An adaptive scheme for suppression of higher harmonics in an undulator

An undulator usually emits radiation at the energy of the fundamental wavelength and the higher harmonics at the same time. Likewise, monochromators with gratings or low-indexed crystals also transmit these higher harmonics with substantial efficiency. Most experiments, however, require a clean monochromatic spectrum and use mirrors or a detuning of the monochromator to suppress the higher harmonics in the spectrum. A quasi-periodic undulator is a well- established device which shifts the position of the higher harmonics to non-integer multiples of the fundamental and helps to improve the signal-to-background ratio in the measurement significantly.We have implemented a new alternative concept for suppression of higher harmonics which can be adapted on the fly during operation of the undulator. It is based on creating a distinct phase step in the center of the magnet structure which leads to a tailored distortion of the emitted spectrum. For that purpose, a permanent magnet phase shifter is embedded into the magnet structure of a planar hybrid undulator. We discuss the principle and technical implementation of this scheme, and present first experimental results