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From page 17... ... ABSTRACTS AND R E V I E W S 245 latter by high-speed Schheren streak photography I t is found that the diflSculties of the method are associated wi th the fimte duration of the laser pulse I n the time between the onset of breakdown and the end of the pulse, the plasma front facmg the incident beam absorbs the incoming energy, leading to the production of an extended and unsymmetrical imtiating source, an absorption threshold which may already be in excess of the mimmum igmtion energy, and the formation of a blast wave which may be powerful enough to imtiate a detonation rather than a deflagration For a pulse of approx 20 nanosecond half-width, these effects become senous above about half an atmosphere for stoichiometric methane-air mixtures For lower pressures, or near hmit mixtures, the promise of the method is borne out and both igmtion energies and quenching distances fal l below those determined by the electric spark method For higher pressures and faster reacting mixtures, i t wi l l be necessary to decrease the duration of the laser pulse and the size of the plasma C Detection of Fires D Read the entire page →

From page 17...

... ABSTRACTS AND R E V I E W S 245 latter by high-speed Schheren streak photography I t is found that the diflSculties of the method are associated wi th the fimte duration of the laser pulse I n the time between the onset of breakdown and the end of the pulse, the plasma front facmg the incident beam absorbs the incoming energy, leading to the production of an extended and unsymmetrical imtiating source, an absorption threshold which may already be in excess of the mimmum igmtion energy, and the formation of a blast wave which may be powerful enough to imtiate a detonation rather than a deflagration For a pulse of approx 20 nanosecond half-width, these effects become senous above about half an atmosphere for stoichiometric methane-air mixtures For lower pressures, or near hmit mixtures, the promise of the method is borne out and both igmtion energies and quenching distances fal l below those determined by the electric spark method For higher pressures and faster reacting mixtures, i t wi l l be necessary to decrease the duration of the laser pulse and the size of the plasma C Detection of Fires D

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