All-Optical Plasma Channels and Electron Injection with Spatio-temporal Control

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。在较小的规模上，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采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加速器内电击穿的限制，因此将粒子加速到高能量需要非常大的设备：同步加速器机器大约有足球场那么大，而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，并建立一个跟随激光脉冲的等离子体波;这种行为类似于水的尾流，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。事实上，这个电场可以比传统加速器中使用的最大电场大大约1000倍，这意味着等离子体加速器可以短1000倍，并且仍然产生相同能量的粒子。在这个研究计划中，我们寻求解决激光驱动等离子体加速器的两个挑战。第一个中心的事实，即驱动激光脉冲必须保持集中在几厘米长的等离子体加速器。一束光会自然地散开（“发散”），在仅仅几毫米后将光束的强度降低到一个低值。为了克服这种倾向，激光脉冲必须被引导，就像用于传输通信数据的光在光纤中被引导一样。然而，驱动等离子体加速器所需的巨大激光强度会破坏光纤。换句话说，所需的激光强度相当于将地球上所有发电站的输出功率集中到比人类头发横截面还小的区域！因此，我们计划开发一种由等离子体而不是玻璃制成的新型光学“纤维”。这项研究的目的是创造等离子体纤维的性能非常适合激光驱动的等离子体加速器。我们将解决的第二个挑战是提高激光等离子体加速器产生的电子束的质量。在大多数实验中，被加速的电子通过一系列复杂的高度非线性过程被注入并被捕获在等离子体波中。虽然这种方法在产生电子束方面非常成功，但是这些束通常具有对于最具挑战性的应用而言太大的能量分布，并且电子束的性质波动很大。为了克服这些问题，我们将研究控制电子如何被注入和捕获在等离子体wave.Both拟议的计划的两个部分将利用最新进展的方法来控制在空间和时间的脉冲的传递，一个过程被称为“时空”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 within the accelerator, so that accelerating particles to high energies requires a very large device: synchrotron machines are about the size of a football stadium, and 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 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. In fact, 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 and still produce particles of the same energy.In this research programme we seek to address two challenges for laser-driven plasma accelerators. The first centres on the fact that the driving laser pulse must remain focused over the several centimetre length of the plasma accelerator. A beam of light will naturally spread out ("diffract"), reducing the intensity of the beam to a low value after only a few millimetres. To overcome this tendency the laser pulses must be guided, just as the light used to transmit communications data is guided in an optical fibre. However, an optical fibre would be destroyed by the enormous laser intensity needed to drive a plasma accelerator. To put this into context, the laser intensity needed is equivalent to focusing the output power of all the power stations on Earth to an area smaller than the cross-section of a human hair! We therefore plan to develop a new type of optical "fibre" made from plasma rather than from glass. The research is aimed at creating plasma fibres with properties ideally suited to laser-driven plasma accelerators.The second challenge we will address is to improve the quality of the electron beams generated by laser-plasma accelerators. In most experiments the accelerated electrons are injected and trapped in the plasma wave through a complex sequence of highly nonlinear processes. Although this approach has been very successful at generating electron beams, these beams typically have a spread in energies which is too large for the most challenging applications, and the properties of the electron beam fluctuate to a large degree. To overcome these problems we will investigate methods to control how electrons are injected and trapped in the plasma wave.Both parts of the proposed programme will take advantage of recent advances in methods to control the delivery of pulses of in both space and time, a process known as "spatio-temporal" control.