High repetition rate femtosecond X-ray studies of dynamically compressed matter

The use of x-ray diffraction to study materials under rapid dynamic compression has a long history, going back well over half a century. The last two decades have seen particularly spectacular advances, with high power optical nanosecond lasers and other drivers coupled to synchrotrons and x-ray free electron lasers (FELs), such that single-shot diffraction patterns from shocked and ramp-compressed samples can now be obtained on a timescale shorter than the period of even the fastest phonon in the system. In principle this can allow the study of matter in regimes of pressure and temperature inaccessible by almost any other means, and of direct relevance to the structure of planets in our own solar system and beyond. Experiments using such systems have started to lead to important advances in our understanding of phase transformations and plasticity at the lattice level. One major obstacle to date has been the low repetition rate of the optical lasers (that typically have drive energies of 100’s of joules in nanosecond pulses) compared with the x-ray sources. For example, at the FEL at LCLS the optical laser fires once every 3 minutes, whilst the x-ray pulses even from the warm linac are produced at 120 Hz. Coupling a higher repetition rate optical laser to the x-ray source would not only increases the overall throughput of experiments and acquisition of data, but will open up novel opportunities for photon-hungry experiments, such as spectrally resolving the inelastic x-rays scattered from phonons, or the use of resonant inelastic x-ray scattering (RIXS) which can provide information on the electronic structure of these transiently compressed materials. Early last year the first user experiments were performed that combined x-ray pulses from the European XFEL with the DiPOLE-100X optical laser, which in principle can operate at 10 Hz, thus transforming the landscape for research in this area. Representative and preliminary results from these experiments will be presented.