Controlled injection for Multi-pulse laser wakefield accelerators

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级中将电子（原则上还有正电子）加速到多GeV的能量。然而，在这些系统可以实现其巨大的前景之前，必须克服几个问题：（i）聚束参数的发射到发射的抖动对于许多应用来说太高;（ii）用于驱动加速器的激光器仅以每秒几个脉冲操作，这对于诸如驱动紧凑光源的应用来说太低;以及（iii）驱动激光器的效率非常低（低于0.1%），这对于诸如驱动粒子对撞机的长期应用将是一个问题。为了解决目前使用的驱动激光器的低重复率和效率，我们的小组提出了一系列低能量激光脉冲可以用来驱动等离子体波，而不是单个高能量脉冲。如果脉冲串中的脉冲由等离子体周期间隔开，则尾场相干地相加以产生具有朝向脉冲串的后部增长的幅度的等离子体波。这种称为多脉冲激光韦克菲尔德加速（MP-LWFA）的方法可以使新型激光器能够被采用，该新型激光器不能容易地产生高能量脉冲，但是可以以几千赫兹的脉冲重复率以高效率产生几毫焦耳的激光脉冲。因此，MP-LWFA方法提供了一种实现能够以高重复率产生GeV级电子聚束的紧凑型加速器的途径，并进而提供了新一代紧凑型光源的途径。为了解决聚束参数的高发射点抖动的问题，将有必要开发用于控制电子到等离子体韦克菲尔德中的注入和捕获的策略。这对于由MP-LWFA驱动的线性或准线性等离子体尾场尤其具有挑战性。这项工作的目标是开发控制电子注入到准线性尾场产生的MP-LWFAs的方法。方法学：将探索两种控制注入的方法：（i）使用附加激光脉冲的电离注入;和（ii）等离子体密度斜坡注入。这些可能的方法将通过实验和数值模拟进行探索。实验工作将在牛津大学的实验室和国家激光设施，如卢瑟福阿普尔顿实验室的中央激光设施进行。将利用在国家和地方计算机集群设施上运行的粒子单元代码进行数值模拟。

Context, aims and objectivesLaser-driven plasma accelerators can already accelerate electrons (and, in principle, positrons) to multi-GeV energies in accelerator stages only a few centimetres long. However, several issues must be overcome before these systems can realize their enormous promise: (i) the shot-to-shot jitter of the bunch parameters is too high for many applications; (ii) the lasers used to drive the accelerator operate at only a few pulses per second, which is too low for applications such as driving compact light sources; and (iii) the driving lasers are very inefficient (below 0.1%), which will be an issue for long-term applications such as driving particle colliders. In an effort to address the low repetition rate and efficiency of the driving lasers used at present, our group has proposed that a train of low-energy laser pulses could be used to drive the plasma wave, rather than a single high energy pulse. If the pulses in the train are spaced by the plasma period then the wakefields add coherently to produce a plasma wave with an amplitude which grows towards the back of the pulse train. This approach, known, as multi-pulse laser wakefield acceleration (MP-LWFA), could enable novel lasers to be employed which cannot easily generate high energy pulses, but which can generate few-milijoule laser pulses at mulit-kilohertz pulse repetition rates, with high efficiency. The MP-LWFA approach therefore offers a route to achieving compact accelerators able to generate GeV-scale electron bunches at high repetition rates, and in turn a route to a new generation of compact light sources. In order to address the issues of high shot-to-jot jitter of the bunch parameters it will be necessary to develop strategies for controlling the injection and trapping of electrons into the plasma wakefield. This is especially challenging for the linear, or quasi-linear, plasma wakefields driven by a MP-LWFA. The objectives of this work are to develop methods for controlling electron injection into the quasi-linear wakefields produced by MP-LWFAs. Methodology: Two methods for controlling injection will be explored: (i) ionization injection using additional laser pulses; and (ii) injection at plasma density ramps. These possible methods will be explored both experimentally and through numerical simulations. The experimental work will be undertaken in our labs in Oxford and at national laser facilities such as the Central Laser Facility at Rutherford Appleton Laboratory. Numerical simulations will be undertaken using particle-in-cell (PIC) codes run on national and local computer cluster facilities.