Air attenuation of high power XFEL beams

The Linac Coherent Light Source II, a high-repetition-rate X-ray free-electron laser (FEL), produces high average power as well as high peak power. Air is a key radiation safety element, helping to contain the FEL beams from reaching accessible areas. At high average power, air absorption can produce a high-temperature and low-density channel along the X-ray beam path. In this case, the X-ray attenuation no longer follows the Beer-Lambert equation, exp(-μx). Air attenuation measurements were performed with X-rays focused into a gas cell at the LCLS time-resolved AMO instrument. In the measurements, several parameters were varied: the repetition rate from 102 Hz to 33.2 kHz, the pulse energy from 18 to 340 µJ and the photon energy at 316 and 346 eV. The air transmission was observed to significantly increase with X-ray power. Simulations were also performed, which included the thermal processes related to the high average power. In comparison with the measurements, the calculated air transmission is higher, confirming that the simulations are conservative and suitable for radiation safety analyses.