Density modulated electron beam tube with enhanced gain

A linear-beam electron tube comprising:

• a cathode with an electron-emissive surface,
• an electron-permeable conductive grid spaced from said emissive surface and generally parallel to said emissive surface,
• means for applying an electromagnetic field of a desired radio frequency between said grid and said cathode for generating a current-modulated beam of electrons emerging said grid,
• an anode spaced from said grid opposite said cathode, said anode comprising an aperture for passage of said beam,
• said means for applying said radio-frequency field comprising resonant means for applying from a single source a first field between said cathode and said grid and a second field between said grid and said anode, said first and second fields being approximately of opposite phases with respect to the direction of flow of said beam, thus providing for regenerative unloading of said source,
• a hollow conductive drift tube for transmission of said beam from said anode aperture away from said cathode,
• a gap in said drift tube for applying the electromagnetic field of a surrounding cavity, resonant near said desired frequency, across said gap, the length of said drift tube between said aperture and the beginning of said gap being greater than the diameter of said drift tube, whereby the space between said grid and said anode is substantially shielded from fields of said cavity, and
• means for collecting said beam downstream of said gap.