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 pioneering studies that have used IXS from laser-compressed matter have focused only on the electron subsystem, measuring in the eV to keV energy transfer range. The properties of the ionic subsystem remain unmeasured.

In this research proposal I will develop techniques using inelastic X-ray scattering to directly measure the dynamic structure factor with the milli-electronvolt energy transfer range. From these measurements I will determine temperature from matter at extreme conditions. In addition I will measure properties such as viscosity and thermal diffusivity, essential for creating accurate models of planetary formation and evolution.

References
[1] Fletcher et al., Nat. Phot., 9 274 (2015)
[2] Millot et al., Science, 347 418 (2015)
[3] Cho et al., PRL, 106 167601 (2011)
[4] Engelhorn et al., PRB 91, 214305 (2015)
[5] Ping et al., HEDP, 4 246 (2010)
[6] Gauthier et al., PRL, 110 125003 (2013)
[7] Patel et al., PRL., 91 125004 (2003)
[8] Mancic et al., PRL., 104 035002 (2010)
[9] UK FEL Science Case (2020) (https://stfc.ukri.org/news/uk-xfel-draft-science-case-consultation/)
[10] Kraus et al., Nat. Ast., 1 606 (2017)
[11] Gorman et al., PRL. 115 095701 (2015)
[12] McBride et al., Nat. Phys. 15 89 (2019)