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

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.