Simulation of a relativistic magnetron with a varying axial magnetic field

Summary form only given. We simulate the University of Michigan´s 6-cavity relativistic magnetron with various configurations of axial magnetic field. The purpose of varying the magnetic field is to increase the growth rate of the pi-mode and to enhance the mode purity of the saturated pi-mode. This effect has already been demonstrated experimentally on non-relativistic magnetrons and simulated in 2D for relativistic magnetrons. We first present 3D particle-in-cell (PIC) results obtained using ICEPIC with an idealized axial magnetic field that is uniform in the axial direction and varies in the azimuthal direction in the form B0+B1 cos(N theta+phi), where we have varied B1 to be 1 percent to 20 percent of the uniform applied field (B0) and N is 3 or 6. Therefore, the magnetic field has the same periodicity of the cavities or half the periodicity of the cavities. We have simulated two values of the phase shift, first with the maximum value of the magnetic field aligned with the vanes and then with the maximum field aligned with the cavities. We compare the growth rate and saturated power with the perturbed magnetic field to that of the uniform field to see the efficacy of this method. We have also simulated the magnetron with an experimentally obtainable field produced from solenoidal coils in the vanes of the magnetron to perturb the applied magnetic field. This magnetic field configuration leads to a small field variation in the axial direction and is compared to ideal simulations and to experiment.