Diagnostic system for resonator length at the JAERI FEL

An optical resonator length of free electron lasers must be exactly matched to the repetition rate of the electron beam from the accelerator driver to obtain the overlap between the incident electron beam and stored light pulses in the resonator. We made the exact resonator length matching using a recently-developed external laser method. The external laser was a mode-locked Ti:Sapphire laser with 80 fs pulse duration and synchronized with the accelerator clock signals. With use of a streak camera and a photo diode simultaneously, we succeeded to make the matching within the accuracy of 0.5 μm or less, quickly and in more direct manner. (author)