综述 | 激光等离子体相互作用的电磁脉冲:产生、探测与抑制

随着激光能量和功率的提升,激光与等离子体相互作用产生的电磁脉冲辐射(EMPs)也越来越强。大能量脉冲的重要应用潜力使其受到了科研人员的广泛关注。

近日,以法国新能源与环境研究中心的Fabrizio Consoli博士和英国卢瑟福实验室的David Neely教授为首的联合研究团队在High Power Laser Science and Engineering发表了综述文章,阐述了到目前为止,在高功率大能量激光脉冲与不同类型的固体靶相互作用中,与EMP的产生、探测和抑制相关的研究成果和遇到的问题(Fabrizio Consoli, Vladimir T. Tikhonchuk, Matthieu Bardon, et al. Laser produced electromagnetic pulses: generation, detection and mitigation[J]. High Power Laser Science and Engineering, 2020, 8(2): 02000e22)。

该文总结了国际上一系列大装置实验室中的最新实验结果。作者认为,通过产生电磁脉冲的强度和频谱可以分析得出其产生机制。电磁脉冲频谱主要集中在GHz频段,这恰巧是造成电子学原件损伤的主要频段,同时也是存在重要应用价值的频段。文中对比了电磁脉冲产生相关的理论模型、数值模拟和实验测量结果,并且讨论了测量和诊断的方法。这对于研究激光等离子体相互作用中深层的物理过程,深入理解电磁脉冲产生机制,进一步发展抑制电磁噪声的技术,避免电子学器件损伤具有重要的意义。

1.EMP产生的物理机制

在高功率激光与固体靶相互作用的过程中,产生的高能电子会率先从靶面逃逸,并在靶面附近形成电荷分离的电场。在这些电子中,能量相对较低的部分容易受到靶面电场的影响回流到靶面上。

2015年Poyé在飞秒装置中将激光入射到一个连接金属杆的靶上(其中金属支杆完美接地),在实验中测量了流过靶杆的电流频谱与辐射的电磁场频谱。研究发现,二者的最强辐射频率几乎一致,均在1 GHz附近。通过计算靶与靶杆的尺寸发现,二者作为整体可以等效近似为一个长度为l+πd/2的天线,其中l为金属支杆的长度,d为靶盘直径。其本征辐射频率满足f = c / 4 (l + πd / 2) = 1.2 GHz,与测量所得辐射频率几乎一致。如图1所示,当靶是一个独立的圆盘时[如图1(a)],靶中心的电子逃逸产生的宽频带辐射如图1(b)所示。增加了金属杆后的靶型[图1(c)]在辐射频率为1 GHz附近存在明显的共振峰[图1(d)]。

图1. (a)独立的和(c)连接金属支杆的金属圆盘靶示意图。对应的辐射频谱分别如(b)和(d)所示。

2018年,中国科学院物理研究所的姜炜曼等在神光-II升级激光装置上测量了在皮秒大能量激光驱动下,辐射场的空间分布、时域波形以及频谱特征。二维电磁模拟说明,大能量的皮秒激光与等离子体相互作用产生的皮秒尺度的THz波在封闭腔室内经过多次反射展宽后(如图2所示),可以转换为功率在GW量级的GHz电磁脉冲。结果表明,电磁波反射共振所产生的EMP辐射,可以通过改善腔室的设计来抑制;另一方面,未来在短脉冲激光等离子体实验中或许可以通过合理设计谐振靶室,产生更高功率、频率及波形可控的高功率微波源。

图2. 不同时刻空间中的电场分布 (a) 1 ns (b) 3.5 ns (c) 21 ns (d) 60 ns;R1,R2和R3为电场的采样位置(与实验中测量位置相同),三个位置均距靶室中心1 m远,与x轴的夹角分别为0°,45°,90°。

2. EMP的诊断方法

实验中通常用D-dot探头探测电磁脉冲的电场强度。D-dot探头直接测量的是电磁脉冲信号在探头处的感应电压随时间的变化,反映了电通量密度D的时间变化率。电场强度E可以通过下式得出

其中u0为探头的输出电压,ε0为自由空间的电容率,Ae为探头在电场方向的等效面积,R为信号传输电缆阻抗。图3(a)为典型的D-dot偶极天线,呈球形,由两个与接地平面相连的半球组成,信号从两个半球流向接地平面,然后再通过50 Ω的同轴电缆传输。图3(b)和(c)为D-dot的典型测量数据,分别为电压信号和积分得到的电场信号。

图3. (a)球状D-dot传感器;(b)典型D-dot测量电压信号;(c)通过电压信号的时间积分得到的电场信号。

3. EMP的抑制方法

一般的电子学元件受到毁伤的电场阈值约为60 kV/m,而在强激光与等离子体相互作用过程中,腔室里的辐射电场达到60 kV/m是极为普遍的现象。实验中常有电磁干扰毁坏仪器设备的情况发生,为此,研究者们也在不断地探索削弱电磁干扰的途径。

2018年,Bradford等在实验中发现,在皮秒激光与100 μm厚的铜平面与靶作用过程中,合理的靶架结构设计可以有效减弱电磁干扰强度。图4(a)为三种不同的靶架结构。实验结果表明,采用标准圆柱(cylinder)靶架、正弦调制(sinusoidal)靶架、螺旋(spiral)靶架,电磁干扰的强度依次减小,如图4(b)所示。在相近的激光条件下,采用标准圆柱靶架得到的电场强度约为采用螺旋靶架的2倍。

图4. (a)三种不同的靶架:1.标准圆柱靶架;2.正弦调制靶架;3.螺旋形靶架;(b) 使用三种不同的靶架时,激光能量归一化的电场强度与激光能量的关系。

目前,人们对激光等离子体中EMP研究还主要是为了实现对其的抑制,从而有效保护仪器设备和实验人员。然而,EMP本身也可以用于脉冲强磁场产生、粒子加速、材料特性表征等方面的研究。该综述文章中介绍的EMP产生机制及探测方法为未来EMP应用技术的发展奠定了基础。

Laser produced electromagnetic pulses: generation, detection and mitigation

Generation of electromagnetic waves was first demonstrated by Heinrich Hertz in 1887 and since then has become a leading subject of research, with an enormous range of applications covering radio communications, electronics, computing, radar technology and multi-wavelength astronomy. The accessible spectrum of electromagnetic emissions continuously extends toward shorter wavelengths from radio waves to microwaves, to optical and X-rays, challenging now the gamma-ray domain. It is also widely recognized that strong electromagnetic waves could be dangerous for health and electronics. Methods of detection of electromagnetic waves and mitigation of their undesirable effects are also in full development.

This paper provides an up-to-date review of the problems related to the generation, detection and mitigation of strong electromagnetic pulses (EMPs) created in the interaction of high-power, high-energy laser pulses with different types of solid targets. It addresses the particular problem of microwaves generated during the interaction, in the domain extending from radiofrequencies (MHz) to terahertz. This work collects the results from many research groups involved in this topic and includes new experimental data obtained independently at several international laboratories.

These electromagnetic pulses are regularly detected in laser–target interactions with laser pulses from the femtosecond to the nanosecond range. They are recognized as a threat to electronics and computers and have stimulated the development of various protective measures. However, this situation has significantly evolved since the invention of chirped pulse amplification (CPA) in lasers and the rapid development of powerful sub-picosecond (sub-ps) laser systems. Paradoxically, the interaction of sub-ps laser pulses with solid targets generates much stronger EMPs in the GHz domain than for nanosecond pulses of comparable energy. This fact has been reported in several publications during the past fifteen years, but an understanding of the underlying physics has been attained only recently.

Top-view scheme of the vacuum chamber; the laser (red beam) is focused on a thin-foil target by an off-axis parabola mirror.

The main source of strong GHz emissions in most interactions has been identified as the return current flowing through the support structure to the target, charged by the intense laser–target interaction. Controlling the geometric and electrical characteristics of the target support has therefore become the major EMP mitigation approach. The understanding of the physics of EMP generation has substantially advanced very recently, and other mechanisms of EMP generation have been identified. Among the related main research topics, we mention: the excitation of chamber resonant modes; the characterization of secondary EMP sources; the scattered radiation.

More accurate and efficient detection methods have been developed and used to deliver improved experimental data. At the same time, construction of a new generation of laser systems with pulsed powers exceeding the petawatt level is opening the possibility of conducting experiments with high repetition rates, creating the need for more reliable and efficient EMP protection and mitigation techniques.

A full comprehension of the physics of EMP generation and the mechanisms of their operation will enable the creation of temporally and spatially controlled electromagnetic fields of high intensity and wide distribution. This would lead to the new and significant employment of laser–plasma interactions for powerful and versatile radiofrequency–microwave sources, which will be of direct interest to particle-acceleration schemes, for which this is indeed of primary importance, as well as to a multidisciplinary range of applications: biological and medical studies of strong microwave interactions with cells; medical engineering; space communication; plasma heating; material and device characterization; EMP-radiation hardening of components; and electromagnetic compatibility studies. Understanding and controlling the sources of EMP radiation is also important for personnel protection.

This review paper summarizes the recent knowledge and experience gained by scientists working with high-power laser systems in many laboratories worldwide. It gives an overview of the

• theoretical understanding of the processes of electric charge accumulation on the target, return current formation and electromagnetic emission;

• advancements in diagnostic techniques for the detection of EMPs, with experimental results obtained on different high power laser facilities and related interpretation;

• techniques of mitigation of EMP effects, with experience accumulated on several high-power laser facilities;

• measured EMP levels on different laser facilities and possible applications of EMPs.