Development of stable laser-accelerated electron beams for radiation generation

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）等机构，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，强烈的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，并建立一个等离子体波，它跟随激光脉冲；这种行为直接类似于船在湖面上的尾迹。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，而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，会通过衍射自然地散焦，从而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。这项技术包括通过一个狭窄的、充满气体的毛细管放电来形成一个所谓的等离子波导。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大约10倍，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，研究以下技术：(1)产生更稳定、更适合应用的电子束；（ii）通过分段等离子体加速器来增加被加速电子的能量，就像在传统加速器系统中经常做的那样。最后，我们将使用改进的电子束特性测量来评估实现激光等离子体加速器可能的第一个应用之一的前景：驱动非常紧凑的可调谐短脉冲x射线源。

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 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 by diffraction 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 a factor of about 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 thesame 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 investigate techniques for: (i) generating more stable electron beams, better suited to applications; (ii) increasing the energy of the accelerated electrons by staging plasma accelerators, just as is routinely done in conventional accelerator systems. Finally we will use improved measurements of the electron beam properties to assess the prospect of realizing one of the likely first applications of laser-plasma accelerators: driving very compact sources of tunable, short-pulse, x-rays.