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

摘要：激光驱动的等离子体加速器已经可以在只有几厘米长的加速器阶段将电子加速到gev规模的能量。然而，迄今为止使用的驱动激光器最多只能以每秒几个脉冲的速度运行，并且具有非常低的壁塞效率。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：多脉冲激光尾流场加速器（MP-LWFA）。在这种方法中，等离子体尾流场由一列低能量激光脉冲驱动。如果这些脉冲的间隔是等离子体周期，那么由脉冲驱动的尾流场就会相干地叠加，产生一个向脉冲序列后部生长的等离子体波。牛津大学的研究小组最近在卢瑟福阿普尔顿实验室（RAL）的实验中首次展示了这种方法。在这项工作中，表明等离子尾流场确实可以被一系列激光脉冲共振激发，证实了这种方法的有效性。然而，到目前为止，由于MP-LWFA驱动的尾流场是线性的，所以电子还没有被这种方法加速，因此不会从背景等离子体中自我捕获电子。该项目的目的是开发控制电子注入到由MP-LWFA驱动的准线性尾流场的技术。将研究两种方法。在第一，使用额外的激光脉冲电离掺杂物质将被探索。如果一个短激光脉冲（“注入”脉冲）被聚焦在由脉冲序列驱动的尾流中，那么被第二个激光脉冲电离的电子可以被困在尾流场中，然后加速。探索的第二种方法是下行注入。在这种方法中，等离子体的密度随纵向位置的增加而迅速减小；当韦克菲尔德沿着斜坡向下移动时，这减慢了它的速度，使电子更容易被捕获，然后加速。这些注射技术将通过粒子池（PIC）代码的数值模拟进行研究，并与牛津、RAL和其他激光设备的实验结果进行比较。如果成功，这项工作将为新一代非常紧凑的等离子体加速器开辟道路，以数千赫兹的脉冲重复率工作。这将立即在驱动非常紧凑的x射线源中得到应用，在超快科学、技术和医学中具有广泛的用途。

Summary: 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 numerical simulations with particle-in-cell (PIC) codes, and compared to the results of experiments performed in Oxford, RAL, and other laser facilities.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.