Theory of Competition between Synchronous and Nonsynchronous Modes in a MagniconOutput Cavity

摘要

In the magnicon amplifier, a scanning electron beam drives a synchronous fast-wave interaction in a gyrotron-like output cavity. The output cavity is designed to support the synchronous TM210 operating mode. However, a number of other TE or TM modes can be excited in the cavity via a nonsynchronous, gyrotron-type interaction. To investigate the possibility of competition from these nonsynchronous modes, a multimode gyrotron simulation theory and code nave been adapted to the magnicon configuration. The gyrotron theory and corresponding code have been generalized to include a synchronous TM mode as well as nonsynchronous TE modes. Proper phase averaging between the modes, and between the modes and the beam electrons, is critical to accurate mode competition calculations. In nonsynchronous interactions this is achieved by averaging with respect to electron entrance time and the orbit guiding center angle. The synchronous mode interaction is invariant with respect to these two averages; however, it is affected by scanning angle spread which is included via a third average over scanning angles. Calculations have been carried out to model a second-harmonic X-Band magnicon experiment which is currently underway at the Naval Research Laboratory (NRL). The output cavity has been optimized for the TM210 mode at 11.4 GHz or twice the drive frequency (wd= 5.7 GHz). The principal competing mode is the TE121 mode. The simulations show that nonsynchronous mode interactions are suppressed by the synchronous interaction if the scanning angle spread is sufficiently small. (AN).