Production of high quality electron bunches in AWAKE Run 2

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, amongst other things, searching 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 advanced proton-driven plasma wakefield experiment (AWAKE) presented here uses a high-energy proton beam, such as those at CERN, to enter 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 future energy-frontier colliders, but in a single stage and with a length of a few km.

In AWAKE Run 1, electrons were accelerated up to 2 GeV in wakefields driven by high energy proton bunches in 10 m of plasma. This observation was documented in a UK-led publication in Nature in 2018 which also received significant attention online and in the media. Given this tremendous success and demonstration of the technique, the AWAKE collaboration is now preparing for a Run 2 with data taking starting with the restart of the CERN accelerator complex in 2021 and continuing for 4 years until its next shutdown in 2024. The main goals of AWAKE Run 2 are to accelerate high-charge bunches of electrons to higher energy whilst preserving beam quality and showing this to be a scalable process. Some of the main challenges of AWAKE Run 2 are:

o The injection of the witness electron bunch is crucial to having high charge capture, therefore detailed modelling is required and an excellent suite of diagnostics is needed.
o Excellent diagnostics will be required to measure the final properties, e.g. energy distribution and spatial extent, of the accelerated electron bunches.
o The development and verification of scalable plasma cells, i.e. plasma cells which can be used over 100s of metres or even kilometres whilst remaining uniform and showing reproducible acceleration.

The ultimate goal is to then be in a position after Run 2 in which an electron beam can be provided for high energy particle physics experiments. Assuming the success of Run 2, high-charge bunches of electrons at ~50 GeV could be delivered for a fixed-target programme to e.g. search for dark photons or a possible electron-proton collider. Other possible particle physics applications of the AWAKE scheme are being investigated.

The AWAKE-UK groups are proposing an ambitious 5-year programme to start at the end of the current grant, April 2020, and run to March 2025, working in all of the three keys areas mentioned above.

This project is naturally a multi-disciplinary pursuit involving accelerator, plasma and particle physicists as well as engineers and technical staff. If AWAKE continues its success, this method of acceleration could provide a new cost-effective route to TeV-scale colliders either of reduced length or increased energy as well as particle physics experiments requiring electrons beams of 10s of GeV. Several possible experiments have been identified which could make use of electron bunches with electrons of order 50 GeV, such as a fixed-target experiment to search for dark photons, measurements of strong-field QED through electron-laser collisions and a possible electron-proton collider.

This UK proposal is for a significant development and is wide-ranging in scope, with some equipment to be purchased from UK-based companies which could increase significantly in the future should this form of technology become a real-world solution. As we in the UK are a significant fraction of the AWAKE collaboration from the start, should the final goal be realised, there is potential for economic stimulus to the UK which building a large-scale research facility brings. This will involve the potential for large industrial contracts, training for students and other staff and knowledge exchange between academic institutes and industry arising from the R&D and the method of plasma wakefield acceleration.

The final aim of this project is to develop an accelerator technology to be used for investigation of fundamental particles and forces, however, the principle of plasma wakefield acceleration could revolutionise accelerators in general. The accelerating gradients achieved are up to three orders of magnitude higher than current techniques allowing a corresponding reduction in the size (and possibly cost) of future accelerators. This could then benefit any branch of science, health or industry which uses particle accelerators. An example is for future free electron laser facilities which could benefit significantly from this technique in which the acceleration of electrons takes place using a much shorter accelerating structure.

Diagnostic techniques developed here could be of benefit to many plasma wakefield experiments with different goals or applications. Therefore the work done here could benefit accelerators planned for other industries using the technique of plasma wakefield acceleration.

Finally, the physics behind the accelerator R&D and the final goal of the next energy-frontier collider will excite future students and captivate the public in much the same way as the Large Hadron Collider has. Having the UK as part of such cutting-edge R&D in order to be leaders of future experiments on the nature of the physical world is essential and beneficial for society. Any economic impact, as mentioned above, can only be achieved through being a strong partner. And the societal benefit of encouraging students to study physics and improving the general public's knowledge of science can best be achieved if we are part of these and future pursuits.