Orbit-Controlled Generation of Two-color Attosecond Mode-locked Free-electron Lasers

TL;DR

This paper addresses producing two-color attosecond X-ray pulse trains from free-electron lasers with independent color control. It introduces an orbit-controlled scheme where a laser-modulated wiggler between two doglegs creates a periodic temporal-transverse structure, enabling fresh-slice, two-stage mode-locked FEL operation for two distinct colors. Three-dimensional simulations show trains of ~250-attosecond pulses at gigawatt power, with tunable color separation ($\sim$2 and $\sim$3 nm) and adjustable delay, demonstrating robustness to realistic timing jitter. The approach broadens the design space for attosecond FELs and can be extended to single-color or cascaded schemes for dual-color X-ray spectroscopy.

Abstract

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 new 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 novel 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, time delay between the two pulse trains can be adjusted over a range of several hundred femtoseconds.

Figures (6)

• Figure 1: Schematic of the proposed scheme. (a) longitudinal phase space and (c) transverse distribution at the end of the wiggler; (b) longitudinal phase space and (d) transverse distribution at the end of the downstream dogleg.
• Figure 2: The (a) longitudinal phase space; (b) density distribution and (c) current profile of the electron beam at the entrance of the undulator.
• Figure 3: Orbit control along the undulators. Average $x$ (a) and average $x^{\prime}$ (b) of the selected electrons along the undulator position $z$. Solid purple and green lines represent the trajectories of the lowest- and highest-energy electrons, respectively; the corresponding light-colored lines show adjacent slices with similar energies.
• Figure 4: FEL performances of the proposed scheme: (a) radiation power profile and (b) spectrum at the end of the first undulator section; (c) radiation power profile and (d) spectrum at the end of the second undulator section. Bunch head is to the right.
• Figure 5: FEL gain curve of the proposed scheme. Purple line represents the gain curve in the first undulator section. Green lines represent the gain curve in the second undulator section under the condition of delay chicane with different values.