Measurement of the wavelength dependence of the threshold of laser-induced gas breakdown

Abstract

Measurements of the threshold intensity for breakdown over the wavelength range 720-840 nm have been made at atmospheric pressure in several gases, by means of a high output power laser pumped, tunable dye laser. Although it may be expected that the resonant phenomena observed in earlier experiments would be more pronounced at lower pressures, the authors have found that over the entire wavelength range studied, the results showed the direct dependence on both wavelength and pulse duration predicted by cascade theory.

Numerical Study of the Threshold Intensity Dependence on Wavelength in Laser Spark Ignition of Molecular Hydrogen Combustion

“…It also comprises the loss terms due to vibration excitation, molecular dissociation besides the electron loss processes through electron recombination. Under the experimental conditions considered in this analysis [5], superelastic collisions from the electronically and vibrationally excited states, as well as radiative recombination of the hydrogen molecule are neglected. Dissociative attachment from the vibrationally excited molecules is also ignored.…”

“…Although a considerable bodies of study on laser induced breakdown in gases have been reported (see for example [1][2][3][4][5][6][7][8]), the breakdown phenomenon of common combustion gases such as hydrogen has not been available. Because of the importance of such phenomenon, in various applications it attracted the attention of many researchers, in particular the mechanism that convert the laser energy to thrust energy or mechanical energy that could be approached via high power laser produced plasma (LPP) in a gas breakdown process [9][10][11][12][13].…”

“…These measurements are investigated by [17]. In an another attempt in [5], Phuoc (2000) measured the threshold intensity of molecular hydrogen laser spark ignition using the fundamental and second harmonic of a Nd:Yag laser source at wavelengths 1064 nm and 532 nm respectively with pulse length 5.5 ns, over a gas pressure range extended from 150 torr to 3000 torr. This measurement showed that the threshold intensities corresponding to the shorter wavelength lie above those obtained for the longer one over the gas pressure range tested experimentally.…”

“…This was attributed to the effect of the high rate of diffusion losses at the shorter laser wavelength. To find out the origin of this high threshold intensities corresponding to the shorter laser wavelength (532 nm), in this work we represents a numerical investigation of the measurements given in this experiment [5]. In doing so, a modified numerical electron cascade model [17,18] previously developed by [19] is applied to clarify the physical processes responsible for the laser spark ignition in molecular hydrogen as a function of both laser wavelength gas pressure.…”

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A numerical investigation of laser wavelength dependence on the threshold intensity of spark ignition in molecular hydrogen over a wide pressure range is presented. A modified electron cascade model (Gamal et al, 1993) is applied under the experimental conditions that carried out by Phuoc (2000) to determine the threshold intensity dependence on gas pressure for spark ignition in hydrogen combustion using two laser wavelengths namely; 1064 nm and 532 nm. The model involves the solution of the time dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations that describe the change of the formed excited molecules population.

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