Generation of an isolated attosecond pulse via current modulation induced by a chirped laser pulse in an x-ray free-electron laser

Abstract

Aiming for the future upgrade of a hard x-ray beamline at the Pohang accelerator laboratory x-ray free-electron laser (PAL-XFEL), we first analyze the scheme recently proposed for the attosecond-terawatt (TW) x-ray pulse [Phys. Rev. Lett. 110, 084801 (2013)]. An x-ray pulse with 32 attosecond full-width half maximum (FWHM) pulse duration and similar to 4 TW in power is shown in simulations using the PAL-XFEL parameters. Furthermore, to remove neighboring radiation pulses and to generate an isolated attosecond-TW x-ray pulse, the role of optical laser frequency chirp is examined on the electron beam current modulation as well as on the x-ray pulse generation in the undulator. Our simulations show an isolated x-ray pulse with 42 attosecond FWHM pulse duration and similar to 3.5 TW in power for the optimal frequency chirp of a 400 nm optical laser. Recently, in a simpler method [J. Synchrotron Radiat. 23, 1273 (2016)] and [Phys. Rev. ST Accel. Beams 8, 040701 (2005)], using a frequency-chirped optical laser and the electron beam delays in between undulator sections, it has been reported that an isolated attosecond-TW x-ray pulse can be obtained at the undulator end. For optimal chirp in this simple setup, we show 50 attosecond FWHM pulse duration and similar to 3 TW in power with similar to 7.5 x 10(10) photons per pulse at 0.1 nm radiation wavelength. Generation of such inherently synchronized, powerful single attosecond x-ray pulses at PAL-XFEL will be advantageous to the pump-probe experiments in the study of ultrafast dynamics.