Desgin of a 94-GHz gyrotron operating at the 4th harmonic, TE5,4-mode

Summary form only given: To evaluate the feasibility of operating a 94-GHz, 30-100 kW gyrotron with a permanent magnet, we have explored various 4^th harmonic configurations, including two- and three-cavity multipliers and single-cavity oscillators. Such devices require a magnetic field of ~9.5 kG, a level that is achievable with readily available permanent magnet materials. Our general conclusion is that in the targeted parameter range, the monotron is preferable to a multiplier configuration provided that a stable operating mode having acceptable wall losses and start-up conditions can be found. Here, we present our results on the design and analysis of a 94 GHz gyromonotron operating on the 4^th cyclotron harmonic, TE_5,4-mode, which satisfies these conditions. The studies were done with the self-consistent time-dependent code MAGY. We have analyzed two cavity configurations: 1) a shorter cavity with an iris near its output and 2) a longer cavity with smooth walls. Also, we have considered two values of beam radii, which correspond to the first two maxima of the beam coupling to the operating mode. Results show that higher efficiency can be achieved when the longer cavity is used. In particular, for a 10 A, 60 kV electron beam with 5% RMS velocity spread injected at the first maximum of the coupling coefficient, the output efficiency can be as large as 11%. From the calculated spent-beam energy distribution, the use of a depressed collector could provide a wall-plug efficiency of about 36.6% for these parameters. Beam injection at the second peak results in a 6.5% output efficiency for a 10 A, 75 kV electron beam with the same value of velocity spread. The wall-plug efficiency in this case can be 32.4 %.