Simple scheme for harmonic suppression by undulator segmentation

In order to suppress harmonic intensity of undulator radiation, a scheme similar to the detuning technique used in the double-crystal monochromator is investigated and found to be effective only when the number of periods of the undulator is small, once the finite emittance of the electron beam and angular acceptance of the beamline are taken into account. Instead, a simple scheme is proposed for undulators with many periods: the undulator is divided into several segments and the optical phase in between is adjusted to shift the fundamental energy without significantly affecting other harmonics.