Production of high quality electron bunches in AWAKE Run 2

在过去的50年里，不断增加的能量和尺寸的加速器使我们能够探索物理世界的基本结构。这在日内瓦欧洲核子研究中心的大型强子对撞机中达到了顶峰，这是一个27公里长的加速器，它发现了希格斯玻色子，并且正在寻找诸如超对称之类的新现象。利用目前的加速器技术，未来的高能对撞机将具有类似的长度，甚至更长。作为替代方案，我们正在寻求一种新技术，该技术可以将长度减少大约十分之一，从而有望大大降低成本。这里介绍的先进质子驱动等离子体尾流场实验（AWAKE）使用高能质子束进入等离子体，就像欧洲核子研究中心（CERN）的那样。等离子体中自由的带负电荷的电子被质子撞出它们的位置，但随后又被带正电荷的离子吸引回来，形成一个高梯度的电“尾流场”，等离子体电子开始振荡运动。已经进行了将激光或电子束撞击等离子体的实验，并观察到比传统加速器高1000倍的加速梯度。考虑到可用质子束的初始能量要高得多，预计它在等离子体中产生的电场可以将尾流场中的电子加速到未来能量前沿对撞机所需的太电子伏特规模，但在单个阶段和几公里的长度上。在AWAKE Run 1中，由10米等离子体中的高能质子束驱动的尾流场中的电子加速到2 GeV。这一观察结果被记录在2018年英国主导的《自然》杂志上，该杂志也在网上和媒体上受到了极大的关注。鉴于这项技术的巨大成功和演示，AWAKE合作项目现在正准备进行第二次运行，从2021年重启CERN加速器综合体开始，持续4年，直到2024年下一次关闭。AWAKE Run 2的主要目标是将高电荷电子束加速到更高的能量，同时保持光束质量，并表明这是一个可扩展的过程。AWAKE Run 2的一些主要挑战是：o见证电子束的注入对于获得高电荷捕获至关重要，因此需要详细的建模和一套出色的诊断。o要测量加速电子束的最终特性，例如能量分布和空间范围，将需要出色的诊断。o发展和验证可扩展的浆细胞，即可以在100米甚至公里的范围内使用的浆细胞，同时保持均匀并显示可重复的加速度。最终目标是在运行2之后，能够为高能粒子物理实验提供电子束。假设Run 2的成功，在~ 50gev的高电荷电子束可以被传送到一个固定的目标程序中，例如寻找暗光子或可能的电子-质子对撞机。AWAKE方案的其他可能的粒子物理应用正在研究中。wake - uk小组提出了一项雄心勃勃的5年计划，从目前的拨款结束时（2020年4月）开始，一直持续到2025年3月，在上述三个关键领域开展工作。

Over the last fifty years, accelerators of ever increasing energy and size have allowed us to probe the fundamental structure of the physical world. This has culminated in the Large Hadron Collider at CERN, Geneva, a 27-km long accelerator which has discovered the Higgs Boson and is, amongst other things, searching for new phenomena such as Supersymmetry. Using current accelerator technology, future high energy colliders will be of similar length or even longer. As an alternative, we are pursuing a new technology which would allow a reduction by about a factor of ten in length and hence would be expected to reduce the cost by a significant fraction.The advanced proton-driven plasma wakefield experiment (AWAKE) presented here uses a high-energy proton beam, such as those at CERN, to enter into a plasma. The free, negatively-charged electrons in the plasma are knocked out of their position by the protons, but are then attracted back by the positively-charged ions, creating a high-gradient electric "wakefield" and an oscillating motion is started by the plasma electrons. Experiments have already been carried out impacting lasers or an electron beam onto a plasma and accelerating gradients have been observed which are 1000 times higher than conventional accelerators. Given the much higher initial energy of available proton beams, it is anticipated that the electric fields it creates in a plasma could accelerate electrons in the wakefield up to the teraelectron-volts scale required for future energy-frontier colliders, but in a single stage and with a length of a few km. In AWAKE Run 1, electrons were accelerated up to 2 GeV in wakefields driven by high energy proton bunches in 10 m of plasma. This observation was documented in a UK-led publication in Nature in 2018 which also received significant attention online and in the media. Given this tremendous success and demonstration of the technique, the AWAKE collaboration is now preparing for a Run 2 with data taking starting with the restart of the CERN accelerator complex in 2021 and continuing for 4 years until its next shutdown in 2024. The main goals of AWAKE Run 2 are to accelerate high-charge bunches of electrons to higher energy whilst preserving beam quality and showing this to be a scalable process. Some of the main challenges of AWAKE Run 2 are:o The injection of the witness electron bunch is crucial to having high charge capture, therefore detailed modelling is required and an excellent suite of diagnostics is needed.o Excellent diagnostics will be required to measure the final properties, e.g. energy distribution and spatial extent, of the accelerated electron bunches.o The development and verification of scalable plasma cells, i.e. plasma cells which can be used over 100s of metres or even kilometres whilst remaining uniform and showing reproducible acceleration.The ultimate goal is to then be in a position after Run 2 in which an electron beam can be provided for high energy particle physics experiments. Assuming the success of Run 2, high-charge bunches of electrons at ~50 GeV could be delivered for a fixed-target programme to e.g. search for dark photons or a possible electron-proton collider. Other possible particle physics applications of the AWAKE scheme are being investigated.The AWAKE-UK groups are proposing an ambitious 5-year programme to start at the end of the current grant, April 2020, and run to March 2025, working in all of the three keys areas mentioned above.