HARMONIC LASER SEEDING AT FLASH ∗ A. Azima, J. B ¨ odewadt, M. Drescher, H. Delsim-Hashemi, S. Khan, T. Maltezopoulos, V. Miltchev † , M. Mittenzwey, J. Rossbach, R. Tarkeshian, M. Wieland (Uni HH, Hamburg) H. Schlarb, S. D ¨ usterer, J. Feldhaus, T. Laarmann (DESY, Hamburg) Abstract Since 2004, the Free-electron-LASer at Hamburg (FLASH) has operated in the Self-Amplified Spontaneous Emission (SASE) mode, delivering to users photon beams with wavelengths between 6.5 nm and 40 nm. In 2009, DESY plans to install a 3.9 GHz RF acceleration section for the production of electron bunches with high peak currents (∼kA), but ten times larger pulse durations compared to the present configuration. The relaxed timing requirements of the new configuration make it possible to externally seed FLASH with high harmonics of an optical laser (sFLASH). The aim of the project is to study the technical feasibility of seeding a free-electron-laser (FEL) at 30 nm with a sta- bility suited for user operation. sFLASH will use 10 m of gap-tuneable undulators installed upstream of the fixed-gap SASE-undulator. A chicane behind the seeding undulators will allow extracting the output radiation for a careful char- acterisation and for first pump-probe experiments with a resolution of the order of 30 fs by combining the FEL and the optical laser pulse. INTRODUCTION Currently FLASH operates in the SASE regime and pro- duces EUV pulses of sub-10 fs duration [1]. Due to its start- up from noise, the SASE radiation consists of a number of uncorrelated modes resulting in reduced longitudinal co- herence and shot-to-shot fluctuations (about 18 % rms [1]) of the output pulse energy. One possibility to decrease the magnitude of these fluctuations is, with the help of a 3.9 GHz RF cavity [2], to produce much longer (∼200 fs) radiation pulses, so that more modes contribute to the FEL output. However, in this case the increased EUV pulse length might not fit to the needs of ultrafast time resolved experiments. An alternative is to operate FLASH as an am- plifier of an injected seed from a high harmonic generation (HHG) source. This approach gives several benefits com- pared to SASE. It makes possible to achieve higher shot- to-shot stability at GW-power level with a pulse duration given by the seed pulse of the order of 20 fs FWHM. The longitudinal coherence is expected to be greatly improved. The FEL output is synchronized with the external seed laser, thus enabling precise pump-probe experiments to be performed. As sketched in Fig.1, sFLASH will be installed at the end of the linac, upstream of the existing fixed-gap SASE-undulators. With the help of a dedicated optical ∗ Work supported by BMBF contract No. 05 ES7GU1 † velizar.miltchev@desy.de Figure 1: Schematic layout of the seeding experiment (not to scale), BC stands for bunch compressor stage. beamline, the HHG seed will be inserted through the col- limator section, making use of the electron beam offset of about 20 cm. After amplification in the sFLASH variable- gap undulators, the output radiation is separated from the electrons by means of a mirror mounted in a small mag- netic chicane downstream. The photons are then reflected towards the experimental area outside the FLASH tunnel. An experiment recently performed by a French-Japanese collaboration in SPring-8 Compact SASE Source [13] has successfully demonstrated HHG seeding at 160 nm. The goal of sFLASH is to study the technical feasibility of the seeding at shorter wavelengths and how to reliably realize it for user operation. In the following the main components of the experimental setup will be reviewed. ELECTRON BEAMLINE General Requirements The aim for a stable seeded operation in the 30-13 nm range imposes certain requirements for the design of the experimental layout and for the electron beam parameters. It is mandatory to obtain a reproducible six-dimensional, {x, y, x ′ ,y ′ , t, λ}, overlap between the seed and the elec- tron bunch. Therefore, the beamline must include proper diagnostics and instrumentation to maintain the overlap within the desired tolerances, which according to the stud- ies performed with GENESIS [3], are of the order of 30 μm and 20 μrad in the transverse plane. In order to minimize the impact of the timing jitter, the electron bunch length should be of the order of 260 fs rms, even though the state- of-the-art synchronisation system (see below) can restrict the jitter to less than 40 fs rms. Such operation mode can be realized only after the installation of the 3 rd harmonic (3.9 GHz) RF cavity. sFLASH has to run in parallel to and without disturbing the SASE operation. The SASE- undulators are fixed-gap devices and the SASE wavelength, given by the electron energy, is defined by the users. There- fore, for tuning the resonant wavelength of sFLASH one needs variable gap undulators. Moreover, the total undula- tor length has to assure that saturation can be reached at all EXPERIMENTAL LAYOUT OF 30 nm HIGH Proceedings of EPAC08, Genoa, Italy MOPC028 02 Synchrotron Light Sources and FELs A06 Free Electron Lasers 127