Nonlinear Simulations Of High Power Gyrotrons

We report our recent results of nonlinear, time dependent multimode simulations of high power conventional cavity gyrotrons. Our model has a self-consistent axial field profile, which satisfies outgoing wave boundary conditions at both ends of the cavity. The model also includes effects of AC space charge, electron pitch angle spread and/or energy spread. Our results show that at high current levels, due to the backward wave instability, lower frequency modes can interact with the electron beam and grow near the throat of the cavity. The growth of these lower frequency modes induces significant energy spread in the electron beam and thereby degrades the extraction efficiency in the cavity. This transverse mode competition, however, is sensitive to the shape of the cavity. By using a sharper taper of the wall radius in the input section of the cavity and a longer straight section, we show that the mode competition can be diminished or suppressed. We will also present comparison of our results with experimental data.