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

TL;DR

This study demonstrates that unfocused laser ignition can reliably ignite high-pressure He-H2-O2 mixtures at energies below previously reported thresholds, likely due to impurity-induced ionization rather than direct absorption.

Contribution

It provides experimental evidence that laser ignition at very low energies is possible in high-pressure mixtures, challenging existing assumptions about minimum ignition energies and absorption mechanisms.

Findings

Ignition occurs at laser irradiances much lower than previous minimums.

No measurable absorption of laser energy by the mixture was detected.

Ignition likely results from impurity-induced seed electrons rather than direct laser absorption.

Abstract

We report consistent ignition of high-pressure (p > 20-30 bar) hydrogen-oxygen mixtures diluted with helium, using an unfocused Nd:YAG laser. This corresponds to laser irradiances several orders of magnitude below the minimum ignition energies reported in the literature. By placing a mirror inside a cylindrical vessel and filling it up to 100 bar with H2-He or O2-He non-combustible mixtures, we obtain the pressure-dependent absorptivity of the combustible He-H2-O2 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). This leads credence to the theory that ignition stems from seed electrons created by autofocusing ionization of dust/impurities in the gas.