Microbunching of relativistic electron beams (REBs) are relevant to many research areas as well as applications, such as attosecond electron diffraction, plasma-based accelerators (PBAs), inverse Compton scattering (ICS) sources, and X-ray free electron laser (XFEL).
We propose a simple and compact scheme to modulate a relativistic electron beam (REB) into three-dimensional (3D)
nanoscale bunches by injecting a rarefied REB into an underdense plasma. This scheme self-consistently integrates
the lateral focusing and axial modulation of REB in its self-driven plasma wakefield. The REB first expels the plasma
electrons in its path to form a wake, where the lateral force of the charge-separation field compresses it to higher density,
so that more plasma electrons are expelled as it propagates. The positive feedback loop is repeated until the REB
becomes a thin electron filament of density hundred times the original. As it continues to propagate in the elongated
electron-free wake bubble, the axial electric field induces an energy chirp on the electron filament, and longitudinally
modulates it into 3D nanoscale bunches by asynchronous envelope oscillations. The excitation conditions of this scheme
with respect to the beam and plasma parameters, as well as the spatial scale of obtained electron bunches, are analyzed
analytically and agree well with particle-in-cell simulations.
 

relativistic electron beam,,plasma wakefields,micro-bunches,plasma focusing