Laser ignition of methane-air mixtures at high pressures

Laser induced plasma ignition of methane-air mixtures at initial pressures up to 4 MPa was investigated experimentally using nanosecond Nd:YAG-laser pulses at 1064, 532 and 355 nm. Variations of the focusing system yielded, independent of the laser wavelength, an optimum focal spot diameter of about 20 mum under the experimental conditions, indicating that small impurity particles provide the seeds for laser plasma generation. The minimum laser pulse energy needed for ignition (ranging from 8 to 15 mJ) decreased with increasing initial pressure and with increasing equivalence ratio in the mixture regime of phi = 0.91-0.56. Corresponding threshold intensities ranged from 10(10) to 10(11) W/cm(2). Evidence for a non-resonant breakdown mechanism was established. (C) 2003 Published by Elsevier Science Inc.