Beam tube with density plus velocity modulation

A method for increasing the efficiency of a beam tube employing density modulation and inductive output circuit, said tube comprising:

• a vacuum envelope;
• a gun for generating a linear beam of electrons, said gun comprising a thermionic cathode, means for heating said cathode, and an electron-permeable grid insulated from said cathode and spaced close to the emissive surface of said cathode;
• means for applying a radio-frequency input signal voltage between said cathode and said grid;
• anode means for drawing a beam of electrons from said gun, said anode means comprising an aperture for passage of said beam through an extended hollow metallic drift tube;
• collector means beyond the end of said drift tube opposite said anode for collecting said electrons and dissipating their remaining energy;
• output circuit means for extracting rf energy from said beam comprising, a transverse output gap in said drift tube, means for coupling a resonant output circuit across said gap, and means for extracting energy from said resonant output circuit;
• a second gap in said drift tube on the anode side of said output gap, and means for coupling an intermediate resonant circuit across said second gap;
• said method comprising:
• applying an amplitude modulated input signal between said grid and said cathode;
• applying dc accelerating voltage between said cathode and said anode;
• applying dc bias voltage between said grid and said cathode such that the emission current is drawn over approximately one-half of each rf cycle;
• tuning said output circuit so that its resonant frequency is approximately at the center of the frequency band of said input signal;
• and
• tuning said intermediate cavity so that its resonant frequency is above said frequency band.