1.Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang 621900, China 2.Departments of Experiments, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang 621900, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11804319, 11805177)
Received Date:16 August 2019
Accepted Date:01 November 2019
Available Online:01 January 2020
Published Online:20 January 2020
Abstract:High intensity laser is an efficient method for shock generator to study the dynamic fragmentation of materials, in which the direct drive is widely utilized. The continuum phase plate is used for smoothing the focal spot of the laser, but the loading region is usually smaller than the designed value. In this work, we study an experimental technique for investigating the dynamic fragmentation of metal via indirectly driving a high-intensity laser. Firstly, the radiation distributions on the sample for four different hohlraums each with a diameter of 2 mm but different length are simulated via the IRAD software, in which the proper hohlraum with a diameter of 2 mm and a height of 2 mm is selected for the experiments. Secondly, the peak temperatures and radiation waves under different laser energy and pulse durations are measured. The peak temperature decreases simultaneously as the laser energy decreases. In addition, the loading shock waves under a peak temperature of 140 eV and different radiation waves are estimated via the hydrodynamic simulation. It is revealed that a peak pressure of several tens of gigapascals is acquired and the peak pressure is greatly increased when the 10 μm CH layer is placed on the sample. In the end, the dynamic fragmentation process via indirect drive is investigated by using the high energy X-ray radiography and photonic Doppler velocimetry. The radiograph is a snapshot at 600 ns and shows a typical result of the spall process. The first layer is measured to be 0.06 mm thick and 0.3 mm away from the unperturbed free surface. It is also exhibited that the hohlraum is expanded to a large extent but is not broken up. The jump-up velocity and time of spall are measured to be 0.65 km/s and 131 ns, respectively. The average velocity of the first layer is estimated to be (0.63 ± 0.1) km/s, obtained via the distance of 0.3 mm divided by the time difference of 469 ns (600 ns minus 131 ns). The one-dimensional loading region is 2 mm, and the flatness is better than 5 %. This work provides a reference for designing new hohlraum, shock wave loading technique and dynamic fragmentation process. Keywords:indirect drive/ dynamic fragmentation/ high energy X-ray radiography
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2.1.实验设计
实验在神光大型激光装置上完成, 实验装置示意图如图1所示. 实验利用上四路纳秒激光注入半柱腔中产生X射线并对锡平面靶进行冲击加载; 在一定的时间延迟后, 利用皮秒激光作用产生微焦点、高能X射线对加载后样品成像[13], 并采用成像板(IP)记录图像;同时利用光子多普勒干涉仪(PDV)对界面速度进行诊断[16]. 高能X射线成像及PDV测速的细节内容已在文献[13,16]中报道, 这里不再赘述. 图 1 激光间接驱动冲击加载物理实验示意图 Figure1. The schematic view of indirect driving shock wave experiments via lasers.
实验中半柱腔为Au腔, 其直径为2 mm, 腔长为2 mm, 注入口直径为0.8 mm, 腔壁厚为0.04 mm. 锡靶厚度为0.5 mm, 光洁表面, 表面粗糙度优于0.1 μm. 为了提高辐射驱动压力, 锡靶上表面粘上CH层(薄膜, 只含C, H元素), 其厚度为10 μm. 背光靶为Au丝靶, Au丝直径为20 μm, 长度为0.5 mm, Au丝放置于CH基底上. 锡靶置于靶室中心(通过靶室的外置基准定位靶室中心的一个点, 定义为靶室中心), 背光靶置于靶室中心偏北20 mm (靶室平面为赤道面, 再按实际方位分东西南北向). 纳秒激光参数为上四束激光, 激光波长为0.351 μm, 脉宽为3 ns或2 ns, CPP束匀滑后焦斑直径为0.45 mm, 实验中每发次激光能量均实测, 数值在800—4800 J范围内, 纳秒激光注入黑腔中. 皮秒激光频率为1.06 μm, 脉宽为10 ps, 能量为450 J, 聚焦光斑Φ50 μm, 注入丝靶中心. 皮秒激光与丝靶相互作用会产生微焦点、高能X射线[13,17,18], 能段范围为50—200 keV, 前期实验表明Φ20 μm丝靶产生X射线可用于高空间分辨成像, 成像空间分辨在20 μm左右. 典型发次的动态诊断实验中, 纳秒激光注入时刻提前皮秒激光600 ns. 22.2.理论设计 -->
利用IRAD软件模拟半柱腔直径为2 mm、不同腔长下样品处峰值辐射温度分布, 结果如图2所示. 在腔长为1 mm条件下, 辐射温度分布图上显示4个局域高温点且温度分布极为不均匀. 在腔长为1.5, 2.0, 2.5 mm下, 整个样品处的辐射温度分布较为均匀. 以直径为2 mm, 腔长为2 mm为例, 中心处辐射温度为152 eV, 边缘处辐射温度为143 eV, 这个辐射场的均匀性在10%以内. 辐射温度在(150 ± 2) eV区域的直径为1.5 mm, 均匀性优于2 %. 考虑到实验中存在着激光注入时束间平衡、激光等离子体效应等诸多效应影响, 实验中半柱腔尺寸设计为直径2 mm, 腔长2 mm. 图 2 不同腔长下样品处的辐射分布 Figure2. Radiation distribution in the surface of the sample for hohlraum with different lengths.
23.2.辐射波形 -->
3.2.辐射波形
利用强激光注入半柱腔中产生均匀辐射场, 通过纳秒针孔相机监测光斑注入正常, 未形成挂边等异常注入情况. 再利用平响应X射线衍射(FXRD)获得不同能量、不同脉宽下辐射波形, 如图3所示. 在激光脉宽为3 ns情况下, 辐射温度峰值时刻为4.0 ns处, 考虑到测量信号的起点为1.1 ns, 实际辐射峰值温度时刻为2.9 ns. 峰值温度在激光能量1000 J时为138 eV, 且随激光能量提高而上升. 在2 ns情况下, 实际辐射温度峰值时刻为1.9 ns. 这里, 激光能量1314 J时辐射温度反而略比844 J低, 经分析发现此发次FXRD的信噪比较大, 但在误差范围内. 图 3 辐射波形 (a)激光脉宽3 ns; (b)激光脉宽2 ns Figure3. Radiation wave at different pulse duration of laser: (a) 3 ns; (b) 2 ns.
23.3.冲击加载波形 -->
3.3.冲击加载波形
冲击加载波形指的是冲击波到达靶后界面时靶内压力分布情况, 此加载波形还与辐射波形与靶厚度有关. 利用一维流体软件模拟不同辐射波形下加载波形, 结果如图4所示. 从模拟结果看出, 3种不同脉宽的辐射波形下, 辐射驱动的加载波形均为三角波结构分布, 且随着脉宽增加, 峰值压力有所增加. 图 4 (a)不同辐射波形; (b)冲击加载波形 Figure4. (a) Radiation wave; (b) loading shock wave at different pulse duration of laser.
实验通过上四束激光注入柱腔中, 产生辐射烧蚀加载样品; 并利用高能X射线诊断加载后样品动态破碎过程. 典型发次的实验结果(总能量为1000 J, 脉宽为3 ns)如图5所示. 动态图像清晰地显示半柱腔已经膨胀到一定程度但尚未完全解体, 在柱腔侧壁上仍能清楚地看到激光弹着点, 柱腔顶端注入口已与侧壁分离并飞行了一段距离. 图 5 高能X射线动态诊断间接驱动的层裂过程 Figure5. High energy X-ray radiography of spall from indirect drive by laser.
从图5还可以看到靶支撑结构和未扰动自由面位置. 冲击加载后的锡样品形成块状层裂片, 其厚度为0.06 mm, 脱离基底向前飞出, 相对未扰动自由面运动的距离为0.3 mm. 在自由面速度曲线中, 该动态过程为典型的层裂行为[20], 起跳速度为0.65 km/s, 起跳时间为131 ns, 平均运动速度为0.55 km/s. 由图中层裂片运动距离为0.3 mm、运动时间为469 ns (诊断时刻600 ns减去自由面起跳时刻131 ns), 由此推出层裂片平均运动速度为(0.63 ± 0.1) km/s, 与PDV的测试结果一致 (如图6所示). 图 6 间接驱动层裂过程的自由面速度历史 Figure6. Velocity of free surface of spall from indirect drive by laser.