Laser spark ignition of chemically reactive gases.

Exploratory experiments on the ignition by laser-induced sparks of chemically reactive gaseous mixtures, both detollable and nondetonable, were performed. It was found that the minimum energy required to induce breakdown was of sufficient magnitude to generate a strong spherical blast wave. The ignition mechanisms are strongly coupled to the dynamics of the decaying blast. For nondetonable gases, it was observed that initially the blast wave is of sufficient strength to result in auto-ignition of the reactive media with the reaction front trailing immediately behind the spherical shock front. As the blast expands, the reaction front decouples rapidly from the shock and propagates as a deflagration wave dependent on the transport properties of the medium. For detonable mixtures there exists a critical energy above which direct initiation of a spherical detonation wave occurs. This critical ignition energy varies inversely with the initial pressure of the mixture showing the same qualitative behavior as the variation of the induction time with initial pressure. For ignition energy well below this critical value, similar behavior of rapid decoupling to a spherical deflagration wave as in the case of nondetonable gases was observed. For ignition energy close to the critical, decoupling followed by re-establishment to a spherical detonation wave occurs. I. Introduction W ITH the recent development of high-powered solid-state lasers, breakdown in gases can readily be achieved at the focal point when the laser beam is collimated by a simple convex lens. The basic mechanisms of electrical breakdown at optical frequencies and the subsequent phenomena associated with laser sparks are discussed in detail in a recent review article by Meyerand. 1 In the present paper some exploratory experiments on the use of laser-induced sparks for the ignition of chemically reactive gases are described. There are a number of important advantages associated with the use of laser-induced sparks for ignition studies. For example, the laser spark is almost a perfect point energy source in which the energy and the rate of its deposition can be controlled and measured accurately. Also, pure chemical ignition, entirely free of contamination by any electrode material, can be achieved. The complete absence of material surfaces in the vicinity of the ignition area eliminates the complex effects of heat transfer on the initial growth of the flame kernel. The previously mentioned properties of the laserinduced spark make it an ideal energy source for the study of the basic mechanisms of ignition and the subsequent combustion phenomena in a chemically reactive gas. It is particularly suitable for ignition experiments in supersonic streams where electrode interference effects would be significant. The present experiments are of a qualitative nature with the objective being to determine the ignition mechanisms and subsequent development of the combustion phenomena from a laser-induced spark in a combustible gaseous mixture. The particular emphasis is centered on detonable gases where it is of interest in the present program to determine the minimum energy level of a free spark that will result in the