Abstract
Laser-driven flyer has been studied for decades as it promises to possess many applications such as in measuring the equation of state (EOS) under ultrahigh pressure, investigating the material dynamic properties under high strain rate, simulating the high-speed impact for aircraft protection, and igniting explosives. However, the planarity and integrity of flyers are determined by indirect velocity lnterferometer system for any reflector (VISAR) or witness slab results due to its high speed and small dimension. For further and wide applications, it is very important to obtain direct experimental proof of the flyer gesture and configuration. Thus, the acceleration and gesture investigation of aluminum flyer driven by laser plasma are studied on Xingguang-III laser facility. The X-ray radiography is achieved by a picosecond laser irradiating the copper wire target. The shadowgraph of flyer and plasma are realized by the incidence of a bunch of infrared laser through the flyer flight path. In additon, photon Doppler velocimetry is employed to measure the flyer velocity simultaneously. The radiography, shadowgraph and velocity of typical small aluminum flyer are obtained. By optimizing the thickness of both CH ablation layer and vacuum gap, the flyer is slowly accelerated via consecutive stress wave produced by plasma colliding. The aluminum flyer has a thickness of 20 μm and diameter of about 500 μm. The whole flyer remains the integrated shape after a great angle of rotation due to uneven plasma loading. The flight distance is about 400 μm, giving an average velocity of 2.2 km/s. The planarity of the flyer is good except a little bend on the two sides due to side rarefaction of plasma. The study verifies that the laser plasma collision can generate a sub-millimeter-diameter metal flyer with integrated shape and a velocity of several kilo-meters per second, showing that it possesses the promising applications in measuring the EOS and igniting explosive .
Highlights
Laser-driven flyer has been studied for decades
as it is promising in many application areas
the planarity and integrity of flyers are determined by indirect VISAR or witness slab results
Summary
强激光烧蚀低密度有机材料形成等离子体射流碰撞可以对材料进行准等熵加 载,比激光冲击加载应变率低,相同压强下可以获得更高的压缩度和更低的 温升,在状态方程、飞片加速等方面有很强的应用前景。在星光III装置上首 次开展了等离子体射流驱动小尺寸铝飞片及姿态诊断联合实验。通过调控有 机材料厚度和真空间隙长度,获得了厚度 20μm、直径约 400μm 的铝飞片, 飞片加速时间长达 200ns。基于 ps 拍瓦激光的高能 X 光背光照相结果显示铝 飞片在飞行约 400μm 距离后仍然保持了很好的飞行姿态和完整性。 直径约 1mm 的铝飞片缓慢加速到 5km/s 的速度[6]。VISAR 干涉条纹光滑连续,一维 平面性良好,流体模拟显示飞片碰撞前温升仅不到 50°C,表明该方法(驱动的飞片) 在状态方程(EOS)的绝对测量方面具有很强的应用潜力。2017 年,Shu 等人利用这 种激光准等熵加载方法开展了铝的状态方程绝对测量[7]。通过调节激光能量,作者将 25μm 厚度、直径约 1mm 的铝飞片加速到 4-12km/s 的速度,通过与铝台阶样品撞击, 同时获得了铝飞片撞击时刻的速度和样品中的冲击波速度,从而实现对铝材料的 EOS 绝对测量。该实验获得了 50-200GPa 压力区间的铝冲击雨贡纽数据,与不同加载方法 获得的实验结果具有很好的一致性,间接证明了这种激光准等熵驱动方式产生的飞片 具有比较好的飞行姿态和较低的温升。同年,本课题研究团队也发表了在原型装置上 开展的激光驱动飞片研究的实验结果[8],将 20-30μm 厚度的铝飞片和钽飞片加速到 3-11km/s 的速度。从上述研究可以看出,利用激光等离子体射流碰撞可以驱动平面性 好、温升较低的飞片,这一点相比传统的激光驱动带窗口约束的飞片具有明显的优势 [9,10,11,12,13]。然而,目前对于飞片平面性和完整性的表征,主要依赖线 VISAR 测量[7,8]和验 证板[13,14]的结果,对飞片的飞行姿态和表观形貌(平面性和完整性)都缺乏直接的观 测结果,而这很大程度上决定了激光驱动飞片能否在 EOS 甚至更多的研究领域(如 激光驱动飞片起爆[10])得到广泛和深入的应用。近两年,基于皮秒拍瓦激光的高分辨 X射线背光照相技术[15]由于具有高亮度、高时空分辨的特点,在惯性约束聚变(ICF) 内爆过程和冲击加载材料微喷过程研究[16,17,18,19]方面已经获得了重要应用。借助这种高时 空分辨的透视成像技术,有望实时捕捉到小尺寸飞片的飞行姿态和表观形貌,为激光 驱动飞片应用研究奠定更坚实的基础。本文基于星光III装置,开展激光等离子体射流 驱动飞片和姿态诊断实验,优化激光参数和靶参数,解决 X 光照相的干扰问题,获得 清晰的飞片姿态图像,并对结果进行细致的分析。 Intensity porfile of ns laser through 1mm CPP Intensity porfile along x direction 60 (b) 50
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