CHARACTERIZATION OF SEEDED FEL PULSES AT FLASH: STATUS, CHALLENGES AND OPPORTUNITIES

Since 2004 the free-electron laser FLASH at DESY has been operated in the Self-Amplified Stimulated Emission mode (SASE), delivering gigawatt pulses with wavelengths between 6.5 nm and 40 nm in the femtosecond domain. In 2009 DESY installed an additional radiofrequency module for controlling the phase space of the electron bunches that gives the possibility to generate bunches with high peak currents but ten times larger pulse durations (�200 fs). The relaxed timing requirements of the new configuration allows to externally seed FLASH with high-order harmonics of an optical laser below 40 nm generated in a gas target (sFLASH). Because in this case the amplification is triggered within the seed pulse length instead of starting from the shot-noise (as in the SASE process), spikes in temporal/spectral pulse profiles should be absent and the tempora l jitter should be eliminated. In this contribution we presen t the current status of the sFLASH photon diagnostics including first commissioning results.