Controlled, staged electron acceleration in plasma waveguides

粒子加速器用于物理和生物科学的许多领域。例如，在欧洲核子研究中心（CERN）这样的机构里，对物质组成部分的基础研究是用巨大的加速器进行的。在较小的范围内，同步加速器使用加速的电子束来产生从红外线到x射线广泛可调的光。这些机器中使用的传统加速器利用射频电场来加速带电粒子。然而，可以使用的最大电场受到束流管道中的电击穿的限制，因此将粒子加速到高能量需要一个非常长的加速器（欧洲核子研究中心最大的加速器周长为27公里！）激光驱动的等离子体加速器提供了一种使粒子加速器更加紧凑的方法。在这些装置中，强激光脉冲通过电离气体（等离子体）传播。当它这样做时，激光脉冲将电子推开，并在激光脉冲后面产生等离子体波；这种行为直接类似于一艘船穿过湖泊时留下的尾流。在等离子体波的情况下，在波峰处有比平均水平更多的电子，在波谷处有更少的电子。由于这种电荷分离，在等离子体波的波峰和波谷之间形成了一个非常大的电场。这个电场可以比传统加速器中使用的最大电场大1000倍左右，这意味着等离子体加速器可以比传统加速器短1000倍，并且仍然产生相同能量的粒子。制造紧凑型加速器的想法最早是在25年前提出的，但直到最近，它们所能达到的能量都相对较低。造成这种情况的主要原因是驱动激光脉冲在通过等离子体传播时自然散焦，使其强度降低到仅在几毫米后加速停止的程度。在过去的几年里，我们的团队开发了一种新技术，可以将强激光脉冲传输到很远的地方。这项技术包括通过狭窄的充满气体的毛细管放电形成所谓的等离子波导。以这种方式形成的等离子体在轴上具有较低的密度，其作用是不断地重新聚焦激光辐射，从而防止其散焦。因此，等离子体波导类似于光纤。最近，我们使用这种通道技术将激光驱动等离子体加速器的长度延长了10倍以上，从而将加速电子的能量增加到10亿电子伏特——也就是说，如果一个电子被两块板加速，两块板之间的电压为10亿伏特，它将获得的能量。这种电子能量与传统的同步加速器差不多，但是等离子加速器只有33毫米长，而传统加速器需要几十米长。目前的研究计划旨在以这些进展为基础，开发增加加速电子能量的技术，并提供对加速过程的更多控制。

Particle accelerators are used in many areas of the physical and biological sciences. For example, fundamental studies ofthe building blocks of matter are carried out with huge accelerators at institutions such as CERN. On a smaller scale,synchrotrons use accelerated electron beams to create light which is widely tunable from the infra-red to x-rays.The conventional accelerators used in these machines employ radio-frequency electric fields to accelerate chargedparticles. However, the maximum electric field that can be used is limited by electrical breakdown in the beam pipes, sothat accelerating particles to high energies requires a very long accelerator (the largest machine at CERN is 27 km incircumference!).Laser-driven plasma accelerators offer a way to make particle accelerators much more compact. In these devices anintense laser pulse propagates through an ionized gas (a plasma). As it does so, the laser pulse pushes the electronsaway from it and sets up a plasma wave which follows behind the laser pulse; this behaviour is directly analogous to thewater wake which trails a boat crossing a lake. In the case of a plasma wave, at the peaks of the wave there are moreelectrons than average, and at the troughs there are fewer. As a result of this charge separation, a very large electric fieldforms between the peaks and troughs of the plasma wave. This field can be about 1000 times larger than the maximumelectric field used in conventional accelerators, which means that a plasma accelerator can be 1000 times shorter than aconventional one and still produce particles of the same energy.This idea for making compact accelerators was first proposed over 25 years ago, but until recently the energies theycould reach were relatively low. The primary reason for this is that the driving laser pulse naturally defocuses as itpropagates through the plasma, reducing its intensity to the extent that acceleration ceases after only a few millimetres.Over the last few years our group has developed a new technique for channelling the intense laser pulses over longdistances. This technique involves forming a so-called plasma waveguide by firing an electrical discharge through anarrow, gas-filled capillary. The plasma formed in this way has a lower density on axis, which acts to continually refocusthe laser radiation and so prevent it from defocusing. The plasma waveguide is therefore similar to an optical fibre.Very recently we used this channelling technique to extend the length of laser-driven plasma accelerators by more than afactor of 10, and so increase the energy of the accelerated electrons to a billion electron volts - that is, the energy anelectron would gain if it were accelerated by two plates with a billion volts between them. This electron energy is about thesame as used in conventional synchrotrons - but the plasma accelerator is only 33 mm long, compared the tens of metresrequired for a conventional accelerator.The present programme of research aims to build on these advances and develop techniques for increasing the energy ofthe accelerated electrons and providing more control of the acceleration process.