Synchrotron radiation (SR) provides a powerful tool for studying materials with X-rays in a wide energy range. A combination of high photon flux and flexibility in photon energy tunability allows exploiting SR for absorption and emission spectroscopy techniques such as XAS, XANES, XES. Some sophisticated techniques like, for example, quick X-ray Absorption Fine structure (qXAFS) spectroscopy allow to quickly measure the XAS spectrum. qXAFS is based on the use of a fast monochromator that scans through a broad spectral range (1 keV) providing data acquisition in less than 100 ms. However, modern SR sources equipped with superconducting undulators require new approaches for generating radiation with such bandwidth and still retain the advantage of undulator radiation compared to other broadband sources.
In this research, we propose to use a stepwise tapered magnetic field configuration for the superconducting undulator: we gradually change the magnetic field along the device. An undulator is effectively split into cells, each with a slightly different magnetic field, such that a given harmonic of the radiation emitted in each cell is shifted by half of its bandwidth. The contribution from the segments sums up to a continuous spectrum with the desired total bandwidth of 1 keV.
We studied the performance of this device both analytically and numerically including propagation of the emitted radiation up to the sample location. We also studied how propagation and spatial filtering affects radiation spectral density. This research serves as a conceptual design for the quick-EXAFS beamline at the Siberian Circular Photon Source (SKIF ― the Russian acronym) in Novosibirsk, Russia.