Experimental progress of laser-driven flyers at the SG-III prototype laser facility

Abstract

Laser-driven flyers have unique advantages of high flyer velocity, low cost, simple facility compared with the flyers driven by other conventional dynamic high-pressure loading techniques. With the fast development of laser technique, launching hypervelocity flyers with high-intensity laser pulse has become more and more prevalent. In this paper, we introduce the recent experiments of laser-driven flyers at the SG-III prototype laser facility. Three ways of launching hypervelocity flyers are developed and introduced, respectively. In the first way, multilayered aluminum flyers are gradually accelerated to a terminal velocity of 8 km/s, which is measured by optical velocimetry, without melting and vaporization. The pressure distribution within the flyer shows that the temporally ramped pulse ablation generates a compression wave, and the flyer is accelerated by this wave and its reverberation within the flyer. In the second way, a strong laser ablates the low-density reservoir foil and generates strong shock in the foil. The shock wave is strong enough, and when the shock breaks out from the free surface, the foil will unload as plasma towards the flyer with a density profile. The plasma decelerates upon colliding the flyer, and the single-layered flyer is gradually accelerated by the momentum transition. In our experiments, single-layered aluminum foil and single-layered tantalum foil are accelerated to 11.5 km/s and 6.5 km/s, respectively. According to the pressure distribution within the flyer, the flyer is also accelerated by the compression wave produced by the plasma collision, which is similar to the case of direct ablation by temporally ramped pulse. However, the way of plasma collision could better reduce X-ray and electron preheat and obtain cleaner flyers. In the last way, the flyers are launched by direct strong short-laser ablation. The multi-layered aluminum foil is accelerated to a high average velocity of 21.3 km/s by using a 3-ns quadrate laser pulse at 351 nm after spatial homogenization. A line-velocity interferometer system for any reflect (VISAR) is employed to monitor the processes of flyer launch and flight in a vacuum gap and the shock velocity associated with phase change in fused silica target after flyer impact is inferred. The reflectivity variations of the VISAR fringe pattern and the shock velocity in the fused silica suggest that the flyer owns a density gradient characteristic. Furthermore, specifically designed multi-layered flyers (polyimide/copper) are accelerated by shock impedance and reverberation techniques to a super high averaged velocity of 55 km/s, which is much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows are obtained, which indicates the good planarity and integrity for the flyer. Compared with single-layer flyers, multi-layered flyers have a good planarity, and a high energy conversion efficiency from laser to flyers. In this paper, we give a comprehensive analysis and comparison of the experimental designs, technique means and data results about laser-driven flyers. This would provide a reference for further experimental study of laser-driven flyers and also verify that the SG-III prototype laser facility is a very promising facility for studying the hypervelocity flyers launching field.