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，对物质构成要素的基础研究是在欧洲核子研究中心等机构的巨大加速器下进行的。在较小的规模上，同步加速器使用加速的电子束产生从红外线到X射线的广泛可调的光。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需要非常长的加速器(欧洲核子研究中心最大的机器周长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。在这样做的过程中，激光脉冲将电子推离它，并在激光脉冲后面产生一个等离子体波；这种行为直接类似于拖曳着一艘船过湖的水尾流。在等离子体波的情况下，在波的尖端有比平均多的电子，而在谷底有更少的电子。这种电荷分离的结果是，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，这意味着等离子体加速器可以比传统加速器短1000倍，但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早是在25年前提出的，但直到最近，它们可以达到的能量都相对较低。其主要原因是驱动激光脉冲在穿过等离子体时自然散焦，使其强度降低到仅在几毫米后加速停止的程度。在过去的几年中，我们团队开发了一种新技术，用于远距离传输强激光脉冲。这项技术包括通过一根狭窄的充满气体的毛细管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，从而防止它散焦。因此，等离子体波导类似于光纤。最近，我们使用这种沟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，从而将被加速的电子的能量增加到10亿电子伏--也就是电子被两个相隔10亿伏的平板加速时所获得的能量。这种电子能量与传统同步加速器中使用的能量大致相同--但等离子体加速器只有33毫米长，而传统加速器需要几十米长。目前的研究计划旨在这些进展的基础上，开发技术来增加加速电子的能量，并对加速过程提供更多控制。

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.