We propose a new structure of Smith–Purcell (SP) emitter for increasing its power radiation. Injection of an electron beam passing over the metallic surface causes excitation of the surface mode for metal grating and subsequently electron acceleration and electromagnetic radiation. This radiation can be achieved in the terahertz spectrum applying suitable parameters in the structure. The grating is in half-cylindrical form, which helps to increase the interaction of a particle with the surface, and because the electron beam moves within the grating, intensity of radiation power, and electric field dramatically accumulate. The proposed geometry utilizes a single instead of the 10-point electron source with smaller dimensions and higher emitted power values compared to the traditional SP emitters. Furthermore, it comes with about 2- and 32-time increase, respectively, in radiation power and electric field. For our simulations, 3-D particle in cell and eigenmode solvers have been used.