Resonant laser-induced breakdown across high voltage gap for air-fuel ignition

A resonant multi-photon scheme has been investigated that could provide laser-induced ignition within a high-voltage gap across an aircraft combustion chamber. The resonant technique could potentially be applied as a laser trigger from a compact low power laser source leading to breakdown and ignition of an aircraft air-fuel flow. Experiments with resonant laser-induced ignition of a moderate-speed flow of an air-propane mixture were conducted by applying a high voltage across the flow chamber that was significantly less than normal self-breakdown. The resulting spark followed the laser pre-ionized path across the chamber within a few microseconds of the laser pulse which led to ignition and combustion on the time scale of milliseconds. This laser scheme involves resonant enhanced multi-photon ionization (REMPI) and subsequent laser field-enhanced electron avalanche to generate a pre-ionized micro-plasma path between flow chamber walls and thus guide the ignition spark through fuel-rich areas of the air-fuel flow. We have determined that a specific resonant ultraviolet wavelength, 287.5 nm, is optimal at relatively low laser pulse energy to photo-ionize oxygen molecules in a fuel-air flow to reliably initiate ignition. This laser wavelength coincides with a 2+1 REMPI transition in molecular oxygen. With this resonant method, sufficient photo-ionization and laser field-enhanced electron avalanche ionization have been generated for inducing air breakdown at a relatively low laser power compared to most laser breakdown concepts. This low power requirement may allow for a laser source to be transmitted to an ignition chamber via fiber optic coupling. Results of this study include detailed spectroscopy of the evolution of the optical emission from laser pre-ionization, spark, and eventual ignition. Spectroscopic results are compared to breakdown in pure air to determine the role of the 2+1 REMPI transition in molecular oxygen compared to hydrocarbon dissociation and ionization processes that may aid ignition. In addition, high speed photography of flame ignition in an air-propane flow was conducted, showing the spatial and temporal evolution of the laser-induced spark and flame kernel leading to combustion.