Optimization of the efficiency in gyrotron backward wave oscillator model via tapered axial magnetic field

The nonlinear evolution of the gyrotron backward wave oscillator (gyro-BWO) has been investigated numerically in the presence of a tapered axial magnetic field. The set of coupled nonlinear differential equations that govern the self-consistent evolution of the TE modes and the trajectories of an ensemble of electrons in a gyrotron have been solved subject to boundary conditions for both the usual BWO (power extracted at the input end) and the reflection-type BWO (power extracted at the input end) and the reflection-type BWO (power extracted in the forward direction) configurations. The space-charge effects were neglected. The calculated saturation efficiency is about 10-15% in the K/sub a/-band with rectangular TE/sub 10/ modes and near 104 GHz with cylindrical TE/sub 10/ modes. However, the efficiency can be greatly enhanced (>30%) by tapering the magnetic field. The frequency of the oscillator can be magnetically tuned over a 15% bandwidth. Beam thermal effects have also been investigated.<>