Mode competition in coaxial cavity gyrotrons

High-power coaxial cavity gyrotrons are the main candidates for application in the next step fusion devises. They use higherly overmoded cavity and operate on very high modes. The problem of mode selection is solved by introduction of the corrugated inner conductor. Usually for modeling the mode competition scenario in coaxial cavity gyrotron with a corrugatede insert the surface impedance modes was used. However for the last prototype of the coaxial cavity gyrotron, it showed some discrepancy in mode competition scenario compared with that observed experimentally. In order to understand the reason of this discrepancy we have developed more sophisticated slow wave nonlinear time dependent multi-frequency theory of coaxial cavity gyrotron which takes into account higher spatial harmonics of the field occurring on each frequency due to corrugation of the inner conductor. Numerical calculations have been performed for the last version of the 170 GHz 2 MW coaxial gyrotron cavity. The values of coupling coefficients for competing modes are calculated as functions of the beam radius. Some differences in coupling coefficients values compared with those calculated on the basis of the surface impedance model have been revealed. It is shown that for the nominal beam radius the coupling coefficients for TE_34,19 ^- (operational) and TE_33,19 ^- (parasitic) modes are largest, while the maximum of the coupling coefficient for the TE_33,19 ^- is situated closer to the nominal beam radius. Therefore the strong concurrence between these modes should be expected. This is in good agreement with the experimental data.