Abstract
To change the wavelength of undulator radiation, variation of the undulator magnetic field amplitude is frequently applied. Another option is changing the undulator period. A corresponding scheme is described. Both the period and number of periods can be changed. For a set of undulator sections (as in the x-ray free-electron lasers ), the mechanical motion of the periods allows doing without phase shifters between the undulator sections. Magnetic field calculations for some undulator parameters of interest were performed. Numerous advantages of the new undulators (fixed gap, strong dependence of the undulator radiation wavelength on the period, relatively low field amplitude variation, and variable number of periods) look very attractive. The prospects for this new type of undulators are discussed.
Highlights
Contemporary storage-ring-based x-ray sources and free-electron lasers use magnetic undulators
For a set of undulator sections, the mechanical motion of the periods allows doing without phase shifters between the undulator sections
For most of the applications, it is necessary to tune the wavelength of the undulator radiation in the forward direction, 1⁄4
Summary
Contemporary storage-ring-based x-ray sources and free-electron lasers use magnetic undulators. A frequent way of tuning the undulator radiation wavelength is varying the field amplitude B0. In electromagnetic undulators it is realized via varying current in the coils, and in permanent magnet undulators, mainly varying the undulator gap. Serious intrinsic drawbacks of this technique are too low field values at short wavelengths and too high ones at long wavelengths The ‘‘simplest’’ way to do it is to replace one undulator with another [4,5] In such a design, fine wavelength tuning can be realized via gap variation. Different electromagnetic variable-period undulators (VPU) were discussed in papers [6,7,8]
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