Effects of geometric azimuthal asymmetries of the PPM stack on electron beam characteristics

The effects of geometric azimuthally asymmetric properties of a periodic permanent magnet (PPM) focusing stack on electron beam characteristics obtained using a fully three dimensional (3D) particle-in-cell (PIC) code will be presented. The simulation model, using MAFIA (Solution of MAxwell's equations by the Finite-Integration-AIgorithm), incorporates 3D behavior of the beam immersed in static fields calculated directly from the exact geometry and material properties ofthe 3D magnetic focusing structure. The Hughes 8916H, 18-40 GHz helical TWT for the millimeter-wave power module (MMPM) was used as a prototype. Firstly, the effects of C-magnets used at the input and output of the TWT to allow for coupling ofthe RF signal into and out of the tube are considered. The 8916H input and output C-magnets differ because coaxial couplers are used at the input and waveguide couplers are used at the output. The repositioning of the beam from its central axis due to the inclusion of the output C-magnet was found to be most significant.