  In this paper, the harmonics suppression effect of QPUs in SASE FEL is
investigated. The numerical results show that the harmonics power is reduced by
using QPUs, but the fundamental radiation power also has a pronounced decrease
as the saturation length gets very long. The cases of employing QPUs as parts
of undulators are studied. The calculations show that if the fraction of QPUs
and their offgap are appropriate in an undulator system, the harmonics
radiation could be suppressed remarkably, meanwhile the fundamental saturation
length does not increase too much.
