  We report the experimental observation of the filamentation effect in gas
devices designed for X-ray Free-electron Lasers. The measurements were carried
out at the Linac Coherent Light Source on the X-ray Correlation Spectroscopy
(XCS) instrument using a Two-bunch FEL beam at 6.5 keV with 122.5 ns separation
passing through an Argon gas cell. The relative intensities of the two pulses
of the Two-bunch beam were measured, after and before the gas cell, from the
X-ray scattering off thin targets by using fast diodes with sufficient temporal
resolution. It was found that the after-to-before ratio of the intensities of
the second pulse was consistently higher than that of the first pulse,
revealing lower effective attenuation of the gas cell due to the heating and
subsequent gas density reduction in the beam path by the first pulse. This
measurement is important in guiding the design and/or mitigating the adverse
effect in gas devices for high repetition-rate FELs such as the LCLS-II and the
European XFEL or other future high repetition-rate upgrade to existing FEL
facilities
