Direct Measurement of bulk temperature 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 extreme states of matter. While X-ray diffraction measurements at large laser facilities or Free Electron Lasers, such as the LCLS, have allowed \textit{in situ} measurement of structure and density, however, the direct measurement of bulk temperature remains a challenge. Here, we present the development of a platform using inelastic X-ray scattering in a Johann geometry to measure temperature by the use of the principle of detailed balance. A proof-of-principle experiment was conducted at the HED beamline at the European XFEL on resistively heated single crystal diamond at 500 K. This technique was then combined with a cryogenic jet of argon compressed with a short pulse laser at the MEC endstation at LCLS, allowing the direct measurement of the temperature of laser compressed matter.