Abstract
The efficiency of a gyrotron backward-wave oscillator (gyro-BWO) in the presence of a tapered axial guide magnetic field is calculated from a three-dimensional self-consistent theory. The saturation efficiency has a low value in the range 10–15% when the external magnetic field is uniform. However, the efficiency can be enhanced to over 30% by a small taper in the magnetic field. Numerical results are shown for operation in the TE11 mode of a cylindrical waveguide.