Strongly tapered field generation by beat frequency undulator

Undulator tapering is an important technique in free-electron lasers and synchrotron radiation sources, used to enhance radiation efficiency, broaden spectral bandwidth, and perform other related functions. However, conventional tapering methods, such as gap variation or electromagnetic control of individual magnets, involve complex mechanical structures or power supply systems, posing challenges in tuning the taper profile while maintaining good field integral performance in practice. This paper presents a beat frequency undulator that enables dynamic control of undulator tapering over a wide range. The concept employs two permanent magnet arrays with slightly different periods, whose magnetic field superposition generates a longitudinal beat frequency envelope, creating a tunable taper profile without gap adjustment or electromagnetic tuning. By longitudinally shifting one array, the taper strength and direction can be reconfigured in real time. This approach offers inherent advantages in mechanical simplicity and maintains a field integral comparable to that of a standard planar undulator, providing a robust and flexible solution for undulator tapering.