KrF激光对高速飞片的驱动特性研究

Research on the driving characteristics of high speed flyer using KrF laser

利用诱导空间非相干技术平滑的KrF准分子(248nm)激光驱动带有烧蚀层的平面靶,研究激光空间均匀性对产生完整飞片的影响,结果表明激光不均匀性在2%以下,能够产生完整的高速飞片,且完整飞片能够维持20ns以上不破裂;当激光不均匀性达到5%,激光引入流体力学不稳定性种子应很强,冲击波在靶内输运过程中不稳定性不断发展增强,到靶背时强到足以使飞片解体甚至气化,不能产生完整的飞片。为了获得尽可能高的飞片速度,采用激光与烧蚀层参数不匹配方法,使冲击波对飞片作多次加速。利用功率密度为1012 W/cm2的KrF激光与含50μm Kapton烧蚀层的5μm铝飞片作用,得到速度约10km/s的高速飞片,与模拟结果吻合得很好。

KrF excimer laser （248nm）has high spatial uniformity by using induced spatial in-coherence technology.After image relay and amplification by optical angular multiplexing tech-nology,non-uniformity of image surface of focal spot is 1 .6%.The spot spatial uniformity decreases with increasing distance of deviating image surface,the spatial non-uniformity is up to 5% at 3mm place deviating image surface.Laser directly drives planar target with ablation layer, the effects of laser spatial uniformity on flyer planarity and integrity were investigated.The results show when the non-uniformity of laser is below 2%,the integrity of inference fringes measured by laser velocity interferometer system for reflector keeps very well,the inference frin-ges last about 20ns after fringes jump.And the flyer can fly about 150 microns.The roughness of discontinuity is 62ps,which indicates that the shock wave planarity is good.The target debris deposited on sputtering film from the target of about 120mm is larger particles,which confirms the flyer has better integrity.When the laser non-uniformity is 5%,the interference fringes dis-appear completely after fringes jump,and the target debris deposited on sputtering film becomes powder,which indicates the shock wave planarity is very poor under this laser condition.The experimental results show hydrodynamic instability seed is weak at good condition of the uni-formity of laser,the development of shock waves is not strong enough to make the flyer disinte-gration when shock waves transfer from the front surface to rear of larget.When the laser non-uniformity is about 5%,hydrodynamic instability seed driven by laser is very strong.When the shock wave transferred to the target back,hydrodynamic instability develops strong enough to make the flyer disintegration even gasification,which leads to the disappearance of interference fringes.So shock planarity driven by laser is strongly dependent on the laser beam spatial uni-formity,and the planarity and integrity of flyers depend on the shock planarity.According to the simulation results using one dimensional HYADES program,the ablation layer thickness should be reduced in order to improve the velocity of flyer.At the condition of thinner ablation layer,la-ser and ablation parameters do not match,shock waves driven by laser can accelerate the flyer multi-times.KrF laser with power density of about 1012 W/cm2 drove flat target of 5μm aluminum with 50μm Kapton ablation.Flyer trajectory was obtained using side-on shadowgraph.The curves of flyer velocity and acceleration were obtained by fitting the trajectory curve.Acceleration reaches 0.23＆#215;109km/s2,the velocity of flyer is about 9.8km/s.And the HYADES simulation shows there are 4 acceleration processes,the flyer velocity is about 10km/s.The results of experiment and simulation agreed very well.At the same conditions,simulations also show when the abla-tion thickness decreases to 30μm,the flyer velocity can be up to 16.5km/s.Target debris from backward jet maybe pollutes the laser focusing system components for too thin ablation.These all need to be verified by experiments.