Mode competition and control in higher-power gyrotron oscillators

One of the most important problems in the design of high-power millimeter-wave sources such as gyrotron oscillators is insuring that the device operates in the desired mode. For high-power and short-wavelength devices the effective mode density is high, in that the current is above threshold for many modes. One then is led to ask whether operation in a single mode is possible and what steps must be taken to maximize the electronic efficiency of the device while ensuring single-mode operation. The answer to the first question has been determined to be yes. Provided that certain conditions are met, single-mode operation is stable. The present results emphasize time-dependent multimode simulations showing how these stable states can be accessed. In particular, the accessibility to the stable single mode with maximum efficiency is studied. Regions of parameter space for which stable single-mode operation is possible are plotted for an annular beam for a closed-cavity gyrotron operating at a high-order whispering-gallery mode (TE_80.4). These results also apply to the quasioptical gyrotron with a pencil electron beam