Towards the direct measurement of bulk temperature in shock-compressed matter using inelastic X-ray scattering at X-ray Free Electron Lasers*

Direct and accurate measurements of thermodynamic and transport properties are essential for understanding the behavior of matter at extreme pressures and temperatures. While X-ray diffraction measurements have allowed in situ measurement of structure and density, the direct measurement of bulk temperature remains a challenge. In shock compression experiments, it is often estimated from hydrodynamic simulations or inferred using streaked optical pyrometry, which requires a priori knowledge of the material properties at extreme conditions. On the time scale of nanosecond shock compression, for temperatures less than 4000 K, the intensity recorded in SOP experiments decreases and the accuracy of the technique degrades. This limitation is particularly hindering for the investigation of high-pressure, moderate temperature states of matter such as the one generated using double shock or quasi-isentropic compression. Furthermore, due to the small penetration depth of optical photons in solid density materials, this technique only gives access to the surface temperature, leaving the bulk temperature unknown.

Here, I will describe experiments conducted at the High Energy Density instrument at the European XFEL and the Matter in Extreme Conditions at the LCLS using high-resolution inelastic X-ray scattering with milli-electronvolt to measure temperature. With the installation of high repetition rate drive lasers at hard X-ray free electron lasers, this technique has the potential to become a powerful to investigate temperature as well as transport of shock compressed matter.