The extension of the FLASH facility at DESY (Hamburg, Germany) – FLASH II Project – is under way. The extension includes a second undulator line with variable gap undulators to allow a more flexible operation, and a new experimental hall for photon experiments. The present FLASH linac will drive the both undulator beamlines. Civil construction of the new buildings has been started in autumn 2011 continuing in several steps until spring 2013. The design of the new electron beamline including the extraction from the FLASH linac and the undulator section is mostly finished, and the manufacturing of the components is under way. Design of the photon beamline and layout of the experimental hall is in an advanced stage. The beamline mounting starts end of 2012, and the commissioning with beam is scheduled for the second half of 2013.

FLASH has been a user facility since 2005, delivering radiation in the wavelength range between 4.1 and 45 nm using the SASE principle. So far the user requests for beam time by far exceeds the time available. In order to increase user beam time and improve the radiation properties delivered to users, a mayor extension of the user facility called FLASH II has been proposed by DESY in collaboration with the HZB, which is a seeded FEL over the parameter range of FLASH. After several years of design, the project now enters its construction phase, which will last approximately 2 years. In the mean time, tests are performed in order to prepare for a multi-undulator operation of the facility. In addition, complete start-to-end simulations will complete the simulations which have been performed so far.

FLASH has been a user facility since 2005, delivering radiation in the wavelength range between 7 and 47 nm using the SASE principle. After the present upgrade, the wavelength range is extended to 4.45 nm. With the third harmonic accelerating module in place to linearize the longitudinal phase space, the stability and reproducibility of the machine is substantially improved. The user requests for beam time by far exceeds the time available. In order to increase user beam time and to improve the radiation properties delivered to users, a major extension of the user facility called FLASH II has been proposed by DESY in collaboration with the HZB. FLASH II is a seeded FEL in the parameter range of FLASH. As logical continuation, the seeding with HHG which started with sFLASH will result in direct seeding. Because in the foreseeable future there will probably not be HHG seed lasers available at high repetition rates down to wavelengths of 4 nm, a cascaded HGHG scheme is proposed to produce short wavelengths.After a first design report, the project now enters its technical design phase. During this time, the FLASH beam parameters after the present upgrade 2009/2010 will be characterized and the present design will be re-evaluated and adjusted. In addition, start-to-end simulations will complete the simulations which have been performed so far, including a design of the extraction area. (C) 2010 Elsevier B.V. All rights reserved.

High-gain free-electron lasers (FELs) are capable of generating femtosecond x-ray pulses with peak brilliances many orders of magnitude higher than at other existing x-ray sources. In order to fully exploit the opportunities offered by these femtosecond light pulses in time-resolved experiments, an unprecedented synchronization accuracy is required. In this Letter, we distributed the pulse train of a mode-locked fiber laser with femtosecond stability to different locations in the linear accelerator of the soft x-ray FEL FLASH. A novel electro-optic detection scheme was applied to measure the electron bunch arrival time with an as yet unrivaled precision of 6 fs (rms). With two beam-based feedback systems we succeeded in stabilizing both the arrival time and the electron bunch compression process within two magnetic chicanes, yielding a significant reduction of the FEL pulse energy jitter.

The Free Electron Laser FLASH has been upgraded during winter 2009/10. Amongst other components, a third harmonic module operating at 3.9 GHz (ACC39) has been installed. Together with the energy chirp induced by off-crest operation, it allows for a linearization of the longitudinal phase space, leading to a uniform compression of the electron bunch with final bunch lengths of 150 µm rms. In contrast to the old non-linear compression scheme, peak current and bunch length are extremely sensitive to the phases of ACC39 and ACC1 and have to be monitored continuously. The foreseen bunch length is within the resolution of electro-optic spectral decoding methods. An ytterbium fibre laser system in combination with a 175 µm thick GaP crystal is used to achieve a good match between the electric field phase velocity and the laser pulse group velocity in the electro-optic crystal. This ensures a large modulation of the polarisation of the chirped laser pulse in the EO crystal. The information on the electron bunch length carried by the laser pulse is decoded in a spectrometer and read out with an InGaAs line scan camera.

At FLASH, UV and soft X-ray pulses with durations in the order of 10 fs are generated. To fully exploit the opportunities provided by these short laser pulses, an optical synchronization system to synchronize external lasers and stabilize the electron bunch arrival time is being constructed. A seeded free-electron laser (FEL) section, called sFLASH, is installed upstream of the existing SASE undulators. After higher-harmonic generation, the femtosecond seed laser pulse needs to be temporally and spatially overlapped with the electron bunch. Furthermore, for time-resolved pump-probe experiments, using an experimental laser and the FEL pulse, the synchronization between pump and probe laser pulses is crucial. While the best performance for synchronizing these lasers within 10 fs will be achieved by using an optical cross-correlator, in this paper we present a precursor that relies on an RF-based locking mechanism. The setup includes a coarse and a fine phase measurement between the laser pulses of the reference and the synchronized system after their conversion to an RF signal.

Next generation FEL light sources like the European XFEL require timing stability of 10-20 fs between different subsystems. In optical synchronization systems, the timing information is distributed across the facilities via sub-ps laser pulses travelling on length stabilized optical fibers. Several methods are available for RF extraction from the pulse train. In this paper, we characterize the long-term phase stability of a 1.3 GHz signal produced from the direct conversion of a higher harmonic of the pulse repetition frequency and from a voltage controlled oscillator locked with a PLL that uses a Sagnac-Loop as balanced optical- microwave phase detector.

FLASH (The Free-Electron Laser in Hamburg) is a High-Gain SASE-FEL (Self Amplified Spontaneous Emission) providing ultrashort pulses with a central wavelength of 6.5 to 40 nm. Measuring and controlling the longitudinal shape of the electron bunches can help considerably to understand and to improve the stability of the lasing process. Non-destructive electro-optical (EO) bunch profile diagnostics have proved to work with resolutions down to 100 fs (rms). Bunch arrival time diagnostic works with comparable precision. The electro- optical setup at FLASH relies presently on a Ti:sapphire laser delivering 80 fs pulses with 4 nJ pulse energy. For practical and physical reasons (i.e., space, cost, ability of permanent operation) an alternative, ytterbium fiber laser system has been developed. This laser system is designed to produce pulse energies of 4.5 nJ at a centre wavelength of 1030 nm with a spectral bandwidth of up to 80 nm. Active repetition rate control allows to lock the laser to the accelerator RF or later to an optical synchronisation system (1). First results from the prototype Yb laser system will be presented.