High-intensity X-ray sources are essential diagnostic tools for science, technology and medicine. Such X-ray sources can be produced in laser-plasma accelerators, where electrons emit short-wavelength radiation due to their betatron oscillations in the plasma wake of a laser pulse. Contemporary available betatron radiation X-ray sources can deliver a collimated X-ray pulse of duration on the order of several femtoseconds from a source size of the order of several micrometres. In this paper we demonstrate, through particle-in-cell simulations, that the temporal resolution of such a source can be enhanced by an order of magnitude by a spatial modulation of the emitting relativistic electron bunch. The modulation is achieved by the interaction of the that electron bunch with a co-propagating laser beam which results in the generation of a train of equidistant sub-femtosecond X-ray pulses. The distance between the single pulses of a train is tuned by the wavelength of the modulation laser pulse. The modelled experimental setup is achievable with current technologies. Potential applications include stroboscopic sampling of ultrafast fundamental processes.

Department of Physics Chalmers University of Technology 412 96 Gothenburg Sweden

Institute of Physics Czech Academy of Sciences ELI BEAMLINES Na Slovance 1999 2 182 21 Praha 8 Czech Republic

Institute of Plasma Physics Czech Academy of Sciences Za Slovankou 1782 3 182 00 Praha 8 Czech Republic

Key Laboratory for Laser Plasmas School of Physics and Astronomy Shanghai Jiao Tong University Shanghai 200240 China

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