The higher harmonics of conventional undulators emit at multiples of the fundamental. For low photon energies they are transmitted through the monochromator and contaminate the experimental data. In a quasiperiodic magnetic structure the position of the harmonics is slightly shifted and the higher harmonics are efficiently blocked by the monochromator.
The quasiperiodic design has been invented by S. Sasaki and several pure permanent magnet structures based on this idea have been built world wide. The HZB-U125-2 is the first hybrid type undulator which has a quasiperiodic structure.