Double chirp-taper x-ray free-electron laser for attosecond pump-probe experiments

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 similar to 0.4 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.