Progress on Generating Bunched Electron Beams Using Modulated Density Downramp Injection

One of the long-term applications of plasma-based accelerators is the fifth-generation light source such as a compact free electron laser (FEL), which requires the generation of ultrahigh-brightness electron bunches [1]. Recently, self-amplified spontaneous emission (SASE) by bunches from both laser- and beam-driven plasma accelerators have been observed [2], [3]. If the drive electron bunch from a plasma accelerator is pre-bunched on the scale of the radiated wavelength, it is then possible to substantially enhance the longitudinal coherence of XFELs by superradiant amplified spontaneous emission. A possible way of generating pre-bunched electron beams is using modulated density downramp injection as recently proposed by Xu et al [4]. Here we report progress on a proof-of-concept experimental realization of this idea, with emphases on a practical way of generating a modulated density downramp using ionization induced plasma grating [5] and preliminary evidence of detection of bunched electrons.