激光驱动电磁脉冲的“自探测”

High Power Laser Science and Engineering 5, e 4 (2017)

激光驱动粒子加速以其在大幅缩减加速器规模和成本方面所具有的潜在优势,在过去二十年中吸引了众多关注。激光产生的等离子体可以维持非常高的加速梯度,由此可以实现带电粒子的有效加速。特别是近年来利用尾波场技术实现电子加速已经取得重要进展,其所产生的电子束品质与传统加速器产生的几乎可以相比拟。
       
      目前实验上也在开展激光驱动离子加速方面的努力,这主要是通过所谓鞘场加速机制来实现的:激光辐照箔片,靶后形成的鞘层电场使得靶后表面的离子被加速。利用这一机制产生的离子束具有一些独特的性质,但是其在能谱、能散以及发散角方面也存在一些局限性,这些严重阻碍了它们的实际应用。

在2016年发表的一篇文章[Nat. Commun., 7, 10792 (2016)]中,Satya Kar等人通过在箔片靶后表面附加线圈装置,在实验上实现了激光驱动的离子束的进一步加速。线圈不但能够提高离子的能量,同时也可在一个窄的能量区间内实现离子的准直。另外,通过依次排布线圈和靶,可以构建具有束流动态准直和能量选择性功能的级联加速器。该文作者之一、英国贝尔法斯特女王大学的Satya Kar博士认为,“这一进展对于构建下一代超紧凑、低成本的粒子加速器奠定了基础,为先进加速器技术小型化提供了助力”。

实验中,线圈主要是通过引导超短电磁脉冲沿其螺旋路径方向传输来工作,而激光驱动离子则沿线圈轴向前进。电磁脉冲电场的径向分量强度足以将质子束缚在线圈轴附近,同时电场的纵向分量会加速导引离子。正如上述论文中所报道的,原理验证实验采用高校实验室规模的激光器,便实现了出射质子的有效后加速,加速效率为500 MeV/m,远高于传统加速器技术所能实现的。

这一方案的成功很大程度上依赖于对电磁脉冲及其沿线圈传播的理解。在发表于High Power Laser Science and Engineering 2017年第1期的一篇论文中,来自英国贝尔法斯特女王大学和德国杜塞尔多夫大学的研究人员采用一种自探测技术方案,利用激光驱动质子分别从横向和纵向两个方面研究了电磁脉冲在螺旋线圈中的传播。

研究人员通过横向探测模式,对沿螺旋线圈传输的电磁脉冲的时域分布进行了表征。实验结果表明,其特征与此前在平面几何情形测量的结果类似,如图1所示。另一方面,线圈的纵向探测阐明了电磁脉冲的超短特性对质子束产生的影响,即,电磁脉冲产生的场会使得质子束流的发散度减小,这一效应具有能量依赖性。通过增加线圈长度,聚焦场在更长的时间内发挥作用,由此可以实现质子束的高度聚焦。这些结果有助于理解螺旋线圈靶选择性导引离子的内在机制,同时对于该技术的进一步发展也大有裨益。


图片说明:电磁脉冲沿螺旋线圈传输的质子横向探测。(a)实验设置示意图;(b)靶和线圈的正面图;(c)(d)利用能量为5.5 MeV和3.0 MeV的质子束得到的螺旋线圈的影像。

Proton probing of laser-driven EM pulses travelling in helical coils

All-optical approaches to particle acceleration have attracted very significant interest in the scientific community over the past two decades, in light of the potential significant reductions in accelerators’ cost and footprint, which could result from the extremely high accelerating gradients that can be sustained in a laser-produced plasma. Particularly important progress towards the production of particle beams of quality comparable to conventional accelerators has been obtained using wakefield techniques for electron acceleration. There has also been a very significant experimental effort in laser-driven ion acceleration, mainly employing the so called Target Normal Sheath Acceleration (TNSA) mechanism, where ions are accelerated from the surface of laser-irradiated foils. The ion beams resulting from this mechanism possess some unique properties, but also present limitations in terms of energy, energy spread and beam divergence, which have severely hindered their applicative use.

An article recently published in Nature Communication [S. Kar, et al., Nat. Commun., 7, 10792 (2016)] sets out the details of an experiment showing how the ions driven by the laser, out of a foil target, can be accelerated further by means of a coil device attached to the back surface of the foil. Not only does the coil boost the energy of the ions, but it also has the intrinsic advantage of collimating ions within a narrow range of energy. Furthermore, by carefully arranging coils and targets in sequence, one can devise a multi-stage accelerator with dynamic beam collimation and energy selection. Dr. Satya Kar of Queen’s University Belfast (UK) said: “This development sets a cornerstone for a next-generation of extremely compact and cost-effective particle accelerators, which complements the current drive for miniaturization in advanced accelerator technology.”

The coil works by transporting an ultra-short electromagnetic (EM) pulse along its helical path, while the laser-driven ions propagate along the coil axis. The radial component of the electric field generated by the EM pulse is strong enough to constrain the protons near the axis of the coil, while the longitudinal component of the electric field accelerates the guided ions. As reported in the Nature Communication publication, the proof-of-principle experiment employing a university scale laser showed efficient post-acceleration of the transiting protons at a rate of 500 MeV/m, which is well beyond what can be sustained by conventional accelerator technologies.

The success of this scheme depends hugely on our understanding of the EM pulse and its propagation along the coil, which was studied in the article by Hamad et al. published in High Power Laser Science and Engineering (HPL, Vol. 5, e4). As shown in Fig. 1 below, and in the image on the cover page, the propagation of the EM pulses in a helical coil was studied in situ by probing the coil both transversely and longitudinally using a self-probing technique employing laser driven protons [H. Hamad, et al., Nucl. Instrum. Methods A 82, 172 (2016)]. The paper in HPL describes in detail the results obtained in an experiment performed by researchers from Queen’s University Belfast and Heinrich-Heine-Universität, Dusseldorf, Germany. In particular, the temporal profile of the EM pulse travelling along the helical coil was characterised from the transverse probing, and found to be similar to what was previously measured in a planar wire geometry. On the other hand, the longitudinal probing of the coil elucidated the effect of the ultra-short burst nature of the EM pulse – i.e. the energy dependent reduction of the proton beam divergence. By doubling the length of the coil, the focusing field was applied for a longer time, which resulted in a tightly focussed proton beam. These results aid underpinning the underlying mechanism of selective guiding ions by the helical coil targets, and are highly beneficial for further development of the technique.


Graphic description: Transverse proton probing of the EM pulse propagating along a helical coil. (a) Schematic of the experimental setup; (b) Front view of the target; (c) and (d) Radiographs of the helical coil obtained by 5.5 and 3.0 MeV protons, respectively.