Compact, laser-driven ion beamlines for interdisciplinary applications

强激光脉冲与各种靶型的相互作用产生具有MeV尺度能量的离子束。2000年，第一个实验报告了由于这些相互作用导致的多mev质子加速，从那时起，较重的离子也被加速了。实验中使用了各种各样的目标配置，例如厚的（尺度在100微米左右）金属箔，其中质子的来源是自然发生的吸附在箔表面的碳氢化合物和水，气体射流，以及低温氢和氘带。大多数的工作集中在靶正常鞘层加速（TNSA）机制上，在这种机制中，由于电子在相互作用过程中从靶逃逸而产生的大电位，离子被加速。然而，也有其他加速机制，如辐射压力加速（RPA），其中加速是由于入射激光光子施加的压力而发生的。由此产生的离子束具有潜在的应用前景，如在癌症治疗中的强子治疗，以及在快点火惯性约束聚变实验中作为点爆束。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原理，而且还要优化产生的光束。例如，TNSA机制提供了高亮度的光束，但是光束发散并且具有广泛的指数能谱。强子治疗还要求质子束具有高能量（约100兆电子伏）。最近，Higginson等人（2018）通过超薄箔的辐照，利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94MeV的能量，并在《自然通讯》上发表了报道。还有一些工作是校准和增强TNSA产生的质子束的能量，例如Kar等人（2016）在《自然通讯》上发表的研究。在固体目标的背面采用螺旋线圈结构，激光相互作用时产生的电磁脉冲可以通过螺旋线圈结构传播。线圈中的电场由于这种电磁脉冲的行为来准直质子束，并被证明在不到一厘米的传播中提高质子能量约5MeV。本项目旨在进一步发展螺旋线圈等目标装置。它将评估该过程的稳定性，例如调查产生的光束的方向性和准直性，以及确定最佳线圈配置。考虑离开单次射击目标的可能性也将是有用的，以便增加实验中的重复率，从而增加离子传递的速度。事实上，这可能需要一种目标设计，其中线圈与辐照箔分离，研究领域将是EM脉冲从目标到线圈通过该间隙的传输。除此之外，还可以部署进一步的束线设备，例如四极磁铁，以帮助离子束的聚焦。这项研究将依赖于使用蒙特卡罗编码对这些过程的模拟，以及在大学规模的激光系统内部进行的实际实验，以及在中央激光设施等较大的实验室进行的实验。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.