Plasma switch as a temporal overlap tool for pump-probe experiments at FEL facilities

We have developed an easy-to-use and reliable timing tool to determine the arrival time of an optical laser and a free electron laser (FEL) pulses within the jitter limitation. This timing tool can be used from XUV to X-rays and exploits high FELs intensities. It uses a shadowgraph technique where we optically (at 800 nm) image a plasma created by an intense XUV or X-ray FEL pulse on a transparent sample (glass slide) directly placed at the pump - probe sample position. It is based on the physical principle that the optical properties of the material are drastically changed when its free electron density reaches the critical density. At this point the excited glass sample becomes opaque to the optical laser pulse. The ultra-short and intense XUV or X-ray FEL pulse ensures that a critical electron density can be reached via photoionization and subsequent collisional ionization within the XUV or X-ray FEL pulse duration or even faster. This technique allows to determine the relative arrival time between the optical laser and the FEL pulses in only few single shots with an accuracy mainly limited by the optical laser pulse duration and the jitter between the FEL and the optical laser. Considering the major interest in pump-probe experiments at FEL facilities in general, such a femtosecond resolution timing tool is of utmost importance.