Abstract
In this paper we propose a double chirp-taper method to produce two subfemtosecond x-ray free-electron lasers (XFELs) using electron beams with sinusoidal energy modulation. The taper of the undulator is optimized to control the power profile of each XFEL pulse to obtain a single-spike power shape. We present start-to-end numerical simulations of this method to demonstrate the generation of two XFEL pulses with $\ensuremath{\sim}0.4\text{ }\text{ }\mathrm{fs}$ full width at half maximum, with a time separation tunable from 0 to tens of fs and energy separation limited by the available undulator tuning range. The output of this method provides a powerful tool for XFEL pump-probe experiments at the attosecond timescale.
Highlights
X-ray free-electron lasers (XFELs) are routinely used for resolving dynamical processes in molecules and materials at the femtosecond timescale [1,2]
Compared to other multiundulator two-color schemes [7,8,9] the proposed method is similar to the fresh-slice method [10]: in both schemes the two colors are generated in two different undulators using two different regions of an electron bunch, which results in higher peak power and improved time separation range
This is because in the double chirp-taper scheme the separation of the two lasing slices is given by half of the modulation wavelength, which in turn is locked to the resonant condition in the wiggler, effectively decoupling the time separation from the peak current jitter of the electron bunch
Summary
X-ray free-electron lasers (XFELs) are routinely used for resolving dynamical processes in molecules and materials at the femtosecond timescale [1,2]. In this paper we propose a method to generate pairs of sub-fs x-ray pulses with attosecond timing stability and controllable separation based on a double chirp/taper scheme driven by self-modulation in a magnetic wiggler technique [13,14]. Since this method relies on variable-gap undulators to generate two independent FEL pulses, the energy of the two pulses can be varied arbitrarily, allowing for pump/probe experiments at different atomic sites and enabling the study of charge and energy transfer in molecules at the attosecond timescale.
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