Generation of attosecond x-ray pulses with a multicycle two-color enhanced self-amplified spontaneous emission scheme

Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. Several schemes using extremely short laser pulses to manipulate the electron bunches have been proposed. In this paper, we extend the attosecond two-color enhanced self-amplified spontaneous emission scheme proposed by Zholents et al. to the long optical cycle regime using a second detuned laser and a tapered undulator. Both lasers can be about ten optical cycles long, with the second laser frequency adjustable and set to optimize the contrast between the central and side current spikes. A tapered undulator mitigates the degradation effect of the longitudinal space charge force in the undulator and suppresses the FEL gain of all side current peaks. Simulations using the Linac Coherent Light Source parameters show a single attosecond x-ray spike of ∼100  attoseconds can be produced. The second laser can also be detuned to coherently control the number of the side x-ray spikes and the length of the radiation pulse.