High power attosecond (as) X-ray pulses are in great demand for ultrafast dynamics\nand high resolution microscopy. We numerically demonstrate the generation of a ~230 attosecond,\n1.5 terawatt (TW) pulse at a photon energy of 1 keV, and a 115 attosecond, 1.2 TW pulse at a photon\nenergy of 12.4 keV, using the realistic electron beam parameters such as those of Korean X-ray\nfree electron laser (XFEL) in a tapered undulator configuration. To compensate the energy loss of\nthe electron beam and maximize its radiation power, a tapering is introduced in the downstream\nsection of the undulator. It is found that the tapering helps in not only amplifying a target radiation\npulse but also suppressing the growth of satellite radiation pulses. Tapering allows one to achieve\na terawatt-attosecond pulse only with a 60 m long undulator. Such an attosecond X-ray pulse is\ninherently synchronized to a driving optical laser pulse; hence, it is well suited for the pump-probe\nexperiments for studying the electron dynamics in atoms, molecules, and solids on the attosecond\ntime-scale. For the realization of these experiments, a high level of synchronization up to attosecond\nprecision between optical laser and X-ray pulse is demanded, which can be possible by using an\ninterferometric feedback loop.
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