高重频高功率激光实验中的靶:需求、挑战和前景

High Power Laser Science and Engineering 5, e 17 (2017)

“建立可靠的高质量靶供应链是极其重要的挑战,也是实现新装置创新潜能的核心要求。”

——Thomas E. Cowan教授

亥姆霍兹德累斯顿罗森多夫研究中心(HZDR)物理辐射研究所

20世纪80年代,当第一台高功率激光器上线时,激光与物质相互作用的基础研究主要受激光发展的驱动,此时运用的是较简单的薄箔或厚的样品作为靶。有关靶制备的研究主要集中在惯性约束聚变上,需要性能好控制的复杂的靶。在过去的二十年间,用于基础研究的靶变得愈加复杂。它们可能会被设计成达到相图上的特定区域,例如在动态压缩和等容加热实验中的极高压力和温度,以探究相对论等离子体和靶本身的电子动力学,并更好地控制激光驱动粒子和辐射源。相关综述发表在High Power Laser Science and Engineering 2017年第3期上。

靶的构造可以多种多样,几乎不受限制,从箔片到三维结构,再到复合靶组装。靶的性质必须可重复并且易控制,从而避免不希望出现的效应(例如等离子体不稳定或样品预加热)。因此,在靶制备中定量测量扮演着至关重要的角色。实验需要对靶的组分、晶体结构、形态学、密度和表面质量等性能进行表征。靶的制备和性能表征通常需要结合多种材料科学的最新技术和研发项目,可能很具挑战性。之前的二十年里,来自各大学、研究中心和公司的靶专家团体已经壮大起来,以合作或商业为基础的途径让靶制备和其性能表征的需求得到了满足。

目前,研究高功率激光的同行团体正面临一个将影响靶制备和运输的新挑战:能够以1-10 Hz重复频率运行的高功率激光器的试运转。近期欧洲已有和即将建成的先进用户装置包括欧洲X射线自由电子激光器(European XFEL)的HED、极端光基础设施(ELI)、欧洲同步辐射实验室(ESRF)的HPLF,国家级装置包括英国的Gemini、法国的Apollon、西班牙的CLPU。1-10 Hz的重复频率意味着每小时60-600个靶,那就等于说每天需用数千个靶。要在维持成本相对低廉的同时大规模制靶是一个极大的挑战,还可能成为研制出高重频装置的瓶颈。

如果高重频实验的目标是产生可重复的激光辐射和粒子束,或者需要大量完全相同的数据点来采集统计数据,那么就需要大规模生产完全一致的靶。从另一方面来说,在探索性活动中,性质有细微不同的靶可以用于参数研究,因此需要快速制备靶初样。主要的制靶实验室正在开发可以满足以上要求的方案。用涂层技术处理大片的原材料,再置于靶托上进行切割和组装,可制备出平面多层靶。“为未来高重频实验提供大量高性能多层靶是巨大的挑战,”Dominik Kraus博士说道,“但是在未来装置上研究极端条件下(几百GPa的压力和数千K的温度)物质的性质又急迫需要这些靶。”

微纳结构的靶可通过在基底材料上光刻来进行大量制备,而半导体行业通常用这个方法生产微电子系统。“这类靶能够为不同激光与物理相互作用过程提供诊断上的新认识,”Thomas Kluge博士说道,“所以制靶用户通常需要与微纳制备团队建立起紧密的联系。比如,我们与两个具有电子束光刻经验的团队开展了合作——德国耶拿莱布尼茨光子技术研究所(IPHT)和HZDR离子束物理和材料研究所,重点放在离子束切割和扫描电镜上,力求制备出纳米光栅靶、微米丝靶、纳米涂层靶和理想的台阶靶等多种类型的靶。”

此外,对大量靶进行高效精准的装校还需要用到自动化手段。美国通用原子能公司(General Atomics)和英国中心激光装置(Central Laser Facility)等地已具备了这样的能力,并开发出相应的夹具用于靶生产。

不仅如此,对大量靶进行性能表征也是很大的挑战。当靶间距大且在大气压强下运行时才能用上自动化性能表征技术。更可能的是,受制备过程限制,生产出的每一批靶中只有一小部分可以进行详细表征,以通过额定参数和系统变量评估统计偏差。

高重频的固体样品辐照引起了其他与靶相关的技术难题。这样的靶辐照机制要求快速定位和微米级的精确准直。靶托不能因热负荷发生振动、活化或膨胀。此外,低重复频率实验中用于保护定位阶段不受回流和电磁脉冲影响的防护措施,被认为在1-10Hz重复频率运行时不可靠。

其他阻碍高重频实验运行的严重问题还有靶碎片对元器件和其他靶造成的损害。来自靶的熔化物和高动能弹片会在靶室中覆盖和损害没有屏蔽的光学元件、防溅射板与诊断设备。高重频实验需要频繁更换防溅射板,这一解决措施既费钱又耗时。此外,打靶过程中引起的冲击波,以及烧蚀物的再沉积和脉冲旁瓣中的强光都会损害距焦斑仅厘米远的靶。

综上所述,高重频的靶供应和靶辐照向科研团体抛出了数个难题。建立可持续性靶供应链需要制靶团队和激光器装置团队合作,包括共同的战略计划、发展和壮大现有基础建设的设备支持,请靶专家介入到实验设计和项目评估当中等。协同合作科研活动能够避免重复性工作、获得使用激光器进行靶测试的时间,有助于研发高重频实验的技术解决方案。

图:通过纳秒脉冲激光沉积的碳泡沫的扫描电镜图。图片显示,该泡沫是由直径约15 nm的纳米颗粒链组成。碳泡沫靶已被用来增强激光驱动离子加速实验里的质子数量和能量。这就是复杂靶需要实验和研发,从而在微米尺度控制、表征其材料性能的最好例子。


2016年8月28-30日在德国德累斯顿举办的EUCALL Target Workshop合影。HPL上发表的综述文章Targets for high repetition rate facilities: needs, challenges and perspectives反映了该次研讨会的讨论结果。作者在此感谢所有参会者的贡献。
 

Targets for high-repetition rate facilities: needs, challenges and perspectives

“An important challenge will be the development of a reliable supply chain of high quality targets,” said Prof. Thomas E. Cowan from Institute of Radiation Physics, Helmholtz–Zentrum Dresden–Rossendorf (HZDR), “which is a central need for the realization of the innovative potential of the new facilities.”Relevant review article has been published in High Power Laser Science and Engineering, Vol. 5, No. 3, e17, 2017.

In the 1980s, when the first high power laser came online, fundamental research on laser-matter interaction was driven by laser development and relatively simple targets were used (foils and thick samples). Target fabrication research focused mainly on samples for inertial confinement fusion that required complex targets with very well controlled properties. In the last two decades, targets for fundamental research have become more and more complex. Targets can be designed to access specific regions of the phase diagram, for example extremely high pressures or temperatures in dynamic compression and isochoric heating experiments, to investigate electron dynamics in relativistic plasmas and in the target itself, and to better control laser-driven particle and radiation sources.

The variety of possible target configurations is virtually unlimited and ranges from foils to 3D structures and multi-target assemblies. Target properties need to be reproducible and well controlled to avoid undesired effects (for example plasma instabilities or sample pre-heating), therefore metrology plays a crucial role in target preparation. Depending on the experiment, characterization could be required for target properties such as composition, crystalline structure, morphology, density, and surface quality. Target production and characterization can be challenging and often require a combination of multiple state of the art materials science techniques and research and development programs. The community of target experts from universities, research centers and companies has grown considerably in the last 20 years and makes these competences and capabilities available to experimentalists on a collaborative or commercial basis.

At present, the high power laser community is facing a new challenge that will also affect target fabrication and delivery: the commissioning of high power lasers, which are able to operate with repetition rates of 1-10 Hz. Recent and upcoming advanced user facilities in Europe include not only the HED instrument at European XFEL, the ELI infrastructure, the ESRF HPLF beamline, but also national facilities like Gemini (UK), Apollon (France) and CLPU (Spain). A repetition rate of 1-10 Hz corresponds to a requirement of 60-600 targets/hour, i.e. thousands targets per day. Scaling production of solid targets to large numbers while maintaining costs relatively low is a huge challenge for the target community and could become a bottleneck for the success of high repetition rate facilities.

High repetition rate experiments require mass production of identical targets if the goal is the generation of reproducible laser-driven radiation and particle beams or if a very large number of identical data points are required to collect statistics. On the other hand, fast prototyping is required for exploratory campaigns in which targets with slightly different properties can be used for parametric investigation. The main target fabrication laboratories are developing possible solutions to address these needs. Planar multilayer targets can be produced in large sheets with coating techniques, then cut and assembled on target holders. “Providing large quantities of well-characterized multilayer targets for future high-rep-rate experiments presents a big challenge,” said Dr. Dominik Kraus “but is urgently required for investigating properties of matter at extreme pressure and temperature conditions (100s of GPa and 1000s of K) at future research facilities.”

Micro and nano-structured targets can be produced in large numbers on wafer by lithographic techniques commonly used in the semiconductor industry for microelectronic systems. “These types of targets enable new diagnostic insights on different laser-matter interaction processes,” said Dr. Thomas Kluge, “which is why users often need to establish close bonds with micro and nanofabrication groups. For example, we developed a collaboration with two external groups with experience in electron beam lithography, focused ion-beam cutting and scanning electron microscopy (Leibniz Institute of Photonic Technology Jena and the Institute of Ion Beam Physics and Materials Research at the Helmholtz-Zentrum Dresden-Rossendorf) to produce wide ranges of targets: e.g. nano-gratings, micro wires, nanometer coatings, and perfect steps”.

In addition, efficient precision assembly of large numbers of targets will require automated solutions. Robotic capabilities and mounting jigs have been developed to address this need and are currently used for example at General Atomics (USA) and Central Laser Facility (UK).

Moreover, scaling characterization to large numbers of targets is challenging. Automation can only be applied to characterization techniques with large standoff distances and operating at atmospheric pressure. More likely, thorough characterization will be performed only for a small number of targets for each batch to evaluate statistic deviations from the nominal parameters and systematic variations due to the production process.

Irradiation of solid samples at high repetition rates poses other target-related technical challenges. Target irradiation in this regime requires fast positioning and alignment with precision in the µm range. The target holder should not vibrate, become activated or expand due to thermal loads. In addition, precautions used for the protection of positioning stages from return currents and electromagnetic pulses in low repetition rate experiments are expected to be unreliable in the 1-10 Hz operation regime.

Other crucial issues hindering high repetition rate operation are target debris and target fratricide. Ablated material and high kinetic energy shrapnel from the target can coat and damage unshielded optics, debris shields and diagnostics in the interaction chamber. High repetition rate experiments would require frequent replacement of debris shields, a solution both expensive and time consuming. In addition, the shock launched in the target, as well as re-deposition of ablated material and high intensity light in the pulse wings can damage targets cm away from the focal spot.

In conclusion, target supply and irradiation at high repetition rates pose several challenges to the research community. The development of a sustainable target supply chain requires coordination between target fabrication groups and laser facilities, including common strategic planning, facility support to the development and expansion of the current infrastructure, involvement of target experts in experiment design and proposal evaluation. Collaborations and joint research activities would be helpful to develop technical solutions for high repetition rate experiments by avoiding effort duplication and obtain dedicated laser time.

Scanning Electron Micrograph of a carbon foam deposited by ns pulsed laser deposition. The image shows that the foam is formed by chains of nanoparticles (diameter ~ 15 nm). Carbon foam targets have been used to enhance the number and energy of protons in laser-driven ion acceleration experiments. This is an example of complex targets requiring research and development activities to control and characterize the material properties at the microscale.

Group picture of the EUCALL Target Workshop held in Dresden on August 28-30th 2016. The review paper “Targets for high repetition rate facilities: needs, challenges and perspectives” reflects the results of the workshop discussions. The authors thank all the participants for their contributions.