Free-electron-driven vortex beam generator based on compound helical grating

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 compou...