Microbunch Amplification in the European XFEL

Linac-based X-ray-free-electron lasers (XFELs) require very short bunches of highbrightness electron beams with peak currents of the order of kilo-Amperes. These bunches cannot be produced directly in guns because space charge forces would destroy the brilliance within a short distance [1]. Increasing bunch peak currents with longitudinal compression at beam energies high enough to suppress space charge forces sufficiently to avoid emittance increase cannot be achieved alone by inducing velocity differences along the bunch. Instead or in addition, dispersive sections like magnet chicanes where particle path length depends on energy have to be employed. An energy chirp induced to the bunch with an rf system upstream of the chicane will then result in longitudinal bunch compression and peak current increase. Bunch compression is usually done in stages. On one hand the energy has to be high enough to avoid too strong self effects, not only by space charge forces but also by coherent radiation and compression work. A too strong compression in the first stage leads to tight tolerances, and a too high energy level of that stage increases the effort for a higher harmonic system needed to linearize and compensate rf curvature and non linear dispersion. Too high energy in further stages is limited by magnet strengths, magnet dimensions, chicane dimensions, remaining energy chirp and finally by effects due to incoherent radiation. In total, the bunch compression system consists of a sequence of straight acceleration sections, where initially strong space charge forces are reduced by the energy gain, and chicanes which increase the peak current, which restores the space charge forces roughly to their initial strength. Then the bunch enters the next stage and the process is repeated. It was shown that this staged bunch compression with its interplay of straight sections where space charge forces are present and magnet chicanes where path length depends on electron energy can very effective in amplifying small initial intensity modulations of the bunch [2]. The gain of that amplification at certain wavelengths can be