Controlled injection for Multi-pulse laser wakefield accelerators

背景、目标和目标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(原则上是正电子)加速到只有几厘米长的多GeV能量。然而，在这些系统能够实现其巨大的前景之前，必须克服几个问题：(I)对于许多应用来说，束团参数的点对点抖动太高；(Ii)用于驱动加速器的激光器仅以每秒几个脉冲的速度工作，这对于诸如驱动紧凑型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频率和低效率问题，我们的团队提出了可以使用一系列低能量激光脉冲来驱动等离子体波，而不是单一的高能脉冲。如果序列中的脉冲被等离子体周期隔开，则尾迹场相干地相加以产生具有朝向脉冲序列背面增长的幅度的等离子体波。这种被称为多脉冲激光尾场加速(MP-LWFA)的方法可以使人们使用新型激光器，这种激光器不容易产生高能脉冲，但可以以高效率产生几千赫兹脉冲重复频率的少毫焦耳激光脉冲。因此，MP-LWFA方法提供了一条实现紧凑型加速器的途径，该加速器能够以高重复频率产生GeV规模的电子束，进而提供了通向新一代紧凑型光源的途径。为了解决束团参数的高抖动问题，有必要制定控制电子注入和俘获到等离子体尾场的策略。这对于由MP-LWFA驱动的线性或准线性等离子体尾迹场尤其具有挑战性。这项工作的目标是开发控制电子注入到MP-LWFA产生的准线性尾迹场的方法。方法：将探索两种控制注射的方法：(I)使用额外激光脉冲的电离注射；(Ii)在等离子体密度梯度下注射。这些可能的方法将通过实验和数值模拟来探索。实验工作将在我们在牛津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。将使用在国家和地方计算机集群设施上运行的粒子单元(PIC)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.