A Field Analysis for Identification of the Spurious Modes of the Coaxial Magnetron

The electromagnetic system of the coaxial magnetron has been analyzed as a concatenation of radial waveguides extending from the cathode through the anode vanes and backwall slots into the coaxial cavity. The analysis yields, for an arbitrary frequency, an admittance at the slot as reference terminal. This admittance obeys the Foster reactance theorem; its zeroes disclose system resonances. The rate of change of this susceptance with frequency discloses the stored energy and enables calculations of the Rsh/Q of the resonance. In addition to resonances which are essentially cavity resonances, there are conditions such that the cavity and vane system resonate as a folded line having a length equal to a multiple of half-waves. The principal pi mode occurs for two half-waves. Other pi modes occur at a lower frequency (one half-wave) and higher frequencies (3 or more half-waves). The analysis was compared with measurements made on the VMS-1178 coaxial magnetron. Virtually all of the observed spurious oscillations are confirmed by the analysis.