Nonlinear contraction of field profile in the gyro-BWO and its effect on frequency tuning

Summary form only given. Physical processes in the gyrotron backward-wave oscillator (gyro-BWO) are investigated with a numerical approach which evaluates the self-consistent field profile by employing outgoing-wave boundary conditions at the open ends. Results indicate a sharp contrast between the linear and saturated field profiles. The former extends over the entire length of the interaction structure, whereas the latter contracts to approximately half of a guide wavelength regardless of the structural length. As a result, the saturated efficiency remains nearly a constant while the start-oscillation current drops sharply with increased structural length. It is shown that this distinctive feature substantially influences the basic properties of the gyro-BWO including the start-oscillation current, efficiency, power scaling, and stability of tuning.