Abstract
THz waves have potential in medicine, biophysics, and imaging. In order to fabricate a simple device with high efficiency, here we propose a special kind of Smith-Purcell grating backward-wave oscillator (BWO) for THz generation. First, the boundary conditions of the Smith-Purcell grating are solved by the Matlab code, and the operating frequency is determined by the dispersion relation and the beam line on the Brillouin diagram. Then we simulate the grating-structure by MAGIC 2D to observe the interaction between the structure and the electrons, and the results of the operating frequency in MAGIC 2D agree with the results in Matlab. Also, we simulate the grating with different current densities and beam-grating distances, and find that the restricted beam condition while operating at THz region. In order to enhance the coupling efficiency, also to make the structure easy to manufacture, “semi-open” grating is proposed. From the result of MAGIC 2D simulation, we find the magnitude of the B_z field is 4 times as large as the conventional Smith-Purcell grating BWO on the surface of the grating. In thick beam simulation, the parameters of the E-gun in NSRRC are applied to the ASTRA simulation to determine the magnetic field for beam focusing. The calculated magnetic field of the solenoids are 130gauss, 120 gauss and 170 gauss at 21.5cm, 33.5cm and 50cm respectively and the rms beam size equals to 0.41mm at 80.5cm. Yet in MAGIC 2D simulation, the upper portion of the electrons is not well modulated due to the asymmetric excited mode profile. We also propose another structure that contains both an optical grating and a dielectric coating metal plate. The dielectric layer provides the condition for stimulated Cherenkov radiation, and the excited magnetic field is 6 times of the field of single-side structure on the surface of the grating. We suppose it is a promising structure for enhancing the energy coupling between the beam and waves.