Production of high quality electron bunches in AWAKE Run 2

在过去的50年里，不断增加的能量和规模的加速器使我们能够探索物理世界的基本结构。这在位于日内瓦的欧洲核子研究中心的大型强子对撞机上达到了高潮，这是一个27公里长的加速器，它发现了希格斯玻色子，并且正在寻找新的现象，如超对称性。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的技术，这将允许减少约十倍的长度，因此预计将减少成本的一个显着的fractions. Advanced质子驱动等离子体韦克菲尔德实验（AWAKE）这里介绍使用高能质子束，如在欧洲核子研究中心，进入等离子体。等离子体中的自由负电荷电子被质子撞出它们的位置，但随后被正电荷离子吸引回来，产生高梯度电“韦克菲尔德”，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到等离子体上的实验，并且已经观察到比常规加速器高1000倍的加速梯度。考虑到可用质子束的初始能量高得多，预计它在等离子体中产生的电场可以将韦克菲尔德中的电子加速到未来能量前沿对撞机所需的太电子伏特尺度，但在单个阶段中，长度为几公里。在10 m等离子体中的高能质子束驱动的尾场中，电子被加速到2GeV。这一观察结果记录在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。鉴于这一巨大的成功和技术的演示，AWAKE合作现在正在准备运行2，从2021年欧洲核子研究中心加速器复合体的重新启动开始数据采集，并持续4年，直到2024年下一次关闭。唤醒运行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，并显示这是一个可扩展的过程。AWAKE Run 2的一些主要挑战是：o见证电子聚束的注入对于具有高电荷捕获是至关重要的，因此需要详细的建模并且需要一套出色的诊断。o将需要出色的诊断来测量加速电子聚束的最终性质，例如能量分布和空间范围。也就是说，等离子体电池可以使用超过100米甚至公里，同时保持均匀并显示可再现的加速度。最终目标是在运行2之后处于这样的位置，其中可以为高能粒子提供电子束。物理实验假设运行2成功，大约50 GeV的高电荷电子束可以用于固定目标计划，例如寻找暗光子或可能的电子质子对撞机。AWAKE计划的其他可能的粒子物理应用正在研究中。AWAKE-英国小组提出了一个雄心勃勃的5年计划，从2020年4月当前拨款结束时开始，一直持续到2025年3月，在上述所有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.