Exploiting the European XFEL for a New Generation of High Energy Density and Materials Science

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X-ray free electron lasers (XFELs) produce quasi-monochromatic, highly-coherent, sub-100-fsec pulses of x-rays that are a billion times brighter than any synchrotron. The LCLS XFEL in the USA first lased in 2009, and research there is fundamentally changing how x-ray diffraction, spectroscopy and imaging are used by providing the ability to probe and understand the dynamics of matter on atomic length- and time-scales. The ultra-short x-ray pulse lengths from XFELs mean that they are ideal for making detailed x-ray scattering studies of extreme states of matter that exist for only a few billionths of a second. But progress at the LCLS has been constrained by the relatively low repetition rate of the LCLS, and the quality and repetition rate of the optical lasers used to create the short-lived states. As a result, while the transformative capabilities of XFELs have been demonstrated by the applicants at the LCLS over the last 5 years, progress has been limited.The European x-ray free electron laser (EXFEL) in Hamburg commenced operations in Sept 2017, and produces high-energy x-rays at unprecedented repetition rates. Furthermore, the UK-supplied DiPOLE diode-pumped optical laser that will be installed at EXFEL in 2019 will be vastly superior to the LCLS equivalent, having 5 times the energy, 2000 times the repetition rate, and the ability to change the laser pulse shape as required in real time. To exploit the £30M capital being invested by BEIS in the EXFEL project, the £8M invested by STFC and EPSRC in DiPOLE, and the £3M pa. UK contribution to EXFEL running costs, we have brought together a team of the leading UK researchers in XFEL and high-pressure science with the aim of combining EXFEL and DiPOLE to make transformative x-ray scattering studies of high energy density solids and liquids and metastable phases. We aim to understand how the remarkably complex properties of different phases of matter emerge from the correlations of the atomic or electronic constituents, and how to control and tailor these properties so that metastable states might be recovered to ambient conditions, leading to a whole new field with a gamut of practical applications.