Nonstationary behavior in gyrotron oscillators

Nonstationary behavior that occurs in the gyrotron originates from many causes, such as mode competition or transients during the initial buildup. In various ways, it imposes severe limitations on the performance of the gyrotron. In the present study, we restrict our consideration to the nonstationary behavior resulting from the over drive of a single mode in the gyrotron oscillator. The threshold current of such a nonstationary state, as predicted in a number of theoretical investigations, is typically several times higher than the start-oscillation current. Immediately above the threshold, the field energy bounces back and forth between the ends of the interaction structure, resulting in self-modulation of the output power. In a recent theoretical study of the gyrotron backward-wave oscillator (gyro-BWO), however, it was shown that the axial field profile contracts in the nonlinear regime which therefore suggests the possibility of stationary (saturated) operation at a beam current as high as two orders of magnitude above the self-modulation threshold predicted by early theories. In this talk, we present the experimental characterization and theoretical interpretation of the self-modulation behavior of the gyromonotron as well as the experimental verification of the much broader stationary operating range of the gyro-BWO which is shown to result from the self-adjustment of the feedback path length.