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

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Journal of the Optical Society of America B: Optical Physics, ISSN: 1520-8540, Vol: 35, Issue: 4, Page: A75-A83

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Kumar, Sandeep; Lee, Jaeyu; Hur, Min Sup; Chung, Moses
The Optical Society; OPTICAL SOC AMER
Physics and Astronomy
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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 ∼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 ∼3.5 TWin 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 frequencychirped 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 ∼3 TW in power with ∼7.5 × 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.