AWAKE: a proton-driven plasma wakefield acceleration experiment at CERN

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，已经发现了希格斯玻色子，并即将开始寻找新的现象，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，这种技术将允许长度减少约十分之一，因此预计将显著降低成本。这里提出的想法是将高能质子束(如欧洲核子研究中心的质子束)撞击到等离子体中。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对撞机所需的太电子伏级，但只需一个阶段，长度为几公里。然而，这样的对撞机还需要很多年的时间，首先需要测试实验。觉醒合作公司将在欧洲核子研究中心进行第一次原理验证实验。该实验将使用高能质子束撞击约10米长的等离子体单元，并测量在质子束后面移动的一束电子的能量变化。观察电子的显著能量变化将证明这种形式的加速的概念，到目前为止只在模拟中进行研究。英国有几个小组(中央激光设施、Cockcroft研究所、帝国理工学院、约翰·亚当斯研究所、Strathclyde和伦敦大学学院)在欧洲核子研究中心合作准备唤醒实验。我们提出了一个方案，以开发广泛的仪器，这将使我们能够成功地建立实验，并提取必要的物理，以证明这种方法的力量。一个关键的部分是能够建立一个在长度上具有均匀密度的等离子体细胞，其长度比以前尝试的要长得多。我们还将在准确的时间将电子源的元素发射到等离子体中，以便感受到质子束在尾波场中可能产生的最大加速梯度。为了确定实验的成功与否，我们将测量等离子体的性质以及电子束在等离子体中加速后的能量和空间分布。最后，我们的结果将改进等离子体尾流场的模拟，让我们对更大规模的实验预期更有信心，并帮助我们最好地优化其布局和能力。如果成功，这项实验将带来一个更大规模的项目，用高粒子数加速小空间范围的电子束，并最终产生一种新的加速形式，这可能会导致未来的能量前沿粒子物理实验。这项技术有可能从根本上改变高能物理的前沿，使用加速器作为当前计划或要求的执行剂，但长度和成本是目前计划或要求的十分之一。随着更大的加速梯度和更小的空间范围，基于等离子体的加速器技术也可以导致更小的同步加速器光源，例如用于医院或工业的低能量蛋白质和桌面加速器的结构探测。

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 about to embark on searches 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 idea presented here is to impact a high-energy proton beam, such as those at CERN, 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 a future collider, but in a single stage and with a length of a few km. Such a collider is, however, many years in the future and test experiments are first needed.The AWAKE collaboration will perform a first proof-of-principle experiment at CERN. The experiment will use a high-energy proton beam to impact on a plasma cell of about 10 m and measure the energy change in a bunch of electrons which will travel behind the proton beam. Observing significant energy changes in the electrons would demonstrate the concept of this form of acceleration which has so far only been studied in simulation.The UK has several groups (Central Laser Facilities, Cockcroft Institute, Imperial College, John Adams Institute, Strathclyde and UCL) in the collaboration preparing the AWAKE experiment in CERN. We propose a programme to develop a wide-range of instrumentation which will the allow us to successfully build the experiment and extract the physics necessary to demonstrate the power of this approach. A crucial part is being able to build a plasma cell with a uniform density over lengths much longer than previously tried. We will also deliver elements of the electron source to be fired into the plasma at exactly the right time so as to feel the largest possible accelerating gradient in the wakefield created by the proton beam. To determine the success of the experiment, we will measure the properties of the plasma and the energy and spatial profile of the electron beam after it has been accelerated in the plasma. Finally, our results will improve simulations of plasma wakefields to give us more confidence in our expectations of a larger-scale experiment and help us best optimise its layout and capabilities. If successful, this experiment will lead to a further larger-scale project to accelerate bunches of electrons of small spatial extent with high particle numbers and ultimately a new form of acceleration which could lead to future, energy-frontier particle physics experiments. This technique has the potential to radically alter the frontier of high energy physics with accelerators as performant as currently planned or required, but at a tenth of the length and hence cost. With the significantly larger acceleration gradients and smaller spatial extent, plasma-based accelerator technology could also lead to vastly smaller synchrotron light sources which probe the structure of e.g. proteins and table-top accelerators of lower energy for use in hospitals or industry.