Figure 1.
Schematic of the experimental setup, consisting of a gas cell of diameter 2R and effective length L, filled with argon gas to a pressure P. The cell was capped off by two 50 µm-thick Kapton windows W 1 and W 2 each inclined by 45° to the direction of an X-ray two-bunch FEL beam coming from the left and being attenuated. The X-ray scattering off the Kapton windows was measured as the intensities of p 1 and p 2 by two fast diodes D 1 and D 2 positioned perpendicular to the beam and each connected to a different channel of a fast digitizer.