Single-shot beam profile diagnostics for X-ray FEL's using gas fluorescence

We report experimental demonstration of capturing single-shot X-ray Free-electron Laser (FEL) beam profiles using gas fluorescence. The measurement was carried out at the Linac Coherent Light Source using a hard X-ray beam propagating through ambient air. The nitrogen fluorescence trail emitted upon the passage of the X-ray FEL beam was imaged using a highly sensitive optical setup, and there was sufficient optical yield that single-shot measurements were feasible. The fluorescence image can be used for determining the beam position in a non-invasive manner, and is best suited for photon energies in the soft X-ray regime. The integrated intensity of the images could also serve as a non-invasive intensity monitor, complementary to current implementations of gas- and solid-based monitors. High repetition-rate Free-electron Lasers could greatly benefit from such a new diagnostic tool for mitigating potential thermal damages.