Production of high quality electron bunches in AWAKE Run 2

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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.