Undulator scheme with variable composition of the magnetic field harmonics

A method of variation of the undulator radiation wavelength is considered. The method is based on the influence of the undulator magnetic field shape on the radiation frequency. It has been shown that the change of the partial contribution of higher harmonics (mainly of the third harmonic) into the magnetic field shape leads to smooth tuning of the radiation frequency within a range adequate to three-fold changes of the undulator period. An undulator scheme providing such properties without reconstruction of the undulator design has been proposed.