Induction Phenomena of Laser Ignition

Abstract

A preliminary study on induction phenomena of laser-ignition was conducted. For observation of the initial phenomena, an experimental study on effects of a focused laser pulse (Nd:YAG, 335mJ/pulse, pulse width 5nsec) into a hydrogen-air mixture was conducted. Temporal evolutions of typical line spectra of a laser-induced plasma of the mixture were measured with the photodiode through specific band-pass filters for each spectrum for N⁺, H, and O, and also with a time-resolved ICCD spectroscopic analyzer. As the results, it was found that phenomena occurring after laser irradiation up to combustion completion were divided into three processes; i) absorption process of an incident laser pulse (2 x 10^-9~ 10^-8 sec), ii) plasma formation process (ionic emission by 400 nsec followed by atomic emission) (10^-8 ~ 10^-6 sec), and iii) ignition (10^-5 ~ 10^-4 sec), having various scales of the characteristic times. In addition, temporal evolution of electronic excitation temperature of N⁺ ions was estimated from the Boltzmann plots. It was shown that the temperature was about 25,000 K at initial 200 ~ 400 nsec, and then gradually decreased 20,000 K at 900 nsec. Moreover, electron densities were also estimated from the Stark broadening widths of H_α and H_β spectra. Temporal evolution of the electron density showed the electron density of about 4 x 10¹⁷ cm^-3 at initial 300 nsec, and then, down to 1.3 x 10¹⁷ cm^-3 at 900 nsec.