Proton-driven plasma wakefield acceleration - a new route to a TeV e+e- collider

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，希望发现像希格斯玻色子这样的新粒子或发现像超对称性这样的新现象。使用目前的加速器技术，下一台对撞机，如线性电子-正电子对撞机，将长30-50公里，这将需要巨大的投资。作为另一种选择，我们正在寻求一种新的超紧凑技术，这种技术可以将长度减少约十分之一，从而大大降低成本。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。第一次原理验证实验将在未来5年内在欧洲核子研究中心进行。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，到目前为止还只在模拟中进行研究。英国有七个小组(ASTEC、中央激光设备、考克罗夫特研究所、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)合作准备在欧洲核子研究中心进行这项测试实验。我们提出了一个解决各种技术问题的方案，并开发了一系列的仪器，使我们能够成功地建立测试实验。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。我们还将设计电子粒子源，在正确的时间发射到等离子体中，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，我们将设计诊断工具，测量尾波场的大小，以及电子束在等离子体中加速后的能量和空间分布。最后，我们的结果将改进对等离子体尾流场的模拟，让我们对更大规模的实验的预期更有信心，并帮助我们最好地优化其布局和能力。如果成功，这项实验将导致进一步的更大规模的项目，以加速具有高粒子数的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 hopes to discover new particles such as the Higgs Boson or new phenomena such as Supersymmetry. Using current accelerator technology, a next collider such as a linear electron-positron collider would 30-50 km long which would require immense investment. As an alternative, we are pursuing a new ultra-compact technology which would allow a reduction by about a factor of ten in length and hence would 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.A first proof-of-principle experiment will be performed at CERN over the next 5 years. 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 seven groups (ASTeC, Central Laser Facility, Cockcroft Institute, Imperial College, John Adams Institute, Strathclyde and UCL) in the collaboration preparing for this test experiment in CERN. We propose a programme to answer various technical issues and develop a wide-range of instrumentation which will the allow us to successfully build the test experiment. 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 design the electron particle 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 design diagnostic tools which will measure the size of the wakefield 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.