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.