The 2016 STFC FEL strategic review recognised the revolutionary potential of X-ray Free-Electron Lasers (XFELs) for science and industry internationally. In order for the UK to stay competitive, a recommendation was made to join the European XFEL, based in Hamburg. This was enacted and the UK formally joined in March 2018. In the longer term, it was foreseen that the UK would need its own XFEL to meet the anticipated demand and to have a source matched to the needs of UK researchers. To this end ASTeC has led the development of a UK-wide R&D programme since 2017, informed by ongoing consultation with UK academia and industry. CLARA is a major part of this effort, being a dedicated accelerator test facility aimed at developing technologies and techniques to improve the radiation output of FELs.
In April 2019, STFC commissioned a project to develop the Science Case for a UK XFEL, led by Professor Jon Marangos (Imperial College) and featuring an expert team of UK scientists, together with an array of international advisors. ASTeC has been strongly engaged throughout, starting at the project's launch at a Town Meeting at the Royal Society, where Professor Jim Clarke gave a presentation on 'The Shape of FELs to Come'.
Since then, ASTeC has led the process of liaising with the science team to establish the requirements and aspirations of this pioneering user community, and to highlight potential future accelerator developments. Based upon these discussions ASTeC has led the development of concept facility outlines to best address the identified needs and to ensure that, should the UK decide to proceed with building a national facility, it would have world-leading and unique capabilities and so give UK researchers and industry a competitive edge.
The Science Case, including ASTeC's concept outlines, was launched by STFC in July 2020 for further consultation with the scientific community, prior to a review by an expert international panel later in the year. Subject to approval, the project will proceed to a conceptual design phase in which a preferred machine option will be developed and refined. Work in this phase would also start to establish cost estimates and the business case for the facility, and would identify a specific R&D plan for future phases. The continued guidance of the UK science community will be vital.
The present R&D programme covers most of the key FEL sub-systems, including electron beam generation, acceleration, transport and diagnostics, as well as theoretical studies into the development of new ways of creating higher quality light from FELs. For example, ASTeC staff are studying how to make pulses of light that are shorter, brighter, and more stable than currently available elsewhere by manipulating the electron bunch using magnets and lasers. UK users of XFELs have also highlighted the importance of combining FEL output with other sources, particularly high power lasers, as developed by STFC's Central Laser Facility. Users need the high quality FEL pulses to be synchronised with other sources to the femtosecond level in order to study extraordinarily fast atomic processes that determine how matter behaves. To meet these requirements ASTeC staff are engaged in understanding the fundamental performance of the accelerating sub-systems, and in studying and improving their combined performance on CLARA.
Two other areas of XFEL R&D that ASTeC leads are in potential recirculation and energy recovery schemes, and in the development of superconducting undulators for FELs. Both of these areas could potentially have a positive impact on the costs required to build and operate a future XFEL.