Application of a modified chirp-taper scheme for generation of attosecond pulses in extreme ultraviolet and soft x-ray free electron lasers

Typically, in self-Amplified spontaneous emission free electron laser (SASE FEL) based short-pulse schemes, pulse duration is limited by FEL coherence time. For hard x-ray FELs, coherence time is in a few hundred attosecond range while for XUV and soft x-ray FELs it is in the femtosecond regime. In this paper the modification of so-called chirp-taper scheme is developed that allows to overcome the coherence time barrier. Numerical simulations for XUV and soft x-ray FEL user facility FLASH demonstrate that one can generate a few hundred attosecond long pulses in the wavelength range 2-10 nm with peak power reaching hundreds of megawatts. With several thousand pulses per second this can be a unique source for attosecond science.