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

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，希望能发现新的粒子，如希格斯玻色子或新的现象，如超对称性。使用目前的加速器技术，下一个对撞机，如线性电子-正电子对撞机，将有30-50公里长，这将需要巨大的投资。作为一种替代方案，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10倍，从而将成本降低很大一部分。这里提出的想法是将高能质子束，如欧洲核子研究中心的那些，撞击成等离子体。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。考虑到现有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韦克菲尔德中的电子加速到未来对撞机所需的太电子伏特级，但需要一个单级，长度为几公里。然而，这样的对撞机还需要很多年的时间，首先需要进行测试实验，第一个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米的等离子体池，并测量质子束后面的电子束的能量变化。观察电子中显著的能量变化将证明这种形式的加速的概念，到目前为止只在模拟中进行了研究。英国有七个小组（ASTeC，中央激光设施，Cockcroft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UCL）在合作准备欧洲核子研究中心的这个测试实验。我们提出了一个方案，以回答各种技术问题，并开发了广泛的仪器，这将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等离子体细胞与均匀的密度超过长度比以前尝试过。我们也将设计电子粒子源，使其在正确的时间射入电浆，以便在质子束所产生的韦克菲尔德中感受到最大可能的加速梯度。为了确定实验的成功，我们将设计诊断工具，测量韦克菲尔德的大小和电子束在等离子体中加速后的能量和空间分布。最后，我们的结果将改善等离子体尾场的模拟，使我们对更大规模实验的期望更有信心，并帮助我们最好地优化其布局和能力。如果成功，这个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，并最终导致未来的新形式的加速，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学的前沿，加速器的性能与目前计划或要求的一样，但长度和成本只有十分之一。由于明显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还可以导致更小的同步加速器光源，其探测例如蛋白质的结构和用于医院或工业的较低能量的台式加速器。

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 Facilities, 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.