Plasma Accelerators Driven In Waveguides: Training the Next Generation of Facility Users

粒子加速器被用于物理和生物科学的许多领域。例如，对物质组成部分的基础研究是在欧洲核子研究中心（CERN）等机构的巨大加速器上进行的。在较小的范围内，同步加速器使用加速电子束来产生从红外线到x射线的广泛可调谐的光，这些机器中使用的传统加速器使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管中电击穿的限制，因此将粒子加速到高能量需要非常长的加速器（CERN最大的机器周长为27公里！）。激光驱动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。在这些设备中，强激光脉冲通过电离气体（等离子体）传播。当它这样做时，激光脉冲将电子从它身边推开，并在激光脉冲后面建立等离子体波;这种行为直接类似于水尾流，它尾随着一艘穿过湖泊的船。在等离子体波的情况下，在波的峰值处有比平均值更多的电子，并且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，这意味着等离子体加速器可以比传统加速器短1000倍，并且仍然可以产生相同能量的粒子。这种制造紧凑型加速器的想法最初是在25年前提出的，但直到最近，它们可以达到的能量相对较低。其主要原因是驱动激光脉冲在等离子体中传播时会自然散焦，使其强度降低到仅在几毫米后就停止加速的程度。在过去的几年里，我们的小组开发了一种新技术，可以将强激光脉冲引导到长距离。这种技术涉及通过狭窄的充气毛细管发射放电来形成所谓的等离子体波导。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，最近我们利用这种通道技术将激光驱动的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提高到10亿电子伏特--也就是说，如果电子被两个之间有10亿伏特电压的平板加速，它将获得的能量。这种电子能量与传统同步加速器所使用的电子能量大致相同，但等离子体加速器只有33毫米长，而传统加速器所需的长度为数十米。目前的研究方案旨在利用这些进展，开发提高加速电子能量的技术，并对加速过程提供更多的控制。

Particle accelerators are used in many areas of the physical and biological sciences. For example, fundamental studies of the 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 charged particles. However, the maximum electric field that can be used is limited by electrical breakdown in the beam pipes, so that accelerating particles to high energies requires a very long accelerator (the largest machine at CERN is 27 km in circumference!).Laser-driven plasma accelerators offer a way to make particle accelerators much more compact. In these devices an intense laser pulse propagates through an ionized gas (a plasma). As it does so, the laser pulse pushes the electrons away from it and sets up a plasma wave which follows behind the laser pulse; this behaviour is directly analogous to the water wake which trails a boat crossing a lake. In the case of a plasma wave, at the peaks of the wave there are more electrons than average, and at the troughs there are fewer. As a result of this charge separation, a very large electric field forms between the peaks and troughs of the plasma wave. This field can be about 1000 times larger than the maximum electric field used in conventional accelerators, which means that a plasma accelerator can be 1000 times shorter than a conventional 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 they could reach were relatively low. The primary reason for this is that the driving laser pulse naturally defocuses as it propagates 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 long distances. This technique involves forming a so-called plasma waveguide by firing an electrical discharge through a narrow, gas-filled capillary. The plasma formed in this way has a lower density on axis, which acts to continually refocus the 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 a factor of 10, and so increase the energy of the accelerated electrons to a billion electron volts - that is, the energy an electron would gain if it were accelerated by two plates with a billion volts between them. This electron energy is about the same as used in conventional synchrotrons - but the plasma accelerator is only 33 mm long, compared the tens of metres required for a conventional accelerator.The present programme of research aims to build on these advances and develop techniques for increasing the energy of the accelerated electrons and providing more control of the acceleration process.