In this paper, a gyrotron oscillator operating at 0.42 THz is studied by Particle-in-cell simulation. Under the condition of a beam voltage of 12 kV, beam current of 160 mA and magnetic field of 7.665 T, the gyrotron can operate stably in the TE06 mode at the second-harmonic. Its output power of continuous wave (CW) is up to 82 W corresponding to an efficiency rate of 4.1%

A free electron laser (FEL) amplifier with a curved parallel plate waveguide and a planar wiggler is presented. A set of operating equations for this FEL amplifier is derived by using the three-dimensional nonlinear theory. The characteristics including the evolution of power, efficiency and bandwidth of the FEL operating at the frequency of 94 GHz are numerically analyzed. The effects of electron energy spread and wiggler taper on saturation efficiency are also studied.