Abstract
A free-electron-driven orbital angular momentum (OAM) emitter is introduced in this paper. When the free electron passes over the cylindrical grating, the spoof surface plasmon (SSP) is stimulated in this system. With loading a helical wire which induces a gradient azimuthal phase distribution the system, the SSP mode will be converted into the spatial OAM radiation. The operating frequency of the OAM mode can be tuned by the electron beam energy of the surface plasmon. Compared with the Smith-Purcell radiation from the helical grating, the field intensity is enhanced by about 50 times. This method provides an efficient way to generate the OAM mode, and may find applications in a communication system.