Orbit-controlled generation of two-color attosecond mode-locked free-electron lasers

The generation of attosecond x-ray pulses has garnered significant attention within the x-ray free-electron laser (FEL) community due to their potential for ultrafast time-resolved studies. Such pulses enable the investigation of electron dynamics with unprecedented temporal resolution, opening avenues in fields such as quantum control and ultrafast spectroscopy. In an FEL, the mode-locking technique synthesizes a comb of longitudinal modes by applying spatiotemporal shifts between the co-propagating radiation and the electron bunch. Here, we propose a scheme for generating two-color attosecond mode-locked FEL pulses via orbit control in two-stage mode-locked undulators. Specifically, a chicane with a wiggler inserted in the middle generates periodic temporal-transverse modulation in the electron beam. In this configuration, the low-energy and high-energy components of the beam lase in separate undulator sections, with each producing attosecond mode-locked FEL pulses. Three-dimensional simulations with realistic parameters confirm that trains of 250-attosecond soft x-ray pulses at the gigawatt level can be independently generated in each undulator section. Furthermore, the time delay between the two pulse trains can be adjusted over a range of several hundred femtoseconds.