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

在过去的50年里，能量和尺寸不断增加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中达到了顶峰，这台长达27公里的加速器发现了希格斯玻色子，并即将开始寻找超对称等新现象。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，从而有望大幅降低成本。这里提出的想法是将高能质子束（比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。人们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。鉴于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对撞机所需的太电子伏特规模，但这是在单个阶段，长度只有几公里。然而，这样的对撞机还需要许多年的时间，首先需要进行测试实验。AWAKE合作项目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米的浆细胞，并测量质子束后面行进的一束电子的能量变化。观察电子中显著的能量变化将证明这种形式的加速度的概念，迄今为止只在模拟中研究过。英国有几个小组（中央激光设施、科克罗夫特研究所、帝国理工学院、约翰亚当斯研究所、斯特拉斯克莱德和伦敦大学学院）在欧洲核子研究中心合作准备AWAKE实验。我们提出了一个开发各种仪器的计划，这将使我们能够成功地建立实验并提取必要的物理来证明这种方法的力量。一个关键的部分是能够建立一个密度均匀的浆细胞，其长度比以前尝试的要长得多。我们还将在正确的时间将电子源的元素发射到等离子体中，以便在质子束产生的尾流场中感受到最大可能的加速梯度。为了确定实验是否成功，我们将测量等离子体的特性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，使我们对更大规模实验的期望更有信心，并帮助我们最佳地优化其布局和功能。如果成功，这个实验将导致一个更大规模的项目，加速小空间范围内的高粒子数的电子束，并最终形成一种新的加速形式，这可能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，使加速器的性能达到目前计划或需要的水平，但长度和成本只有目前的十分之一。等离子体加速器技术具有更大的加速度梯度和更小的空间范围，也可以产生更小的同步加速器光源，用于探测蛋白质等结构，以及用于医院或工业的低能量台式加速器。

Over the last fifty years, accelerators of ever increasing energy and size have allowed us to probe the fundamentalstructure of the physical world. This has culminated in the Large Hadron Collider at CERN, Geneva, a 27-km longaccelerator which has discovered the Higgs Boson and is about to embark on searches for new phenomena such asSupersymmetry. 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 andhence 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 bythe positively-charged ions, creating a high-gradient electric "wakefield" and an oscillating motion is started by the plasmaelectrons. Experiments have already been carried out impacting lasers or an electron beam onto a plasma and acceleratinggradients 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 inthe wakefield up to the teraelectron-volts scale required for a future collider, but in a single stage and with a length of a fewkm. 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-energyproton beam to impact on a plasma cell of about 10 m and measure the energy change in a bunch of electrons which willtravel behind the proton beam. Observing significant energy changes in the electrons would demonstrate the concept ofthis 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 todevelop a wide-range of instrumentation which will the allow us to successfully build the experiment and extract the physicsnecessary to demonstrate the power of this approach. A crucial part is being able to build a plasma cell with a uniformdensity over lengths much longer than previously tried. We will also deliver elements of the electron source to be fired intothe plasma at exactly the right time so as to feel the largest possible accelerating gradient in the wakefield created by theproton beam. To determine the success of the experiment, we will measure the properties of the plasma and the energyand spatial profile of the electron beam after it has been accelerated in the plasma. Finally, our results will improvesimulations of plasma wakefields to give us more confidence in our expectations of a larger-scale experiment and help usbest optimise its layout and capabilities.If successful, this experiment will lead to a further larger-scale project to accelerate bunches of electrons of small spatialextent with high particle numbers and ultimately a new form of acceleration which could lead to future, energy-frontierparticle physics experiments. This technique has the potential to radically alter the frontier of high energy physics withaccelerators as performant as currently planned or required, but at a tenth of the length and hence cost. With thesignificantly larger acceleration gradients and smaller spatial extent, plasma-based accelerator technology could also leadto vastly smaller synchrotron light sources which probe the structure of e.g. proteins and table-top accelerators of lowerenergy for use in hospitals or industry.