Fund Project:Project supported by the Joint Fund of the National Natural Science Foundation of China and the China Academy of Engineering Physics (Grant No. U1930108), the Science Challenge Project, China (Grant No.TZ2016005), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA25030700)
Received Date:21 July 2020
Accepted Date:13 August 2020
Available Online:27 November 2020
Published Online:20 December 2020
Abstract:Kelvin-Helmholtz instability is the basic physical process of fluids and plasmas. It is widely present in natural, astrophysical, and high energy density physical phenomena. With the construction of strong laser facilities, the research on high energy density physics has gained new impetus. However, in recent years the magnetized Kelvin-Helmholtz instability was rarely studied experimentally. In this work, we propose a new experimental scheme, in which a long-pulsed nanosecond laser beam is generated by a domestic starlight III laser facility. The whole target consists of two parts: the upper part that is the CH modulation layer with lower density, and the lower part that is the Al modulation layer with higher density. The laser beam is injected from one side of the CH modulation layer and generates a CH plasma outflow at the back of the target. During the transmission of the CH plasma outflow, the Al modulation layer is radiated and ionized, which makes the Al modulation layer generate an Al plasma outflow. The interaction between the Al plasma outflow and the CH plasma outflow produces a velocity shear layer, and then Kelvin-Helmholtz instability will gradually form near the Al modulation layer. In this paper, the open-source FLASH simulation program is used to conduct a two-dimensional numerical simulation of the Kelvin-Helmholtz instability generated by the laser-driven modulation target. We use the FLASH code, which is an adaptive mesh refinement program, developed by the Flash Center at the University of Chicago, and is well-known in astrophysics and space geophysics, to create a reference to the magnetohydrodynamic solution in our experiment. At present, this code introduces a complete high-energy-density physical modeling module, which is especially suitable for simulating intense laser ablation experiments. The equation of state and opacity tables of targets are based on the IONMIX4 database. The evolution of Kelvin-Helmholtz vortices, separately, in the Biermann self-generated magnetic field, the external magnetic field, and no magnetic field are investigated and compared with each other. It is found that the self-generated magnetic field hardly changes the morphology of the Kelvin-Helmholtz vortex during the evolution of Kelvin-Helmholtz instability. The external magnetic field parallel to the fluid direction can stabilize the shear flow. The magnetic field mainly stabilizes the long wave disturbance. The study results in this work can provide theoretical guidance for the next step of the Kelvin-Helmholtz experiment under a strong magnetic environment in the high energy density laser facility. Keywords:magnetic field/ Kelvin-Helmholtz instability/ high energy density/ laser
其中Te为电子温度, $ \ln \varLambda = 10 $为库仑对数, Z = 3.5 为原子序数. 根据Farmer等[26]的结果, 有$ \omega_{{\rm{e}}} \cdot \tau_{\rm{ei}} \ll 1 $, 所以电子热传导的各向异性很小. 在垂直于磁场以及平行于磁场的情况下, CH等离子体出流整体的发展是各向同性的. 由于各向异性电阻率系数与各向异性的热传导相关, 在电子热传导基本各向同性的情况下, 可以基本忽略磁场对电阻率各向异性带来的影响. 为了更详细地分析外部磁场对KHI涡旋的抑制作用, 图5为0—120 ns不同时刻内磁场强度的分布情况. 图5中白色线段表示磁力线, 颜色分布表征在x-y平面内的磁场强度. 图5(a)为激光打靶前在x方向上外加磁场的初始状态即静态磁场分布. 外加磁场的初始值为0.4 T. 图5(b)—(d)分别对应40, 80和120 ns时的磁场分布. 图5(b)显示, 当靶后CH等离子体流到达Al调制层和外加磁场区域时同时参与两个过程, 一方面靶后CH等离子体流受到了x方向外加磁场施加的磁压力和磁张力影响向中心准直, 另一方面由于靶后CH等离子体流和Al调制层之间的挤压效应, 使得外加磁场发生变形并被放大至1 T左右. 随着靶后CH等离子体流与Al调制层不断的相互作用, 在调制层界面处生长出KHI涡旋的同时, KHI涡旋处的磁场也在不断地被挤压和放大, 其中的一部分等离子体流的动能转化为了外加磁场的磁能. 如图5(d)所示, KHI涡旋处的磁场剪切最为剧烈, 中心磁场强度约2.4 T, 在磁压力和磁张力共同影响下, KHI涡旋的生长受到抑制. 这里需要说明的是, 外加磁场需要穿透一个完全电离的超声速等离子体, 以确保磁场和等离子体流出之间的相互作用, 那么电子的拉莫尔半径需要远小于系统尺度[27]. 可以用单流体电阻磁流体力学来分析电子参数. 经压缩放大后, 外加磁场为2.4 T, 如图5(d)所示, 电子拉莫尔半径为$ r_{\rm {L e}} \!=\! \left(2 K m_{\rm e} / e B\right)^{1 / 2} $ = $7.12 \times 10^{-6}\;{\rm{mm}}$$(K \!=\! m_{\rm e} v^{2} / 2$, $ v = 3 \;{\rm{km}} / {\rm{s}} $为等离子体沿x方向的速度). 可以选择靶后CH等离子体流在120 ns时在靶后外加磁场区域前进距离作为系统尺度$ {{D}} = 1.2\;{\rm{mm}} $, 电子拉莫尔半径与系统尺寸的比值$ r_{\rm {L e}} / D $约为$ 5.93\times 10^{-6} $. 因此电子的拉莫尔半径比系统尺度小得多, 这说明电子被完全磁化, 磁场可以穿透等离子体. 图5(b)—(d)中未发展和形成KHI的扰动区域也显示出磁场发生了变化, 可以看到调制层的结构, 这是因为尽管这一区域没有明显的流体间的相互作用, 但是仍然有部分离化了的等离子体, 在这附近存在的等离子体密度差异、压强差异进而导致等离子体的运动, 从而改变初始的外加磁场结构. 图 5x方向外加0.4 T磁场时不同时刻外加磁场的分布情况 (a)静态参考图像(0 ns); (b) 40 ns的磁场分布图像; (c) 80 ns的磁场分布图像; (d) 120 ns的磁场分布图像 Figure5. Snapshots of the magnetic field distribution at different delay times with 0.4 T in x direction: (a) Reference image (0 ns); (b) 40 ns. (c) 80 ns; (d) 120 ns
图6给出了在120 ns时磁压力和磁张力的分布情况. 图6(a)中的结构与图3(d)中的结构相似, 这表明, 外加磁场作用整个KHI演化区域. 对比图6(a)和图3(d)可以发现, 磁压力集中在整个扰动界面上, 最大值可以达到$10^{11}\; {\rm{dyn}} · {{\rm cm}^{-3}}$, 由于界面被卷起, 导致磁力线扭曲, 局部放大了磁场能量, 因此界面上的磁能梯度大, 使得磁压力变大, 以阻止扰动界面两侧流体间的相互作用. 在图6(b)中, 磁张力主要集中在弯曲的界面附近, 磁张力的作用是将弯曲磁力线拉直. 这些模拟结果表明, 在磁张力和磁压力的共同作用下, KHI的演化受到了抑制. 通过比较图6(a)和图6(b), 可以看出磁压力明显大于磁张力, 在对KHI的抑制过程中占主导地位. 图 6 (a) 120 ns时磁压力的分布情况; (b) 120 ns的磁张力的分布情况 Figure6. (a) Distribution of magnetic pressure at 120 ns; (b) distribution of magnetic tension at 120 ns.
图7给出了定量的外加磁场对KHI的抑制情况, 为便于简化分析, 在有无外加磁场两种情况下, 针对相同区间(0—1200 μm) KHI涡旋的平均高度进行了对比. 图7(a)中, 红线对应无磁场情况, 黑线对应x方向初始时刻外加0.4 T磁场, 这里比较了KHI涡旋高度随时间的变化. 从图7(a)可以看出, KHI涡旋高度的变化分别对应前面描述的三个演化阶段. 对于无磁场情况, 在0—15 ns内KHI涡旋高度没有变化, 15—20 ns内KHI涡旋高度明显降低, 这表明依次经过了激光驱动产生靶后CH等离子体流以及靶后CH等离子体流离化Al调制层并相互作用这两个过程, 严格地说此时尚未形成KHI. 如图7(a)所示, 在20—80 ns, KHI涡旋随时间逐渐增长对应KHI涡旋线性生长过程和KHI涡旋破裂向湍流转化过程. 对于外加磁场情况, 如图7(a)所示, KHI的演化相对滞后, 直至35 ns时靶后CH等离子体流才与Al调制层发生相互作用, 在50—120 ns内KHI涡旋随时间缓慢增长, 涡旋高度明显低于无磁场情况. 为了更深入地分析, 分别模拟了相同外加磁场(x方向外加0.4 T), 不同初始扰动波长情况下, KHI涡旋的生长情况. 图7(b)中黑线对应图7(a)中的有外加磁场情况(即黑线), 红线、蓝线和绿线分别对应初始扰动波长为600, 800和1000 μm. 结果表明KHI的演化遵从相似的演化规律, 但是随着扰动波长的增加, 磁场对KHI的抑制效果越来越明显, 说明外加磁场主要抑制长波扰动, 而短波可能需要更强的外加磁场来抑制. 图 7 (a) 有无外加磁场的情况下, KHI涡旋的生长情况; (b) x方向外加0.4 T磁场, 初始扰动波长不同时, KHI涡旋的生长情况 Figure7. (a) Growth of the KHI vortex with or without an external magnetic field; (b) the growth of the KHI vortex when a 0.4 T magnetic field is applied in the x direction and the initial disturbance wavelength is different.