Generally HPM devices like gyrotrons have relativistic electron beam which are generated by MIG guns. These beam have the energy mostly in transverse direction and these beam rotate in a cycloidal path in presence of constant axial magnetic field. In order to have beam wave interaction RF wave should have transverse electric field and it should be circularly polarized. For further reference please refer the book by A. S. Gilmour Titled "Klystrons, Traveling Wave Tubes, Magnetrons, Crossed-Field Amplifiers, and Gyrotrons"