Photo of a FLASH undulator magnet. The gap height is 12 mm, the period is λu = 27 mm and the peak magnetic field is B0 = 0.47 T.
The undulator is a long periodic arrangement of short dipole magnets with alternating polarity. The undulators at FLASH are made from iron pole shoes with NndFeB permanent magnets
in between. To achieve FEL saturation in a single pass the undulator must be more than 20 m long for wavelengths in the 10-nanometer regime. The FLASH undulator system consists of six magnets of 4.5 m length each.
In the 60-cm space between the segments, quadrupole lenses are installed for beam focusing alongside beam diagnostics tools. An excellent field quality has been achieved in the undulator, the deviation of the electrons from the ideal orbit is less than 10 μm. This ensures a good overlap between the electron beam and the light wave, which is a prerequisite for achieving a high gain in the lasing process.
