Production of high quality electron bunches in AWAKE Run 2

在过去的五十年里，不断增长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，发现了希格斯玻色子，并正在寻找新的现象，如超对称。使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。作为另一种选择，我们正在寻求一种新技术，它将允许长度减少约十分之一，因此有望显著降低成本。这里提出的先进的质子驱动等离子体尾场实验(ACEKEND)使用高能质子束进入等离子体，如欧洲核子研究中心的那些实验。等离子体中的自由、带负电荷的电子被质子击退，但随后又被带正电荷的离子吸引回来，形成了一个高梯度的电尾场，等离子体电子启动了一种振荡运动。已经进行了激光或电子束撞击等离子体的实验，观察到了比传统加速器高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波场中的电子加速到未来能量前沿对撞机所需的太电子伏尺度，但只需一个阶段，长度为几公里。在觉醒运行1中，由10米等离子体中的高能质子束驱动的尾波场中，电子被加速到2GeV。这一观察结果被记录在2018年英国主导的《自然》杂志上，该出版物在网络和媒体上也得到了极大的关注。鉴于这一巨大的成功和该技术的演示，Awake Collaboration现在正准备进行第二轮数据采集，从2021年CERN加速器综合体重新启动开始，持续4年，直到2024年下一次关闭。觉醒运行2的主要目标是将高电荷束电子加速到更高的能量，同时保持束流质量，并表明这是一个可扩展的过程。O唤醒运行2的一些主要挑战是：o见证电子束的注入对具有高电荷捕获至关重要，因此需要详细的建模和一套优秀的诊断程序。o需要优秀的诊断程序来测量加速的电子束的最终性质，例如能量分布和空间范围。o开发和验证可缩放的等离子体细胞，即可以在保持均匀并显示可重复加速的情况下使用100秒以上的等离子体细胞。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，大约50GeV的高电荷束电子可以被送到一个固定目标计划中，例如搜索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调查中。Awake-UK组织提出了一个雄心勃勃的5年计划，从目前的拨款结束，即2020年4月开始，一直持续到2025年3月，在上面提到的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.