Abstract

The plasma produced by focusing a passively Q-switched 30 J ruby laser beam is studied as regards its suitability for minimum ignition energy measurement. By comparison with electric spark discharges, it would appear to offer shorter times and smaller volumes of the initiating plasma, as well as freedom from energy losses to electrodes and elsewhere within the circuit. The growth of the initiating plasma as well as that of the flame kernel are studied as a function of pressure and composition, the former using an optical delay line with a schlieren system and the latter by high-speed schlieren streak photography. It is found that the difficulties of the method are associated with the finite duration of the laser pulse. In the time between the onset of breakdown and the end of the pulse the plasma front facing the incident beam absorbs the incoming energy, leading to the production of an extended and unsymmetrical initiating source, an absorption threshold which may already be in excess of the minimum ignition energy, and the formation of a blast wave which may be powerful enough to initiate a detonation rather than a deflagration. For a pulse of approx. 20 nano second half width these effects become serious above about half an atmosphere for stoichiometric methane-air mixtures. For lower pressures, or near limit mixtures, the promise of the method is borne out and both ignition energies and quenching distances fall below those determined by the electric spark method. For higher pressures and faster reacting mixtures it will be necessary to decrease the duration of the laser pulse and the size of the plasma.

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.