This study suppresses effectively some unwanted harmonics and broadens
the preferred harmonics by introducing a periodic gap between undulat
ors and properly selecting the gap size. This effect is ascribed to th
e interference between each part of the separated undulator. Closely e
xamining the envelop term (i.e. the diffraction term in the case of a
grating) of the undulator radiation spectrum reveals the possibility o
f this energy redistribution. Analytical results and numerical results
are presented to confirm the idea. (C) 1998 Elsevier Science B.V. All
rights reserved.