Two-Color Operation of FLASH2 Undulator

Abstract

FLASH is the first soft X-ray FEL user facility, routinely providing brilliant photon beams for users since 2005. The second undulator branch of this facility, FLASH2, is gaptunable which allows to test and use advanced lasing concepts.In particular, we tested recently a two-color mode of operation based on the alternation of tunes of the undulator segments (every other segment is tuned to the second wavelength).This scheme is advantageous in comparison with a subsequent generation of two colors in two consecutive sections of the undulator line. First, source positions of the two FEL beams are close to each other which makes it easier to handle them. Second, the amplification is more efficientin this configuration since the segments with respectively"wrong" wavelength still act as bunchers. We developed methods for online intensity measurements of the two colors simultaneously that require a combination of two detectors.We present some examples of such measurements in the XUV and soft X-ray regimes