Investigating electron dynamics and radiation transport in solid-density plasmas using X-ray FELs

The advent of 4th generation light sources - X-ray free-electron lasers (FELs) - is revolutionising the way we investigate matter in extreme conditions by providing ultra-bright, femtosecond, nearly monochromatic X-rays at tuneable photon energies from the XUV to the hard X-ray spectral region. When focused to micron-sized spots, intensities exceeding 10^17 W/cm^3 can be generated at X-ray wavelengths for the first time. We showed recently that such high intensities are sufficient to heat solid systems to temperatures of several million Kelvin within a few tens of femtoseconds, i.e., to temperature and density conditions similar to those found half way into the centre of the Sun, thus paving the way to novel investigations of extreme states of matter of broad interest to astrophysics, planetary science, inertial confinement fusion research, and national security applications.

Alongside generating hot-dense plasmas, intense X-ray interactions with matter give rise to a well-controlled source of non-thermal (hot) electrons which are generated either directly by photoionization, or via inner-shell atomic recombination processes such as Auger decay. Because ponderomotive energies are negligible at X-ray wavelengths, these are the only 'hot' electrons generated during the irradiation, leading to a non-thermal electron distribution that can be controlled directly by modulating the X-ray wavelength and intensity, and is also far simpler to model theoretically than hot electrons produced in intense optical laser-plasma interactions.

In this project we aim to use these unique characteristics of X-ray FEL pulses to experimentally create a tailored non-thermal electron distribution within a hot-dense plasma, and track its evolution and equilibration dynamics on ultra-fast timescales. These measurements will not only provide some of the first measurements of electron-electron collisionality in strongly-coupled systems, but will also more broadly assess the validity of the Coulomb Logarithm framework commonly used to model a wide range of electron interaction processes, including bremsstrahlung emission, conductivity, thermal transport and stopping power.

Importantly, we note that the irradiation of solid samples with intense X-ray light allows us to reach the temperature-density conditions corresponding to the radiation/convection zone boundary of the Sun. By using our recently developed spectroscopic techniques we aim to investigate radiation transport and the opacity of low and mid-Z elements in these extreme conditions, and determine whether the opacity can help address the outstanding disagreement between solar models and the internal structure of the Sun determined by helio-seismic observations. Accurate independent measurements of the Fe opacity in this regime are particularly of interest given the recent experimental results from Bailey et al. (Nature 517, 56, 2015), showing a significant deviation in the experimental opacity from that predicted by plasma opacity models for lower density plasmas.

Our proposed research project will focus on investigating the fundamental aspects of ultra-fast electron dynamics in high-energy density systems and, as explained in the academic beneficiaries section, will primarily impact various fields of academic research. There are, however, several other areas where we envisage our work will have a significant impact.

Firstly, we note that X-ray-matter interactions and the evolution of high-density plasmas on femtosecond timescales is of particular importance to research in structural biology using coherent diffraction imaging techniques on FELs, including serial femtosecond crystallography, X-ray holography and single-molecule imaging, since electron collisional dynamics is one of the most important processes contributing to damage on femtosecond timescales. Here we will investigate some of these dynamical processes for the first time, which will provide unique new insight into electron damage processes and help improve and develop new computational models to be used across the biological imaging community. The capability to image biological systems on ultra-short time scales, and how they fold or interact with light, is of great practical interest not only to structural biologists but also the medical and pharmaceutical industries more broadly, and the successful development of appropriate techniques and the accurate, predictive computational modelling of samples irradiated by intense X-rays will be key to fully exploit the range of future opportunities. We envisage parts of this project could become eventually a standalone computational package with a broad user base and, with limited additional support, be made commercially viable, aimed at a range of public and private enterprise.

An understanding of the physics of dense plasmas is further of considerable interest and importance to MoD and AWE in the context of the CTBT, and the current nuclear security environment that relies on simulation, and thus the underpinning physics models. The Oxford group has strong links with AWE via the OxCHEDS institute (the Oxford University Centre for High Energy Density Science) and will continue to engage with AWE to ensure they are aware in a timely manner of the results of the proposed experiments.

Importantly, we are keen to promote our research more widely making full use of opportunities for public engagement, including writing for the Oxford Science Blog, working with press offices at Oxford and from abroad to produce press releases publicising our research results as widely as possible, and actively engaging with the popular scientific press (our work has in the past been picked up often and has featured in, among others, New Scientist, La Recherche, National Geographic and Scientific American) and with national broadcasters.

Finally, the proposed research will provide a truly exceptional opportunity for young researchers (both at the postdoctoral and graduate student level) to work on billion-dollar flagship science facilities worldwide, and to collaborate internationally with a wide range of world-leading researchers, universities and laboratories. The training of scientists on the newly developed X-ray FEL facilities will not only provide unique opportunities for personal and professional growth, but is also strategically important to enable the UK community more widely to make full use of future opportunities arising from the large investments the UK has in the European XFEL project: alongside direct contributions to the project the UK is (co-)funding dedicated beam lines for serial femtosecond crystallography (SFX) and high energy density-science (HED) via the HiBEF consortium.