Efficient Conversion of the Energy of Intense Relativistic Electron Beams into rf Waves

A novel relativistic klystron amplifier has achieved 50% energy efficiency, a significant advance over the previous state of the art of 20%. A radially converging structure that was inserted in the output gap reduced the potential energy residing in the electron beam and maximized rf output energy. Electrons that stopped inside the gap were intercepted by this structure and were shunted to ground. This is in contrast to classical klytons in which such electrons would have formed a virtual cathode and/or flow backward, regaining energy from the rf field.