Small-spot X-ray Irradiation from Metal Nanowire Target

The experiment was performed on the XGIII facility. The picture shows a young scientist installing the equipment in the chamber.

In 1901, Wilhelm Röntgen received the first Nobel Prize in Physics for his discovery of X-rays. Nowadays, X-ray has been commonly used in medical, industrial detection, security inspection, scientific research, etc. Recently, with the development of TW or PW laser facility, ultrafast X-ray sources based on laser-plasma interaction (Kα X-ray source, γ-ray source and so on) begin to be applied.

High quality energetic electrons are required for all the sources mentioned above. Unfortunately, the divergence angle of the electron beam is too large to produce Kα X-ray and γ-ray source. Therefore, how to reduce the divergence angle of electron beams has been a problem.

It is reported by some research groups that high energy electron beams with small divergence angle can be obtained through laser-metal wire interaction. The generated electrons move forward along the wire, which generates a cold electron return current inside the wire to overcome the Alvén limit. Correspondingly, a quasi-static electromagnetic field is built on the surface of the wire. Under the effects of both electric force and magnetic force, an electron beam with small divergence angle can be achieved. However, the total number of the controlled electrons is restricted by the single wire structure and the brightness of the source is also limited.It is reported by some research groups that high energy electron beams with small divergence angle can be obtained through laser-metal wire interaction. The generated electrons move forward along the wire, which generates a cold electron return current inside the wire to overcome the Alvén limit. Correspondingly, a quasi-static electromagnetic field is built on the surface of the wire. Under the effects of both electric force and magnetic force, an electron beam with small divergence angle can be achieved. However, the total number of the controlled electrons is restricted by the single wire structure and the brightness of the source is also limited.

Researchers in Prof. Yuqiu Gu's group, from Research center of Laser Fusion, China Academy of Engineering Physics, proposed a more efficient Kα X-ray source with metal nanowire target. The multiple nanowire structure helps generate high energy electrons with a smaller divergence angle, which makes it possible to produce a micro-focused X-ray source. It has great potential to be applied in the dynamical shock wave imaging field. It is reported in Chinese Optics Letters Vol.13, No.3, 2015.

Many research groups found that the single wire target could greatly reduce the spot size of the X-ray, but the yield from the wire target would be much lower than that from the planar target. However, small spot size can be achieved with the nanowire target while a high yield can be guaranteed. Therefore, these sources which generated from the laser-nanowire interaction are suitable for diagnosing high-compressed matter, lattice dynamics response, and so on.

In this research, micro-nano materials are used in the X-ray generation. Utilizing the surface electrical field on the nanowire, fast electrons are controlled then a compact spot source is produced. Prof. Gu concludes that the results of this research pave a new way to generate compact spot X-ray source. Micro-nano material may even be used in the fast ignition in the future.

Following works will be focused on promoting the efficiency and monochromatization of the source.

强激光与金属纳米丝阵列作用产生小焦斑X射线源。

图片说明：该实验在新建成的星光III激光装置上开展，图为研究人员在靶室安装实验设备。

1901年，第一届诺贝尔物理学奖被授予伦琴，以表彰他发现X射线。一个多世纪过去了，X射线已广泛应用于医学、工业检测、安防科技和科学研究等领域。近年来，随着百太瓦以及拍瓦级激光器的出现，基于激光等离子体相互作用的X射线源也开始出现并得到了初步的应用，包括Kα射线源和伽马射线源等。

不论是哪一种X射线源，都需要首先将激光能量转换成高品质的高能电子束。然而，对于Kα射线源和伽马射线源，激光和稠密等离子体相互作用将产生具有很大空间发射度的高能电子束，从而不可避免地造成X射线焦斑远大于激光焦斑，因此，如何产生具有小发散角的电子束就成了一个亟待解决的问题。

为解决这一难题，中国工程物理研究院激光聚变研究中心的谷渝秋研究员实验组提出采用纳米金属丝阵列导引激光产生小发散角高能电子束，从而有望产生高亮度微焦点X射线源。其基本原理是纳米金属丝的内部产生冷电子回流，相应地产生一个准静态的磁场，方向向外，而电子束在传输的过程中由于具有横向动量，电子将在金属纳米丝的表面建立一个方向也向外的德拜鞘电场。在电场力和磁场力的共同作用下，高能电子会被束缚在金属纳米丝表面并沿金属纳米丝方向运动，从而获得发散角很小的高能电子束。相关实验结果发表在Chinese Optics Letters 2015年第3期上。

国外的已有的理论和实验基本上是基于单丝模型，由于阿尔芬电流极限的限制，沿着单丝向前传输的电子束的总电量是有限的。采用纳米金属丝阵列靶可以约束更大电荷量的电子，从而有利于产生高亮度微焦点X射线源。

国外同行的研究发现，在相同激光参数下，采用小尺寸单金属丝靶可以减小X射线的焦斑尺寸，但是产生的X光子的数量比采用普通平面金属靶时要低一个数量级。而采用纳米丝金属阵列靶可以在提高激光-X射线转换效率的同时，获得远低于平面靶的X射线焦斑尺寸。这种X射线源在激光聚变高密度压缩诊断、极端条件下材料动力学响应动态诊断等领域有广泛的应用。

谷渝秋研究员认为，该成果的核心思想是利用等离子体界面电磁场实现了对高能电子的输运控制，同时把材料微纳加工方面的最新成果引入激光X射线源研究领域，具有很好的应用发展前景，为将来激光聚变快点火高能电子束的控制以及激光X射线源的推广应用提供了一种有效途径。

目前，通过强激光与金属纳米丝阵列相互作用产生X射线源的亮度仍然有继续提升的空间。通过采用高对比度激光与纳米丝阵列相互作用，将进一步增加光子的产额。同时，在射线源的单色化方面仍然有很多工作要做。