Filamentation effect in a gas attenuator for high-repetition-rate X-ray FELs

• Published in: Journal of synchrotron radiation Vol. 23; no. 1; pp. 21 - 28
• Main Authors: Feng, Yiping, Krzywinski, Jacek, Schafer, Donald W., Ortiz, Eliazar, Rowen, Michael, Raubenheimer, Tor O.
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01-01-2016; John Wiley & Sons, Inc
• Subjects: attenuation; Attenuators; Beams (radiation); Density; Density gradients; Depression; diffusion; FEL; Femtosecond; filamentation; gas; Mathematical analysis; photoabsorption; X-ray; X-rays; diffusion; attenuation; filamentation; gas; X-ray; photoabsorption; FEL
• ISSN: 1600-5775; 0909-0495; 1600-5775
• DOI: 10.1107/S1600577515018408

A sustained filamentation or density depression phenomenon in an argon gas attenuator servicing a high‐repetition femtosecond X‐ray free‐electron laser has been studied using a finite‐difference method applied to the thermal diffusion equation for an ideal gas. A steady‐state solution was obtained by assuming continuous‐wave input of an equivalent time‐averaged beam power and that the pressure of the entire gas volume has reached equilibrium. Both radial and axial temperature/density gradients were found and describable as filamentation or density depression previously reported for a femtosecond optical laser of similar attributes. The effect exhibits complex dependence on the input power, the desired attenuation, and the geometries of the beam and the attenuator. Time‐dependent simulations were carried out to further elucidate the evolution of the temperature/density gradients in between pulses, from which the actual attenuation received by any given pulse can be properly calculated.