The acceleration mechanism of shock wave induced by millisecond-nanosecond combined-pulse laser on silicon

The velocity variation law of shock wave induced by millisecond-nanosecond combined-pulse laser has been investigated experimentally. The pulse delay and laser energy are important experimental variables. The method of laser shadowgraphy is used in the experiment. Experimental results show that when the pulse delay is 2.4 ms, the ms and ns laser energy density is 301 J cm⁻² and 12 J cm⁻², respectively, the velocity of shock wave is 1.09 times faster than that induced by single ns pulse laser. It is inferred that the shock wave propagates in the plasma is faster than that in air. When the ms and ns laser energy density is 414.58 and 24 J cm⁻², the velocity of shock wave shows rising trend with pulse delay in a range of 1.4 ms > Δt > 0.8 ms. It is indicated that with the increase of ns laser energy, the laser energy absorbed by laser-supported absorption wave increases. The mechanism of inverse bremsstrahlung absorption acts with target surface absorption simultaneously during the ns laser irradiation. Thus, the phenomenon of the double shock wave is induced. The numerical results of the phenomenon were accordance with experiment. The results of this research can provide a reference for the field of laser propulsion.