Mode competition in fourth-harmonic magnicon amplifiers

In the magnicon RF amplifier, the drive and gain cavities are cylindrical deflection cavities which operate in a rotating TM/sub 110/-mode and spin up an electron beam to high transverse momentum. As a result of the rotating-mode interaction, the electron beam entry point into the output cavity rotates about the axis at the drive frequency. The gyrotron-like output cavity can be operated at m times the drive frequency by using a mode with an azimuthal index of m, as this mode rotates at m/sup -1/ times its RF frequency, thus maintaining synchronism with the electron beam. Previous frequency-multiplying magnicons have used m=2; in this paper it is shown that magnicons with m=4 may be practical, provided one also operates the output cavity at the m/2 harmonic of the cyclotron frequency. Operation at higher harmonics lowers the frequency of the deflection cavities allowing lower RF fields, reducing cavity breakdown problems; lower magnetic fields, reducing magnet cast and complexity; and a larger electron beam, relaxing beam quality constraints. On the other hand, higher order azimuthal-index magnicon modes interacting at higher order cyclotron interactions are subject to competition with nonsynchronous (gyrotron) modes and are more sensitive to electron beam scanning angle spread. A time-dependent multimode gyrotron code has been modified to examine competition in the output cavity between the phase-synchronous operating mode and other nonsynchronous modes which interact via the conventional gyrotron interaction. Calculations have been carried out for fourth-harmonic magnicons with TM/sub 410/ and TE/sub 411/ mode output cavities and include the effects of mode competition with nonsynchronous modes as well as a spread in entry-point angles of the scanning electron beam.