DRIFT CALIBRATION TECHNIQUES FOR FUTURE FELS

Future FELs (Free-Electron-Lasers) requires a precise detection of the cavity field in the injector section with a resolution of much less than 0.01 in phase and 0.01% in amplitude for a cavity operation frequency at 1.3GHz. Long-term stable SASE (Self Amplified Spontaneous Emission) at FLASH (Free-electron LASer in Hamburg) operation mainly suffers from injector accelerator components and the stability of the reference distribution. Especially thermal instabilities of the distributed cavity field detectors, probe pickup cables and their mechanical vibrations influence the energy stability dramatically on a scale of 0.1%, a scale which is 10 times worse than required. To eliminate the long-term amplitude and phase changes of the field detectors of the order of 0.2%/C and 0.2/C, we made an out-of-loop measurement of the field detector performance by injecting a reference signal prior to the arrival of the cavity field signal. This enabled pulse-to-pulse calibration which compensated for the drifts of the field detectors. By applying the injected calibration method, we demonstrated a dramatic phase and amplitude stability improvement from the ps-range to the 0.008 (peak-to-peak) range in phase and 0.02% (peak-to-peak) in amplitude; this represents an improvement in drifts by a factor of about 100. The injected calibration was successfully employed during FLASH operation.