DESY, Hamburg
TUL-DMCS, Łódź
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 deg in phase and 0.01% in amplitude for a cavity operation frequency at 1.3GHz. Long-term stable SASE (Self Amplified Spontaneous Emission) 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, we injected 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. We demonstrated a dramatic phase and amplitude stability improvement from the ps-range to the 0.008 deg (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.
DESY, Hamburg
CLASSE, Ithaca, New York
Uni HH, Hamburg
At FLASH an optical synchronisation system with femtosecond stability is now being installed and commissioned. The system is based on pulses from a passively modelocked fibre laser which are distributed in length-stabilised fibres to various end-stations. Several modifications and improvements with respect to the original layout, especially concerning permanent operation and reliability, are already incorporated at this stage. The electron bunch arrival-time monitors (BAM), based on electro-optical modulation, are an integral part of the system. Built on the experiences with first prototypes, the most recent version of the BAM, installed prior to the first bunch compressor, includes essential changes affecting the optical layout, the mechanical and thermal stability as well as the electronics for read-out and controls. The revised BAM showed improved performance and will be complemented by a second congenerous BAM after the first bunch compressor during the present FLASH upgrade. The experiences with installation as well as the scope of improvements as to simplification and long-term stability will be presented.
Uni HH, Hamburg
PSI, Villigen
DESY, Hamburg
After its upgrade, the free-electron laser in Hamburg (FLASH) will start operating with an exchanged RF-gun driven by an improved photoinjector laser. Since the SASE FEL process is very sensitive to the RF gun phase it is highly desirable to implement phase stabilization feedback, which, in turn, requires an arrival-time stabilization of the photoinjector laser pulses. In this paper we report on the synchronization of the photoinjector laser system to the optical timing reference using an optical cross-correlation scheme. This enables not only the measurement of the timing jitter, but also the stabilization using adaptive feed-forward algorithms acting on an EOM incorporated in the laser's pulse train oscillator. First results from the commissioning and future plans for a feedback system are discussed.