ENERGY DEPOSITION; IGNITION ENERGY; DRAG REDUCTION; FLOW-CONTROL; SPARK-IGNITION; INDUCED PLASMA; HIGH-SPEED; COMBUSTION; MIXTURES; WAVE

To optimize a laser ignition scheme, absorption rate measurements and Schlieren visualizations are performed on dual-pulse laser-induced breakdowns at incident energies from 50 mJ to 200 mJ and pulse intervals that range from 20 ns to 250 mu s in quiescent air at atmospheric pressure. For comparison, experiments on single-pulse LIBs are also conducted. The shock loss is determined using a semi-empirical model , and quantitative information on the spatial distribution of the hot plume is extracted from Schlieren images using in-house code. The results reveal that multi-location laser ignition can be achieved without reducing the energy absorption or strengthening the shock loss only when the energy of each laser pulse exceeds 200 mJ. This requirement is because the absorption rate of single-pulse LIB decreases significantly when the laser energy is lower than 200 mJ, and the shock loss of single-pulse LIB invariably accounts for approximately 80% of the absorbed laser energy at various incident energies. Compared with single-pulse LIB, dual-pulse LIB with a pulse interval of less than 200 ns is slightly inferior in terms of energy absorption and shock loss; however, the advantages of a larger initial plasma volume and lower energy dissipation can compensate for this deficiency. Therefore, dual-pulse laser ignition is a promising alternative to single-pulse laser ignition. Moreover, ignition reliability can be enhanced by initially releasing the laser pulse with higher energy when the energies of the successive pulses are not the same because of higher energy absorption and lower shock loss. In addition, the spatial distribution of the resulting hot plume is relatively centralized, which helps to reduce energy and radical dissipation. However, a pulse interval longer than 200 ns should be avoided for dual-pulse LIB because the laser energy cannot be utilized efficiently.

INDUCED BREAKDOWN SPECTROSCOPY; STAINLESS-STEEL; PROPAGATION; REMOVAL; MEDIA

High Energy Lasers (HELs) used for defense applications require operational distances ranging from few hundred meters to several kilometers. As the distance increases, the incident beam properties and, consequently, the anticipated effect delivered to the sample become less predictable. Therefore, the direct observation of the event induced by the laser can become an asset. In this paper, we propose a novel spectroscopic method that analyses in real time the spectral components present in the flames produced during the interaction of a HEL with a metallic piece at a long distance. This method was used on aluminum and carbon steel samples placed 200 m away from the laser system. It was discovered that the aluminum and iron oxides created as a by-product of the HEL reaction with the samples emitted clear fingerprint signatures that could be detected remotely using a spectroscopic receiver placed beside the HEL beam director. The real-time assessment of the laser-induced effect can be achieved by monitoring the temporal evolution of the oxide signatures, hence providing information to the operator about the reaction and the nature of the sample illuminated.

INDUCED BREAKDOWN SPECTROSCOPY; CAVITATION BUBBLE; NANOPARTICLES; SIMULATION; UNDERWATER; COLLAPSE; LIQUID; VALIDATION; TRACKING; GROWTH

Laser ablation of a solid target immersed in liquid (such as water) has many important applications such as laser synthesis of nanoparticles, lasermicromachining in water, and laser shock peening. Laser ablation of a solid target in water involves complicated physical processes. One important process often involved is the generation and evolution of a bubble in water and attached to the target surface, which may have significant effects on the target and the ambient water, and hence may greatly affect the relevant practical applications. Some experimental studies were reported in the literature on bubble evolutions induced by laser ablation of a solid target in water. However, the reported previous relevant physics-based modeling work is not sufficient. A physics-based model may help improve the process fundamental understanding and generate valuable information to related applications. In this paper, physics-based modeling work has been performed on the shrinking process of a bubble induced by laser metal ablation in water, together with time-resolved shadowgraph imaging experiments to verify the model. The model-predicted bubble evolution agrees reasonably well with the experimental measurement shown in the paper. Under the studied conditions, it has been found that near the bubble collapse moment (i.e., the moment when the bubble shrinks to a minimum size): (1) the bubble shrinks very fast, and the peak fluid velocity magnitude occurs inside the bubble and can exceed similar to 550 m/s; (2) the temperature inside the bubble increases very quickly and approaches similar to 2000 K; and (3) the pressure inside the bubble becomes very high, and can reach a peak magnitude of similar to 380 MPa at the collapse moment at the bubble center. During the shrinking process, a high-pressure region outside and near the bubble wall is generated near the collapse moment, but the temperature of the region outside the bubble mostly remains low. Published by AIP Publishing.