High gas-pressure apparatus for nonlinear X-ray propagation and reshaping via stimulated X-ray Raman scattering.

We describe an experimental setup for non-linear interaction and propagation of ultrashort and intense x-ray free-electron laser (XFEL) pulses in a dense gas. It allows one to provide high, but adjustable, target-gas pressures of up to 6 bar within a vacuum environment of 3∙10- 3 mbar or better. The setup enables investigation of intense x-ray propagation in an optically thick medium with minimal absorption loss of the unfocused beam outside of the target. As an application, we demonstrate the amplification of spectrally-resolved stimulated x-ray Raman scattering (SXRS) in dense neon gas, where the most intense inner part of the beam is almost completely absorbed. As a result, in the inner part of the beam, the SXRS signal exceeds the residual XFEL pulse by a factor of around two. In principle, this reshaping effect allows for a spatial separation of the two spectral components, i.e., the driving pulse and the SXRS signal.