In the high gain FEL model the changes of the electromagnetic field are strong in a single passage through the undulator. The radiation intensity grows exponentially with a characteristic e-folding length. To start the FEL process, there are two general options. One may start from the spontaneous undulator radiation generated in the first section of the undulator which is amplified by the FEL mechanism in the subsequent undulator sections (the so-called SASE-FEL) or could "seed" the FEL using photons generated by an external laser of the same wavelength as the FEL wavelength. In the latter the FEL undulator will amplify the input signal such that the output laser quality is characterized by the seed.

sFLASH layout

sFLASH is an experiment to introduce a seed laser to FLASH electron-beam in a distance of about 40-meters upstream the SASE-FEL undulators. The seed laser namely light pulses of ~38 nm wavelength, is one of high harmonics which are generated by focusing a near-infrared laser in a noble-gas jet (HHG).
The efficient transport of such a short wavelength laser and the spatial overlap with the electron beam are among the challenging tasks. The interaction of the seed laser and the electron beam takes place in the undulators region. Four hybrid variable-gap undulators are foreseen with the total undulator length of 10 meters. In the space between undulators there are diagnostics devices for both the electron beam and the seed laser. After undulators there is a weak magnetic chicane as a separator of the electron beam and the FEL. A VUV beam-line transports the FEL to the experiments.