Noise reduction and magnetic priming for kW magnetrons by azimuthally varying axial magnetic fields

Summary form only given. Recent experiments have proven that imposition of an azimuthally-varying axial magnetic field to microwave oven magnetrons causes a significant (-30 dB near carrier) decrease in the close-in noise, elimination of sidebands and faster startup. Noise reduction is observed at low, medium and high currents, for fresh and aged, 10-vane, double-strapped magnetrons produced by various manufacturers. This noise reduction technique perturbs the magnetic field by adding a number of permanent magnets to the outside of one of standard circular magnets of the magnetron. The magnitude of the axial magnetic field perturbation was up to 50%, measured at the outside of the cavity structure. Experiments were performed utilizing Panasonic 2M167A-M10 and Toshiba 2M172 J(E) magnetrons, with a low-ripple DC power supply at parameters: voltage about -4 kV, tube current up to 300 mA, and power levels up to 800 W. Several magnetic perturbation arrangements were experimentally and computationally studied. For an N-cavity magnetron operating in the pi-mode, optimal "magnetic priming" for rapid startup employs N/2 azimuthal magnetic variations, as the electrons are kinematically-pre-bunched in N/2 spokes within just one Larmor period. A highly idealized model of magnetic priming revealed a parametric instability, which favors a radial expansion of electrons from the cathode to the anode, in the absence of rf fields and electrostatic effects.