensities are found to be in accordance with the measured ones over the whole pressure range for = 0.248 ?m. However, the agreement for = 0.53 ?m was poor below 200 Torr. Moreover, the study of the electron energy distribution function and its parameters revealed the competing role of multiphoton and cascade collisional ionization mechanisms against loss processes over the pressure range examined in this analysis.

The breakdown of molecular oxygen by brief pulses of laser radiation

The authors report a numerical analysis for the combined action of multi-photon ionisation and cascade ionisation processes in producing ionisation and breakdown of molecular oxygen. The calculations were carried out at two different laser pulses of wavelengths 0.6943 and 0.53 mu m with pulse durations of 18 and 25 ps, respectively, over a pressure range of 102-5*104 Torr. A reasonably good agreement has been obtained between the computed threshold intensities and the measured ones. Computations for nitrogen irradiated with 0.53 mu m laser pulses are also given. This analysis shows that the model is capable of interpreting gas breakdown experiments with short flashes of laser radiation.

Electrical breakdown in argon by powerful pulses of ultraviolet laser radiation

A theoretical investigation of the physical processes associated with the interaction of ultraviolet pulses of laser radiation with argon gas is presented. Analyses are based on an extended electron cascade model which is applied to two sets of experimental conditions, in which argon, in the pressure range of 200-10000 Torr, is illuminated by laser pulses of durations 15 and 500 ns at a wavelength of 0.35 mu m. The predicted threshold intensities as a function of gas pressure showed reasonable agreement with the two sets of experimental data. The strong dependence of the threshold intensity on the gas pressure indicated that ionization growth proceeds mainly via collisional processes. The calculated energy distribution function and its parameters (namely electron density and electron mean energy) revealed information about the importance of loss processes in the breakdown phenomenon. The results of the calculations also provided a satisfactory explanation for the observed reduction of threshold intensity when ultraviolet laser radiation induced breakdown in argon gas.

Breakdown of helium under single-mode ruby laser irradiation

The breakdown of helium, produced by single-mode laser beam with a triangular time-variation, has been calculated on the basis of the cascade mechanism by following the evolution of the electron energy distribution. A simple model of the helium atom has been used-a ground state, one excited and one ionised state, and the collision cross-sections for ionisation, excitation, and ionisation of the excited state have been taken from the literature. With parameters corresponding to a focal spot of diameter 14 mu m, in helium of pressure between one and 12 atm, and no loss processes, the computed breakdown threshold intensities were found to be within a factor of two of those determined experimentally. On making allowance for diffusion losses, the agreement improved to the order of 20%.