Development of a multi-pulse laser wakefield electron accelerator operating at kHz rep-rate

激光驱动的等离子体加速器已经可以在加速器阶段将电子加速到GeV级的能量，而加速器阶段的长度只有几厘米。然而，迄今为止使用的驱动激光器最多只能在每秒几个脉冲的情况下工作，并且壁塞效率非常低。牛津大学的激光-等离子体加速器小组开创了一种克服这些限制的新方法：多脉冲激光尾迹加速器(MP-LWFA)。在这种方法中，等离子体尾迹是由一串低能量激光脉冲驱动的。如果这些脉冲由等离子体周期隔开，则由脉冲驱动的尾迹场相干地相加，产生朝向脉冲序列背面生长的等离子体波。牛津大学的研究小组最近在卢瑟福·阿普尔顿实验室进行的实验中首次展示了这种方法。在这项工作中，证明了等离子体尾迹确实可以被一串激光脉冲共振激发，证实了这种方法的有效性。然而，到目前为止，这种方法还没有加速电子，因为MP-LWFA驱动的尾迹场是线性的，因此不会从背景等离子体中自捕获电子。该项目的目的是开发控制电子注入到MP-LWFA驱动的准线性尾迹场中的技术。将研究两种方法。首先，我们将探讨如何使用额外的激光脉冲来电离一种掺杂物质。如果短激光脉冲(“注入”脉冲)在脉冲序列驱动的尾流中聚焦，那么被第二个激光脉冲电离的电子可以被捕获到尾场中，然后被加速。第二个待探索的方法将是下坡道喷射。在这种方法中，等离子体的密度随着纵向位置的变化而迅速降低；这会减缓尾波场沿斜坡移动的速度，从而使电子更容易被捕获，然后被加速。这些注入技术将通过在牛津大学、拉尔大学以及可能在其他激光设施进行的实验进行研究，并通过粒子在细胞内(PIC)代码的数值模拟来支持。如果成功，这项工作将为以几千赫兹脉冲重复频率工作的新一代非常紧凑的等离子体加速器开辟道路。它们将立即在驱动非常紧凑的X射线源方面得到应用，在超快科学、技术和医学中具有广泛的用途。

Laser-driven plasma accelerators can already accelerate electrons to GeV-scale energies in accelerator stages only a few centimetres long. However, the driving lasers used to date can only operate at best at a few pulses per second, and have very low wall-plug efficiencies. The laser-plasma accelerator group at Oxford has pioneered a novel approach for overcoming these limitations: the multi-pulse laser wakefield accelerator (MP-LWFA). In this approach the plasma wakefield is driven by a train of low energy laser pulses. If these pulses are spaced by the plasma period then the wakefields driven by the pulses add coherently, yielding a plasma wave which grows towards the back of the pulse train. The Oxford team recently demonstrated this approach for the first time in experiments performed at the Rutherford Appleton Laboratory (RAL). In that work it was shown that plasma wakefields could indeed be resonantly excited by a train of laser pulses, confirming this validity of this approach. However, to date electrons have not been accelerated by this method since the wakefields driven by MP-LWFA are linear, and as such do not self-trap electrons from the background plasma. The aim of this project is to develop techniques for controlling the injection of electrons into the quasi-linear wakefields driven by a MP-LWFA. Two approaches will be studied. In the first, the use of additional laser pulses to ionize a dopant species will be explored. If a short laser pulse (the "injection" pulse) is focused in the wake driven by the pulse train, then electrons ionized by the second laser pulse can be trapped into the wakefield, and then accelerated. The second approach to be explored will be down-ramp injection. In this method the density of the plasma decreases rapidly with longitudinal position; this slows the wakefield as it moves down the ramp, making it easier for electrons to be trapped, and then accelerated. These injection techniques will be studied through experiments performed in Oxford, at RAL, and, potentially, at other laser facilities, backed up by numerical simulations with particle-in-cell (PIC) codes. If successful this work will open the route to a new generation of very compact plasma accelerators operating at multi-kilohertz pulse repetition rates. These would immediately find applications in driving very compact X-ray sources, with widespread utility in ultrafast science, technology, and medicine.