Interferometric study on the shock wave collisions during double laser produced plasmas

Summary form only given. Double laser produced plasmas are receiving increasing attentions to improve the performance of different applications, such as the laser-induced breakdown spectroscopy (LIBS) or the tabletop sources of short wavelength light. Previously, the plasmas dynamics during the double laser ablation of metal targets have been studied from the perspective of self-emission, by the fast photography or the optical emission spectroscopy. Jet-like plasmas, and stagnation layers were observed at the interaction zone for the low-Z target material, while interpenetration phenomena were found for the high-Z material. However, interaction characteristics of the shock waves during the double laser-ablated-metal-plasmas expansion into the atmospheric air were hardly ever studied. In this work, we investigated the shock wave collisions during the expansion of plasmas generated by double laser ablating aluminum target. A Nd:YAG laser (1064 nm 40 ns) with the energy of ~ 20 mJ was splitted by a wedge prism first, and then focused by a plano-convex lens onto the target. The two foci created at the target surface were separated by a distance of about 2 mm. The other Nd:YAG laser (532 nm 30 ps) was employed to construct a Mach-Zender interferometer. A DG535 was used to trigger the actions of both the ablation laser and detection laser, with controlled time intervals to get time-resolved interferometric results. The phase shifts were extracted by the 2D FFT method, and then refractive indexes were recovered by the Abel transformation. At last, the electron density as well as the gas density behind the shock waves in the interaction zones were estimated by the refractive indexes. In the end, a compressible fluid model were used to simulate the plasmas and shock waves dynamics, and results from the simulation are compared with experimental measurements.