LASER-IGNITION OF LASER-SHOCK DISPERSED METAL PARTICLES

Understanding the post-detonation ignition and combustion of metal particles loaded in an explosive under the extreme conditions encountered in open-air or internal blast applications is important for maximizing the energy output. Of particular interest is the effect of particle velocity on ignition and sustained combustion. Published laboratory experiments are typically limited to velocities in the range of 0.5 to 10 m/s. These are significantly lower than velocities achieved in detonating explosives which can reach 500 m/s to 1 km/s. The work presented in this paper describes a new technique to examine the ignition and combustion properties of particles travelling at high velocity in an oxidizing atmosphere. The technique utilizes two short (10 ns) IR laser pulses. The first launches the particles at velocities reaching 250 m/s as determined by high-speed digital shadowgraphy. The second, fired after a delay period, heats a section of the resulting particle cloud to ignition. Ignition and combustion are monitored using photodiodes, pyrometry, and time-resolved spectroscopy. The ignition delay for 1 to 15 urn Al particles is found to increase as particle diameter increases and particles larger than 33 km do not ignite under the current conditions. However, 9 km Ni coated Al particles were found to ignite more quickly and with lower laser fluence as compared to pure Al particles of similar size. Wavelength dependent laser absorption efficiency effects are discussed as a potential reason for these differences.