Unfocused laser ignition of high-pressure He-H2-O2 combustible mixtures

We report consistent ignition of high-pressure (p(fill)>20-30 bar) hydrogen-oxygen mixtures diluted with helium in two different combustion vessels, using an unfocused Nd:YAG laser. This corresponds to laser irradiances several orders of magnitude below the minimum ignition energies reported in the literature. This unusual phenomena has led us to try to measure the amount of laser radiation absorbed by the gas medium. By placing a mirror inside a cylindrical vessel and filling it up to 100 bar with He-H-2 or He-O-2 non-combustible mixtures, we obtain the pressure-dependent absorptivity of the combustible He-H-2-O-2 mixture. We find no measurable absorption of the laser signal by the medium, for the overall pressure range, to the experimental apparatus sensitivity (about 1% of the laser irradiance). The exact mechanism for ignition remains henceforth unknown. One possibility could the creation of seed electrons created by autofocusing ionization of dust/impurities in the gas, but this has yet to be experimentally confirmed