用于激光等离子体磁场诊断的紧凑型偏振干涉仪
Active magnetic field diagnosis based on the Faraday rotation effect is widely used in experimental studies on magnetized plasmas. The Faraday rotation angle is correlated with both magnetic field strength and electron density, and magnetic field information can be obtained through simultaneous measurement of interferometric and polarization information. Conventional designs introduce the diagnostic beam into an interferometer and a polarimeter, respectively, which require multiple back-end recording devices. As a result, the complexity of the optical path is increased, and inter-device sensitivity calibration problems are encountered. Therefore, a compact optical path design with multiple probes incident on the same recording device is expected to be able to avoid these problems.
In response to the above needs, this paper proposes a compact polarization interferometer, which is capable of obtaining plasma interference, polarization, and shadowgraph images simultaneously on a single recording device and acquiring magnetic intensity with a single shot measurement. The polarized light enters the target chamber in parallel, and the lens system images the plasma region and relays the images to the target chamber. The prism separates two virtual images, and then the lens images the two virtual images to the CCD. A Nomarski interferometer with Wollaston prisms and polarizers is located before the CCD so that four images are obtained on a CCD, and the images involve the interference pattern containing the plasma electron density information and the polarization pattern containing the Faraday rotation angle information. The Faraday rotation angle obtained from the experimental diagnostics contains the information of the diagnostic light path integral, which cannot directly reflect the three-dimensional magnetic field structure. We calculate the plasma evolution by using magnetohydrodynamic simulation, design a ray-tracing program to simulate the behavior of diagnostic light passing through the plasma and compact polarization interferometer, and finally synthesize the line-integrated image on CCD. The reliability of the diagnostic instrumentation and numerical simulations is cross-validated by comparing the experimental diagnostic images with the numerically synthesized images.
After error calculation and parameter scanning, the compact polarization interferometer reaches the theoretically optimal sensitivity of 0.013°. In the solid target laser-ablation experiment, the instrument successfully diagnoses the signal of the self-generated magnetic field. The measured deflection angle is about 0.7°, and the self-generated magnetic field region is about long and wide. In addition, the electron density is about 1019 cm-3, and the estimated magnetic field strength is about 0.9×106 Gs. The intensity and spatial structure of the magnetic field are in good agreement with the numerical simulation, and the images synthesized by the simulation results show characteristics similar to the experimental diagnostic images.
The compact polarization interferometer has successfully diagnosed self-generated magnetic fields on the order of ten Tesla at a spatial scale of several hundred microns. Numerical simulations interpret the dynamic evolution and three-dimensional structure of the magnetic field. This compact polarization interferometer is expected to reduce the risks associated with imaging device variations and the complexity of diagnostic systems and improve the efficiency of magnetic field diagnosis.
1 引言
激光和固体靶相互作用的场景中,有多种机制可以激发自生磁场,典型的有激光焦斑附近电子斜压项产生的环形场[1-6]和激光烧蚀面流体不稳定性涡旋伴生的磁场[7-8]。一般来说,自生场磁压力在总压中占比较小,但磁场可以改变等离子体输运特性[9-10],从而影响其宏观形态[11-12]。高时空分辨磁场诊断在聚变和高能量密度物理研究中都非常重要。激光等离子体自生磁场具有磁力线闭合、空间尺度小、动态演化快的特点,目前应用于诊断激光等离子体磁场的光学方法有基于高次谐波[13]、塞曼效应[1]、科顿穆顿效应[14]、法拉第旋转[15]等的方法。本文基于法拉第旋转的装置搭建成本低,光学系统结构紧凑,诊断数据解读简单,适合实验室条件下的应用。
在通常的法拉第旋转测量中,需要诊断束分束分别进入干涉仪和偏振仪,后端需要多个记录设备,这增加了整个诊断光路的复杂性,同时还需要通过细致的实验来标定多个记录设备之间的灵敏度差异。为此,提出了一种紧凑型的偏振干涉仪,该仪器能够利用单个记录设备在同一发次实验中同时获取等离子体的干涉图、偏振图以及阴影图,提高了系统的模块化能力、避免了记录设备灵敏度不一致的问题。此干涉仪被成功应用于双焦点激光与固体靶的相互作用实验,得到了等离子体磁场和电子密度信息,诊断结果与流体模拟的预测相符。
2 紧凑型偏振干涉仪设计
紧凑型偏振干涉仪的光路如
图 1. 紧凑型偏振干涉仪光路示意图
Fig. 1. Illustration of compact polarization interferometer optical path
式中:
图 2. 诊断光束在各偏振器件之后的投影情况。(a)WP1之后;(b)WP2之后;(c)检偏器P2之后;(d)棱镜摆放方向说明
Fig. 2. Probe light projection on each polarization component. (a) After WP1; (b) after WP2; (c) after polarization analyzer P2; (d) clarification about prism's placement
下面简单估计等离子体中受自生磁场影响下诊断光偏振面发生旋转的角度。沿磁场方向传播的平面电磁波存在色散关系[16]:
式中:
对于长度为
式中:
法拉第旋转角对应
式中:
3 自生磁场实验诊断和模拟分析
为了保证磁场诊断的空间一致性,同时探索更丰富的等离子体相互作用现象,本实验利用双棱镜将纳秒加热束分为两个焦斑,单个焦斑直径为
式中:
图 3. 自生磁场造成的偏振图像明暗变化现象与实验现象的对比。(a)自生磁场和诊断光几何示意图(实线为WP1光轴,双箭头虚线为诊断光初始偏振方向,双箭头实线为经过等离子后的诊断光偏振方向);(b)双焦点打靶示意图;(c)紧凑型偏振干涉仪典型数据
Fig. 3. Comparison of polarization image brightness variation introduced by self-generated magnetic field with experimental phenomenon. (a) Illustration of self-generated magnetic field and probe (solid line is optical axis of WP1, dashed-line-with-arrow indicates initial polarization of probe beam, and solid-line-with-arrow indicates polarization of probe beam after passing through plasma); (b) illustration of dual-spot shooting; (c) typical result from compact polarization interferometer
偏振通道内的光强变化还会受到等离子体自发辐射,诊断束在等离子体中的折射、吸收,以及等离子体中电子密度梯度导致的诊断束退极化等非磁场因素的影响。从
为了进一步确认偏振通道中的明暗变化是否源自于自生磁场,设计了如下对比实验。首先对诊断束偏振面与棱镜光轴夹角的方向进行如此规定:对于
图 4. 验证自生磁场的对比实验数据。(a) 设定为正值;(b) 设定为负值(插图为另一发次相近时刻的偏振通道图像,表明数据具有良好的重复性)
Fig. 4. Comparison of experimental data for verification of self-generated magnetic field. (a) set to positive; (b) set to negative (inset figure is polarization channel image from similar timed shot and indicates good repeatability)
利用第2节介绍的方法对干涉图像进行处理,求得法拉第旋转角并展示在
图 5. 由归一化强度得到的法拉第转角(诊断光与主激光的延时为3.5 ns)
Fig. 5. Faraday rotation angle infered from normalized intensity (delay between probe and main pulse is 3.5 ns)
双焦点打靶的模拟通过辐射磁流体程序FLASH[17]进行。FLASH在欧拉网格上求解Riemann问题,可以有效处理大梯度、强间断、大动态的等离子体;激光场通过逆轫致机制加热电子;电子、离子、辐射场可以具有不同温度,通过电子离子碰撞、辐射发射、辐射吸收实现组分之间的能量弛豫,FLASH还包含多群扩散模型以处理非平衡辐射场的演化;FLASH电子热传导采用限流的Spitzer-Harm模型,针对本实验条件,采用限流因子
式中:
对于
模拟得到的密度、磁场等关键物理量展示在
图 6. 磁流体模拟结果。(a)模拟所采用的激光波形(E=4.90 J,FWHM为4.6 μm);(b) 的电子等值面,以及密度的侧面投影图;(c)50 T的磁场等值面,以及磁场投影图;(d)~(f)温度、流速、等离子体 的三切面数据
Fig. 6. Results of magneto-hydrodynamics simulation. (a) Temporal pulse-form used in simulation (E=4.90 J, FWHM is 4.6 μm); (b) isosurface of electron density at and density projection on side panels; (c) isosurface of magnetic field at 50 T and magnetic field projection; (d)-(f) 3-slice data of temperature,velocity, and plasma
为了直观对比实验与模拟结果,编写了虚拟诊断程序,从模拟结果合成CCD上各个通道的图像。诊断光波面用大量小光线组成的簇代表,如
图 7. 由模拟结果合成的光学诊断。(a)3D光线追踪示意图,双焦点位于 线上,诊断光沿 轴方向(射线展示了部分光线在等离子体中的轨迹);(b) 处电子密度切片(虚线为距靶面 处的密度分布);(c)CCD阴影通道合成图(收光系统F数为 );(d)法拉第旋转角合成图;(e)偏振通道合成图(全口径收光);(f)偏振通道合成图(收光系统F数为 )
Fig. 7. Synthesized optical diagnostic images from simulation results. (a) 3D ray tracing illustration, double focusing spots are on line , probe is incident along axis (half-lines show trajectory of rays in plasma); (b) electron density slice at (dashed-line shows density above target surface); (c) synthesized CCD shadowgraph channel image (with an aperture F number of ); (d) synthesized Faraday rotation angle; (e) synthesized polarization channel image (with full aperture); (f) synthesized polarization channel image (with an aperture F number of )
4 结论
针对激光等离子体磁场诊断的需要,设计了一套紧凑型偏振干涉系统。利用该紧凑型偏振干涉仪成功地诊断到了几百微米空间尺度、10 T量级的自生磁场。结合数值模拟,分析了磁场的动态演化和三维结构,实现了模拟和诊断的交叉印证。此紧凑型偏振干涉仪有望降低成像设备差异性和诊断系统高复杂性带来的风险,提升磁场诊断的效率。未来计划结合机器学习、压缩采样等方案,针对性地改进偏振干涉系统,提升二维成像到三维磁场的推断精度。
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Article Outline
袁鹏, 陶弢, 郑坚. 用于激光等离子体磁场诊断的紧凑型偏振干涉仪[J]. 光学学报, 2023, 43(9): 0912002. Peng Yuan, Tao Tao, Jian Zheng. Compact Polarization Interferometer for Magnetic Field Diagnostics in Laser-Produced Plasmas[J]. Acta Optica Sinica, 2023, 43(9): 0912002.