Analysis of Thomson scattering data from strongly-driven hydrogen

We present the theory necessary for analysing Thomson scattering data from plasmas in states far from equilibrium. As an example, we compare with experimental data obtained during the interaction of cryogenic hydrogen with the intense VUV radiation of the FLASH free electron laser at DESY in Hamburg. The component due to inelastic scattering from free electrons is treated within a generalised quantum statistical approach which accounts for the nonequilibrium states produced by the strong pump of the FEL radiation. The elastic scattering component is determined by the structure factor of the ions for the initial conditions of a cold, atomic fluid as the ions do not rearrange during the interaction time. Monte Carlo simulations show that this treatment results in a very small elastic scattering feature. Integrating the full nonequilibrium spectrum over time yields excellent agreement with the measured data. Moreover, this treatment gives quantitatively different plasma conditions as may be inferred using an equilibrium analysis.