Design Considerations of Magnetic Devices for Bright Laser-plasma Undulator Radiation in the Water Window Spectral Range

The realization of a tabletop X-ray free-electron laser (XFEL) based on a laser-plasma electron accelerator is a challenging scientific issue. A high-efficient undulator and a compact electron beam line, as well as a stable laser-plasma accelerator, are key elements for this form of state-of-the-art synchrotron source. In particular, X-ray radiation in the water window spectral range is very attractive for biomedical applications due to its unique spectral characteristics. In this paper, we present numerical calculations and simulations for obtaining bright radiation in the water window spectral range from a prototype laser-plasma undulator source. Two compact undulators (planar and helical) and quadrupole-magnet doublets for electron beam focusing are considered. In our results, a helical undulator with a relatively low magnetic field strength shows beneficial radiation performance in terms of total power and on-axis angular flux density. The undulator radiation is then characterized based on its spectral brilliance deduced from an electron beam trajectory that was optimized using a quadrupole-magnet doublet. From the simulation results, the on-axis peak brilliance of the undulator radiation with a quadrupole-magnet doublet having a stronger magnetic field gradient (700 T/m), which can approach an electron source closer, was approximately three orders of magnitude higher than that of a case in which no doublet was used.