Three-dimensional nanoscale microbunching of relativistic electron beam via plasma wakefield for coherent EUV radiation

We propose a 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 the 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 a hundred times that of 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 the obtained electron bunches, are analyzed analytically and agree well with particle-in-cell simulations. In addition, our radiation simulations show that coherent extreme ultraviolet radiation can be generated with such 3D nanoscale bunches.