Direct measurements of Temperature and Transport Properties at Extreme Conditions

Understanding the structure and behaviour of matter at extreme pressures and temperatures is of critical importance to many fundamental physics applications, including geophysics, planetary science and astrophysics, shock and plasma physics, and the search for novel phases of materials and non-equilibrium thermodynamics. Of particular interest is the area of pressure-temperature space that defines Warm Dense Matter (WDM), an intriguing state of matter too hot, and hence too highly-ionised, to be described by condensed matter theories, and too strongly coupled and correlated for classical plasma physics to provide an accurate description. A direct characterisation of matter at such conditions is therefore essential.

Experimentally, the extreme temperature-density space where WDM resides is difficult to create. Often, such states are made by strong shock compression using nanosecond lasers [1, 2], by isochoric heating via femtosecond laser excitation [3-5], or by laser-generated proton beams [6-8]. These states are extremely short-lived and are hence very challenging to probe. Hard X-ray Free Electron Lasers (XFELs), such as the LCLS, USA; European XFEL, Germany; and the proposed UK XFEL[9], have the ability to emit extremely bright, hard X-rays with a pulse length of less than 100 fs, allowing one to capture these short-lived states.

These XFELs, combined with laser-compression drivers, have provided unprecedented insight into the structure of matter at extreme pressures through measurement of the static structure factor [4, 10-12]. To-date, however, remarkably little attention has been paid to developing methods to measure temperature, or transport properties such as sound speed or viscosity, which would be accessible through measurement of the dynamic structure factor by inelastic X-ray scattering (IXS). IXS is a photon in - photon out process covering a broad range of physical phenomena relating to the energy transfer range one chooses to measure. To date, the...