B-field perturbation effects on magnetic priming of a relativistic magnetron

Experiments have been performed testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high power microwave performance. Three high permeability wires were embedded beneath the emission region of a 1.27 cm diameter cathode, spaced 120 degrees apart (for pi-mode symmetry in an 6 vane magnetron) to perturb both the axial and radial magnetic fields near the emission region of the cathode. Magnetic priming was demonstrated to increase the percentage of pi-mode shots by 15% over the baseline case. Mean peak power for pi-mode shots was found to be higher in the magnetically primed case by almost a factor of 2. Increases in mean microwave pulse width were also observed in the magnetically primed case when compared to the unprimed case (66 ns primed versus 50 ns unprimed). Magnetron starting current for the magnetically primed pi-mode exhibited a reduction to 69% of the unprimed baseline starting current. Earlier research by Neculaes (2005) and recent simulation work performed utilizing MAGIC PIC and the Magnum magnetostatics code suggest that using permanent magnets with radially-directed remanence fields centered under the cathode emission region instead of high permeability wires can yield improved magnetron performance. Simulations of magnetically primed magnetrons utilizing permanent magnets with radially-directed remanence fields demonstrated improved performance as compared to simulations of axially-directed remanence fields. Both simulation and experimental results will be presented for the magnetic priming cases described.