Experimental investigation of the shock loss and temporal evolution of hot plume resulting from dual-pulse laser-induced breakdown in quiescent air

Abstract

To optimize a laser ignition scheme, absorption rate measurements and Schlieren visualizations are performed on dual-pulse laser-induced breakdowns (LIBs) at incident energies from 50 mJ to 200 mJ and pulse intervals that range from 20 ns to 250 μs 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 (Jones' 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.