Uni HH, Hamburg
PSI, Villigen
DESY, Hamburg
FLASH (The Free-Electron Laser At Hamburg) is a High-Gain SASE-FEL providing ultrashort pulses with a central wavelength of 6 to 40 nm. Measuring and controling the longitudinal shape of the electron bunches can dramatically improve the stability of the lasing process. Non-destructive electro-optical bunch profile diagnostics have proved to work with resolutions down to 100 fs. The electro-optical (EO) setup at FLASH relies on a standard Ti:sapphire laser delivering 80 fs pulses with 4 nJ pulse energy. For practical and physical reasons (i.e., space, costs, maintenance, performance) a new, ytterbium fiber laser system has been developed. This laser system supports pulse energies of 4.5 nJ and a bandwidth of 100 nm at a center wavelength of 1030 nm. Active repetition rate control allows to lock the laser to the RF based synchronisation system. A better EO signal-to-noise ratio is expected due to the improved group velocity matching in the EO crystal. First results from the prototype Yb laser system and comparison with the Ti:Sa based data will be presented. Furthermore, a structurally engineered version, promising enhanced stability and reliability will be introduced.
DESY, Hamburg
Uni HH, Hamburg
At FLASH an optical synchronisation system with femtosecond stability is now being installed and commissioned. The pulses from an erbium-doped fibre laser being distributed in length-stabilised fibres to various endstations are used to detect the electron bunch arrival time using electro-optical modulators. To determine variations of the arrival time caused by phase changes of the RF gun or by timing changes of the photo-injector laser a beam arrival time monitor has been installed after the first acceleration section, prior to the bunch compressor BC2. A second bunch arrival time monitor installed after the bunch compressor allows for measuring the beam energy with high precision through a time-of-flight detection. Both monitors provide futher insight into the accelerator subsystem stability and opens up the opportunity for a robust fast feedback stabilisation.
DESY, Hamburg
The master laser oscillator (MLO) at FLASH is locked to the master RF oscillator (MO) by mixing a 1.3 GHz signal from an MLO-based photodetector and a 1.3 GHz signal from the MO. The baseband output of the mixer is sent to an ADC-DSP-DAC regulation system that feeds back on a piezo controlled mirror position in the laser. The rms jitter and long term drift stability of the RF-lock circuit alone can be less than 5 fs in the temperature controlled chassis, but it can jump 10 to 15 fs when the temperature regulation of the room is disturbed by people working inside. Out-of-loop and in-loop measurements were also conducted under various environmental conditions.
DESY, Hamburg
A unique, perpendicularly-mounted stripline BPM pickup is installed in the dispersive sections of the FLASH bunch compressors. For 4-5 um resolution, it requires a front-end that can measure the difference between the phases of the beam transient pulses with a resolution that is better than 10-15 fs. Two front-ends have been tested with the pickup: a 10.4 GHz down-mixing scheme and an electro-optical modulator (EOM) based scheme that uses the optical synchronization system. The EOM scheme typically produces 6 to 12 fs resolution. It is, however, expensive, complex, and dependent on an optical infrastructure that is still in a development phase. It was not anticipated that an RF-mixing scheme could deliver the required, sub-15 fs resolution and drift stability, but with a temperature stabilized chassis in a climatized room and sufficiently high frequencies, an RF mixing scheme can deliver resolution that is comparable to that of the EOM scheme for this particular application, the measurement of the relative arrival-times of two ~ps pulses. A direct comparison of beam arrival time measurements with 10.4 GHz down-mixing and EOM sampling is also presented.
Uni HH, Hamburg
DESY, Hamburg
Free-electron lasers like FLASH and the planned European XFEL generate X-ray light pulses with durations in the order of a few ten femtoseconds. For these next-generation light sources, an optical synchronization system has been proposed to enable time-resolved measurements with sub-10 fs resolution and the laser-driven seeded operation mode of the FEL. The system is based on the timing-stabilized distribution of an optical pulse train, from which RF signals can be generated or to which other laser systems can be synchronized. Furthermore, it facilitates several special diagnostic measurements on the sub-10 fs time-scale.
The optical synchronization system at FLASH has recently progressed from a bread-board/test-bench implementation to a more permanent engineered infrastructure. We report on the master laser oscillator, the lock to the master RF oscillator, the free-space distribution unit, four installed fiber links, three bunch arrival-time monitors, one optical cross-correlator and the controls development. We also identified a couple of design issues during the commissioning of the devices.
DESY, Hamburg
Uni HH, Hamburg
At the Free-Electron Laser in Hamburg (FLASH), an optical synchronization system is being installed with a projected point-to-point stability of 10 fs. The system is based on the distribution of reference laser pulses over actively stabilized fiber links using optical cross-correlators. As an alternative to the complex cross-correlation scheme, which can achieve sub fs long-term stability and works well over several 100 m long fiber links, an RF-based technique which is much less complex and expensive could be used. It is based on the power detection of high harmonic frequencies in a balanced arrangement to reduce amplitude noise. For a 20 m long fiber link, it was demonstrated that a sub-5 fs rms long-term stability over 30 hours can be achieved. The system and the most recent measurements are presented here.
FYSIKUM, AlbaNova, Stockholm University, Stockholm
Uppsala University, Uppsala
Uni HH, Hamburg
DELTA, Dortmund
DESY, Hamburg
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin
We present results from the new electron bunch diagnostic tool, Optical Replica Synthesizer [1] (ORS), installed at FLASH. The ORS produces an optical replica of the electron bunch profile, which is analyzed with a Grenouille, a device based on the Frequency Resolved Optical Gating (FROG) technique. This optical replica is generated by inducing a microbunching in the electron bunch and letting it pass through an undulator, called a radiator. The radiator emits coherently at the wavelength of microbunching, 772 nm. In order to create the microbunching a laser pulse is spatially and temporally overlapped with the electron bunch in another undulator, placed before the radiator. This introduces an electron energy modulation which is transformed into a density modulation in a chicane before the microbunched electron bunch is sent into the radiator. We observed an optical replica pulse of approximately 5 microJ corresponding to an electron bunch-spike of about 150 fs FWHM when the accelerators were set at optimal FEL conditions. We also showed that the ORS can run parasitically while maintaining SASE by steering the electron beam around the outcoupling mirror for the radiation.
[1] E. Saldin, E. Schneidmiller, M. Yurkov, “A simple method for the determination of the structure of ultrashort relativistic electron bunches,” Nucl. Inst. and Methods A 539 (2005) 499.