Developing direct measurements of temperature and transport properties at hard X-ray FELs

For extreme states of matter produced in the laboratory, direct and accurate measurements of thermodynamic and transport properties are vital to guide the development of theoretical models. One technique to create such systems is to couple dynamic laser-driven compression and laser heating techniques to access pressure and temperature similar to those found inside the core of planets. While it is possible to directly investigate the density and pressure of a material under dynamic compression, measuring the temperature of the bulk temperature remains a challenge. Common techniques such as streaked optical pyrometry, rely on a priori knowledge of the matter under investigation and give accurate measurements above 4,000 K. In contrast, inelastic X-ray scattering at an X-ray Free Electron Laser offers a unique capability to measure the bulk temperature from low energy collective oscillation of the electron density and the principle of detailed balance. Here, I will discuss the method and its validation from measurements taken at the High Energy Density end station at the European XFEL on resistively heated single crystal Diamond. Finally, I will present the application of this method to measure the ion temperature of shock compressed argon at the Matter in Extreme Condition at the Linac Coherent Light Source.