Abstract
A free-electron-driven vortex beam emitter based on coherent Smith-Purcell radiation (SPR) induced by the compound helical grating is proposed in this paper. Due to the diffraction of the helical periodic system, vortex beams carrying orbital angular momentum are excited when the ring-shaped electron beam flies over the surface of the grating. The dispersion relations of the uniform and the compound helical grating are analyzed. Owing to the periodic modulating of grating width, the dispersion line of the fundamental mode of the uniform helical grating is split to the fast wave region regarding the Brillouin-band folding phenomenon, and the generated SPR vortex beam is coherent accordingly.