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.
