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The self-seeding mechanism: The SASE process is started in the first undulator but not driven into saturation. Subsequently the electron beam is deviated over a bypass where the microbunching is removed, and led to the entrance of the final undulator. The soft X-ray beam produced in the first undulator is transported through a high-resolution monochromator selecting a fully coherent narrow-band but stretched radiation pulse that meets the electron pulse at the entrance of the second undulator and seeds it, i.e. initiates the microbunching process in a controlled way. This way the coherent, narrow-band radiation is amplified to saturation, increasing the peak brilliance by approximately two orders of magnitude at the expense of the pulse duration which is then about 200 femtoseconds.
Improved FEL Radiation Pulses by Self-Seeding
Strong efforts are made worldwide to apply seeding schemes in order to improve the quality of the FEL radiation pulses – for the pioneering work at FLASH, LCLS and the European XFEL the principle of Self-Amplified Spontaneous Emission (SASE) is used, where the FEL process starts from noise. At FLASH a self-seeding scheme will be implemented which is expected to provide a fully coherent beam with flashes of 200 fs duration and to increase the peak brilliance by two orders of magnitude. This mode of operation is very attractive for experiments that do not need the very short pulses, like high-energy density studies or high-resolution spectroscopy, such as Raman spectroscopy on correlated electron systems or samples of biological interest. There is also discussion of the possibilities to create femtosecond or even sub-femtosecond pulses in the wavelength range around 30 nm at FLASH by seeding with an intense external laser pulse produced by High Harmonic Generation (HHG). By splitting the optical seed pulse excellent synchronization will be possible and exciting investigations of many-body dynamics in strong laser fields, as an example, could be performed. The unique selling point of FLASH is in the combination of the extreme peak brilliance typical for all X-ray FELs with a very high average brilliance, which will be between three and four orders of magnitude higher than the best storage ring synchrotron radiation facilities once FLASH reaches its specifications. FLASH is thus especially attractive for experiments with highly diluted targets like spectroscopy of highly charged ions needing high average brilliance to reach the necessary statistical accuracy, especially if a reaction microscope is used as a detector which fully determines the scattering kinematics, or for imaging of single particles flying through the FEL beam, where high luminosity is the key criterion for success. In general, an increasing effort will be needed to prepare specific samples for FEL experiments as well as to develop novel detectors for best use of the FEL radiation.
