Compact, laser-driven ion beamlines for interdisciplinary applications

强激光脉冲与各种靶类型的相互作用产生具有MeV尺度能量的离子束。第一个报道由于这些相互作用而加速多MeV质子的实验是在2000年，从那时起，更重的离子也被加速了。在实验中已经采用了各种靶配置，例如厚的（大约一百微米的尺度）金属箔，其中质子的来源是箔表面上天然存在的吸附烃和水、气体射流以及低温氢和氘带。大部分的工作集中在目标正常鞘层加速（TNSA）机制，在该机制中，离子被加速，由于在相互作用过程中从目标逃逸的电子所建立的大电位。然而，还有其他加速机制，例如辐射压力加速（RPA），其中由于入射激光光子施加的压力而发生加速。由此产生的离子束具有潜在的应用，如癌症治疗中的强子疗法，以及作为点火器束在快速点火惯性约束聚变实验。在强子治疗的例子中，激光驱动的离子加速器可能比传统的同步加速器更紧凑，从而增加了在医院部署这种治疗的实用性。考虑到潜在的应用，实验的目的不仅是探索和理解加速机制的物理学，而且还优化产生的光束。例如，TNSA机制提供具有高亮度的光束，然而光束发散并且具有宽的指数能谱。在强子治疗中还需要质子束具有高能量（100 MeV的量级）。最近，Higginson等人（2018）通过照射超薄箔，使用辐射压力和鞘层加速的混合机制实现了质子加速到超过94 MeV的能量，并在Nature Communications上报道。也有工作来准直和增强TNSA产生的质子束的能量，例如Kar等人在Nature Communications中提出的。这里，在固体靶的后侧采用螺旋线圈结构，在激光相互作用期间产生的电磁（EM）脉冲可以通过该螺旋线圈结构传播。电磁脉冲产生的线圈中的电场用于准直质子束，并在不到一厘米的传播过程中将质子能量提高约5 MeV。该项目旨在进一步开发螺旋线圈等目标设备。它将评估该过程的稳定性，例如研究所产生的光束的方向性和准直性，以及确定最佳线圈配置。考虑远离单次发射目标的可能性也是有用的，以便可以增加实验中的重复率，从而可以增加离子输送的速率。实际上，这可能需要一种目标设计，其中线圈与被照射的箔分离，并且研究的一个领域将是EM脉冲穿过该间隙从目标到线圈的传输。除此之外，可以部署另外的束线设备，例如四极磁体，以帮助离子束的聚焦。该研究将依赖于使用蒙特卡洛代码对这些过程的模拟，以及在大学规模的激光系统以及中央激光设施等大型实验室进行的实际实验。最终，这项研究将有助于开发实用的，紧凑的激光驱动离子加速器，用于许多应用。

The interaction of an intense laser pulse with various target types produces beams of ions with MeV scale energies. The first experiments reporting the acceleration of multi-MeV protons due to these interactions were in 2000, and since then heavier ions have also been accelerated. Various target configurations have been employed in experiments, such as thick (of a scale on the order of a hundred micrometres) metal foils, in which the sources of protons are naturally occurring adsorbed hydrocarbons and water on the foil surface, gas jets, and cryogenic hydrogen and deuterium ribbons. Most of the work focusses on the target normal sheath acceleration (TNSA) mechanism, in which ions are accelerated due to the large potential set up by electrons escaping from the target during the interaction. However, there are also other acceleration mechanisms, such as the radiation pressure acceleration (RPA), in which acceleration occurs due to the pressure exerted by the incident laser photons. The resulting ion beams have potential applications such as hadron therapy in cancer treatment, and as ignitor beams in fast ignition inertial confinement fusion experiments. In the example of hadron therapy, the laser-driven ion accelerator can potentially be built to be more compact than a conventional synchrotron accelerator, thus increasing the practicality of the deployment of this treatment in hospitals.With potential applications in mind, experiments aim not only to explore and understand the physics of the acceleration mechanisms, but also to optimise the beams produced. For example, the TNSA mechanism provides a beam with a high brightness, however the beam diverges and has a broad exponential energy spectrum. There is also the requirement in hadron therapy that the proton beam has a high energy (of the order of 100s of MeV). Recently, acceleration of protons to energies exceeding 94MeV was achieved using a hybrid mechanism of radiation pressure and sheath acceleration by the irradiation of ultra-thin foils by Higginson et al. (2018), and is reported in Nature Communications. There has also been work to collimate and enhance the energy of proton beams produced by TNSA, such as that presented in Nature Communications by Kar et al. (2016). Here, a helical coil structure is employed on the rear side of a solid target, through which the electromagnetic (EM) pulse generated during the laser interaction can propagate. The electric field in the coil due to this EM pulse acts to collimate the proton beam and was shown to enhance the proton energy by around 5MeV over less than a centimetre of propagation.The project will aim to develop further the target apparatus such as the helical coil. It will look to assess the stability of the process, such as investigating the directionality and collimation of the resulting beam, as well as determining the optimum coil configuration. It will also be of use to consider the possibility of moving away from a single-shot target, in order that the repetition rate in the experiments, and thus the rate at which the ions may be delivered, can be increased. Indeed, this may necessitate a target design in which the coil is detached from the irradiated foil, and an area of research would be the transport of the EM pulse across this gap from the target to the coil. In addition to this, further beamline apparatus could be deployed, such as quadrupole magnets, to aid in the focussing of the ion beam. The research will rely on the simulation of these processes using Monte Carlo codes, as well as practical experiments carried out in-house on a university scale laser system, as well as at larger laboratories such as the Central Laser Facility. Ultimately, the research will contribute to the development of practical, compact laser-driven ion accelerators for use in a number of applications.