高功率激光系统的电磁脉冲控制和表征

随着激光装置规模和功率的增加,了解电磁脉冲(EMP)是怎么产生的已经成为一个非常重要的问题。高强度激光能够在固体靶中产生强场(MV•m-1)和大电流(MA),从而产生电磁脉冲辐射干扰电子测量、破坏电子设备。人们提出了许多不同的机制来解释激光驱动EMP的宽光谱分布特性,即从直流电过程到太赫兹频率的跃迁辐射。当高功率激光和物质相互作用时,它们会加速从靶中逸出的热电子并使靶电极化。如果靶接地,则中和电流会通过靶的支架流出靶室。人们认为,这种电流能够发射出频率在千兆赫兹的强烈电磁脉冲,这种脉冲对电子具有破坏性。如今,人们对定向电磁脉冲和快速电流产生方面应用越来越感兴趣,即使世界上诞生了诸如极端光基础设施(ELI)装置这类的高强度、高重频的激光驱动器,用一定的方式限制EMP辐射仍然相当重要。

该研究有两个目标:第一是表征EMP辐射的能量(了解它如何随激光和靶参数而变化)。第二是看看它能否被降低。N. C. Woolsey教授课题组使用卢瑟福阿普尔顿实验室的Vulcan West激光装置进行实验,该装置在最佳焦距时能达到的最大强度为∼2×1019W•cm-2。激光束指向安装在各种支撑杆上的铜靶。为了测量电磁脉冲的能量,该课题组研究人员在相互作用靶室对面的玻璃窗后面安装了三个被动探头。一个Bdot探针和一个Ddot探针面向靶的正面,另一个Bdot探针指向靶的后方。文章第一作者P. Bradford整合兆赫兹和千兆赫兹频率的探测信号得到了总EMP能量。该实验结果发表在High Power Laser Science and Engineering 2018年第6卷第2期上(P. Bradford et al., EMP control and characterization on high-power laser systems)。

实验的第一阶段,学者们观察了在不同的激光和靶参数下EMP能量如何变化,以定性评估实验与理论模型的一致性。当激光能量在7~70 J范围变化时,人们观察到了EMP能量与其变化呈线性关系。该课题组还研究了EMP能量随激光脉冲宽度、预脉冲延迟和离焦的变化。这些结果表明,激光强度越高,或者耦合到等离子体的能量越多,EMP辐射的能量就越大。当研究靶尺寸对EMP能量的影响时,发现较小的箔和丝靶产生的EMP能量急剧减少。实际上,线靶比3 mm×8 mm矩形箔的EMP能量小一个数量级(Ø=25~100 μm)以上。

由于EMP是由电流放电机制(可以被描述为一个射频无线链路控制(RLC)电路)产生的,所以一个关键的实验目标为是否可以通过改变靶架的电阻R、电感L和电容C来改变EMP能量。课题组测试了三种不同的几何设计:圆柱杆、有正弦曲面起伏的支架和螺旋杆(见图1)。首先,用塑料替换了铝制圆柱杆,发现EMP能量有很大的下降(减少了三分之一)。课题组研究人员将其归因于增大的电阻限制了中和电流的大小。然后用塑料螺旋和塑料正弦设计代替圆柱形塑料杆。对于螺旋杆,其效果非常明显:和铝圆柱杆相比,塑料螺旋杆将EMP能量减少了一个数量级以上。同时也可以看到正弦波动杆使EMP能量显著减少,尽管效果没有塑料螺旋杆那么明显。

为了验证电磁脉冲的变化是否与激光和靶的相互作用无关,作者Y. Zhang使用电子光谱仪记录从靶的后表面发射的电子能量。结果表明,使用改进的靶杆对电子发射没有明显的抑制作用。

为了验证改进靶杆减少EMP能量是否是由于经典的RLC效应,作者F. Consoli进行了一系列3D粒子模拟和电磁仿真。实验从一个中心靶发射高能电子锥,并在虚拟腔内的不同点测量EMP能量。模拟表明,当使用绝缘杆而不是导电杆时,所期望的EMP能量的降低值要比观察到的大得多,而几何形状与杆的导电性相比是次要因素。因此,可能需要其他物理机制才能解释课题组观测的结果。比如,来自激光等离子体相互作用的带电粒子和电离辐射可能沿着塑料杆的长度沉积,从而降低绝缘体的作用。这也可以解释为什么改进杆的效果那么好,因为它们具有与众不同的几何形状,能够部分隐蔽杆的表面,阻止入射粒子/辐射,从而防止电击穿。第二组模拟时,将杆的长度减半,结果显示出更高的EMP能量。这为该课题组之前的理论提供了初步支持。然而,由于模拟没有考虑到靶杆的电离,因此,在做出权威的报道之前,课题组还需要进行更多的实验。

实验已经表明,通过简单地改造靶架,能够使EMP能量显著减少。尤其是,塑料螺旋杆与金属棒相比,能使EMP能量减少一个数量级以上。该课题组通过光谱分析,并对不同设计的杆进行实验,从而对杆的有效性做出一个完整的解释。该课题组还将激光和靶参数的扫描结果与主流的EMP理论模型进行比较。该方向的进展取决于区分、理解激光驱动EMP不同机制的能力,以及按需调整辐射量的能力。

激光靶架的三种设计

 

Electromagnetic Pulse control and characterization on high-power laser systems

As laser facilities have grown in size and power, understanding how electromagnetic pulses (EMPs) are generated has become an issue of great practical importance. High intensity lasers can induce strong fields (MV•m-1) and massive currents (MA) in solid targets, producing EMP radiation that disrupts electrical measurements and damages electrical equipment. A number of different mechanisms have been proposed to explain the broad spectral profile of laser-driven EMP, ranging from direct current processes up to transition radiation at terahertz frequency. When high-power lasers interact with materials, they accelerate hot electrons that escape from and electrically polarize the target. If the target is grounded, a neutralization current is pulled out of the chamber through the target support. It is thought that this current is responsible for the emission of intense electromagnetic pulses at gigahertz frequency that are disruptive to electronics. Today, there is growing interest in the applications of directed EMPs and fast current generation, though with the advent of intense, high repetition-rate lasers like the Extreme Light Infrastructure, strategies to limit EMP emission remain of considerable importance.

The research group had two objectives in the study: first to characterize the energy of the EMP emission (to understand how it varied with laser and target parameters) and second to see if it could be reduced. The research group of professor N. C. Woolsey used the Vulcan West laser system at the Rutherford Appleton Laboratory for our experiment, reaching a maximum intensity of ∼2×1019W•cm-2 at best focus. The laser beam was directed onto copper targets mounted on a variety of support stalks. To measure the energy of the EMP, the researchers installed three passive probes behind glass windows on opposite sides of the interaction chamber. A Bdot and Ddot probe were positioned facing the front of the laser target and a further Bdot probe was directed towards the target rear. Probe signals at megahertz and gigahertz frequency were then integrated by first author P. Bradford to produce a measure of the total EMP energy. The results have been published in High Power Laser Science and Engineering, Vol 6, 2018 (P. Bradford et al., EMP control and characterization on high-power laser systems).

The first phase of the experiment looked at how EMP energy scaled with different lasers and target parameters, in order to assess qualitative agreement with theoretical models. Varying the laser energy from 7-70 J, the researchers observed a linear relationship with EMP energy. They also looked at the variation of EMP energy with laser pulse duration, pre-pulse delay and defocus. These scans suggested that the higher the laser intensity, or the more energy coupled to the plasma, the greater the EMP emission. When the researchers examined the effect of target size on EMP, they found that smaller foils and wire targets produced drastically reduced EMP. Indeed, EMP energy was over an order of magnitude less for wire targets (Ø=25-100μm) than for 3 mm×8 mm rectangular foils.

Since the EMP is generated by a current discharge mechanism (which can be pictured as a radio-frequency radio frequency control (RLC) circuit), a key experimental objective was to see if the EMP energy could be modified by changing the resistance, R, inductance, L, and capacitance, C, of the target mount. The research group fielded three different geometrical designs: a cylindrical stalk, a mount with sinusoidal surface undulations and a spiral stalk (see Figure 1). First, the research group replaced Al cylindrical stalks with plastic and found that there was a very significant drop in EMP energy (over one third reduced). The researchers attribute this to increased stalk impedance that limits the size of the neutralization current. Then they replaced the cylindrical plastic stalk with a plastic spiral and plastic sinusoidal design. For the spiral stalk the effect was clear: the researchers found that the plastic spiral stalk reduced the EMP energy by over an order of magnitude compared with Al cylinders. The researchers also saw a significant reduction for the stalk with sinusoidal undulations, though the effect was less pronounced.

To verify whether the change in EMP was independent of the laser-target interaction, author Y. Zhang used an electron spectrometer to record the energy of emitted electrons emitted from the target rear surface. Her results showed that there was no significant reduction in electron emission for shots with the modified stalks.

To see if reduced EMP energy from the modified stalks was due to classical RLC effects, author F. Consoli ran a series of 3D particle-in-cell and electromagnetic simulations in which a cone of energetic electrons was emitted from a central target and the EMP energy measured at different points inside a virtual chamber. The simulations suggest that there will be a greater reduction in EMP than observed when using insulating versus conducting stalks and that geometry is a less important factor than stalk conductivity. It is therefore possible that other physical mechanisms may be required to explain our observations. For instance, charged particles and ionizing radiation from the laser-plasma interaction could be deposited along the length of plastic stalks, reducing the effectiveness of the insulator. This could also explain why the modified stalks were so successful, because their unusual geometry serves to partially shield the stalk surface against incoming particles/radiation and thereby guard against electrical breakdown. A second set of simulations were run with a stalk of half-length which showed much higher EMP energy and therefore provides us with tentative support for this theory. However, since the simulations did not take stalk ionization into account, more experiments are required before any definitive pronouncements can be made.

The experiment has demonstrated that a very significant reduction in EMP can be achieved by a simple modification of the target mount. In particular, a plastic spiral stalk has been shown to reduce the EMP energy by over an order of magnitude versus a metallic rod. The researchers are working on a complete explanation of why the stalks are effective using spectral analysis and by experimenting with other stalk designs. The researchers would also like to compare our laser and target parameter scans with leading theoretical models of EMP. Progress in this field depends on our ability to differentiate between the different mechanisms responsible for laser-driven EMP and, in understanding them, to tailor the emission according to our needs.

Three designs for the laser target mounts.