Modeling of mode purity in high power gyrotrons

Summary form only given. Spurious mode generation at the frequency of the operational mode in a high-power gyrotron can significantly reduce the power handling capability and the stability of a gyrotron oscillator because these modes are usually not matched at the output window and thus have high absorption and reflection rates. To study the generation of this kind of mode, the authors have developed a numerical model based on an existing multimode self-consistent time-dependent computer code. This model includes both TE and TM modes and accounts for mode transformations due to the waveguide inhomogeneity. This tool has been used to study the mode transformation in the gyrotron and the possibility of excitation of parasitic TE and TM modes in the up-taper section due to the gyroklystron mechanism. Preliminary results show moderate excitation of both TE and TM modes at the same frequency as the main operating mode at locations near their cut-off.