Undulator assembly and quality assurance

To assure the constructive interference the gap must be the same within 20 µm all along the undulator. In order to allow the use of the 5th harmonic the reproducibility of gap and shift setting must be in the order of 1 µm. The gap and shift motion have to be realized in the presence of strong and variable magnetic forces reaching tons per meter undulator length.
We achieve these specifications using an as stiff as possible support built around a cast iron structure designed by a bionic optimization procedure. The magnets are held in precision mounts bolted to the aluminum beams. All parts are designed in-house and fabrication drawings are used to contract the production to industry. The actual gaps and shifts are measured using optical encoders. High dynamical servo motors provide the motive force.

All undulators are assembled and commissioned at BESSY. After assembly of an APPLE type undulator the typical remaining phase jitter is less than 4° for all operation modes i.e. the resonance condition is met from pole to pole within less than 1%. Furthermore, there is close agreement between the trajectories from magnetic measurements of the assembled undulator and those from previous magnetic measurements of the individual blocks. This unique result is caused by the high precision techniques developed at BESSY which are applied throughout the manufacturing process.

At such a small phase jitter the brilliance is limited either by diffraction effects leading to transversely coherent beams, or by the finite emittance and energy spread of the electron beam. Thus there is no need to further improve the magnetic field quality of BESSY undulators in this respect. Shimming is done only for the purpose of minimizing higher order multipoles.