TECHNICAL DESIGN OF THE XUV SEEDING EXPERIMENT AT FLASH ∗ V. Miltchev † , A. Azima, J. B ¨ odewadt, F. Curbis, M. Drescher, H. Delsim-Hashemi, T. Maltezopoulos, M. Mittenzwey, J. Rossbach, R. Tarkeshian, M. Wieland, Uni HH, Hamburg S. D¨ usterer, J. Feldhaus, T. Laarmann, H. Schlarb, DESY, Hamburg A. Meseck, Helmholz-Zentrum Berlin, Germany S. Khan, DELTA, Dortmund, Germany R. Ischebeck, PSI, Villigen, Switzerland Abstract The Free-electron-laser at Hamburg (FLASH) oper- ates in the Self-Amplified Spontaneous Emission (SASE) mode, delivering to users photons in the XUV wavelength range. The FEL seeding schemes promise to improve the properties of the generated radiation in terms of stability in intensity and time. Such an experiment using higher harmonics of an optical laser as a seed is currently un- der construction at FLASH. The installation of the XUV seeding experiment (sFLASH) is going to take place in fall 2009. This includes mounting of new variable-gap undu- lators upstream of the existing SASE-undulators, building the XUV-seed source as well as installation of additional photon diagnostics and electron beam instrumentation. In this contribution the layout of sFLASH will be discussed together with the technical design of its major components. 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 pulse energy shot-to-shot fluctu- ations 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 alterna- tive is to operate FLASH as an amplifier of an injected seed from a high harmonic generation (HHG) source. This ap- proach gives several benefits compared to SASE. It enables to achieve higher shot-to-shot stability at a 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 pump-probe experiment with increased temporal resolution. As sketched in Fig.1, sFLASH will be installed at the end of the linac, upstream of the existing fixed-gap SASE- ∗ Supported by the Federal Ministry of Education and Research of Ger- many under contract 05 ES7GU1 † velizar.miltchev@desy.de undulators. With the help of a dedicated optical beamline, the HHG seed will be inserted through the collimator sec- tion, making use of the electron beam offset of about 20 cm. After amplification in the sFLASH variable-gap undula- tors, the output radiation is separated from the electrons by means of a mirror mounted in a small magnetic chicane downstream. The photons are then reflected towards the experimental area outside the FLASH tunnel. An experi- ment recently performed by a French-Japanese collabora- tion at the SPring-8 Compact SASE Source [3] has suc- cessfully demonstrated HHG seeding at 160 nm. The goal of sFLASH is to study the technical feasibility of the seed- ing at shorter wavelengths and how to reliably realize it for user operation. ELECTRON BEAMLINE General Requirements The aim for a stable seeded operation in the 30-13nm 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 [4, 5], are of the order of 30 μm and 20 μrad in both transverse planes. 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 re- strict the jitter to less than 40 fs rms. Such operation mode can be realized only after the installation of the 3 rd har- monic (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 seeding wavelengths. Finally, since the HHG seeding will share the same beamline with other setups (e.g. ORS [6] and LOLA[7]), one has to provide compatibility between the different experiments. Proceedings of FEL2009, Liverpool, UK WEPC05 FEL Technology II: Post-accelerator 503