High-power S-band fundamental-mode eight-beam klystron and gun design

Summary form only given. In multiple-beam amplifiers, beamlets are transported in individual beam tunnels but interact with RF fields in a common interaction region. This approach enables the designer to have the best of both worlds: individual beamlets can have low perveance which is conducive to efficient bunching and beam transport, leading to higher gain, electronic efficiencies, and average power, while the aggregate beam current can be high, facilitating high beam and RF power, and also broad bandwidth. We will report on the design of a beam forming system for high-average-power broadband S-band multiple-beam amplifiers to be developed at the Naval Research Laboratory (NRL). These amplifiers will utilize eight individual electron beams and operates in the fundamental TM/sub 01/ mode. The singly convergent electron gun topology has fourfold symmetry with four inner and four outer emitters, interlaced 90/spl deg/ apart. The operating voltage is 45 kV with a total beam current of 32 A, evenly divided among the beamlets. Each individual beam perveance is 0.42 micro-pervs, for a total beam microperveance of 3.35. Cathode loading is kept below 10 A/cm/sup 2/ (space charge limited) for lifetime considerations. The cathode is magnetically shielded and the magnetic field in the interaction region is 1.1-1.8 kG. A key design feature is the magnetic focusing system, designed to ensure minimal beam corkscrewing, which limits the beam clearance in the beam tunnel. The primary computational tools used in this design were the 3-D gun code, MICHELLE, and the magnetic code, MAXWELL-3D. Beam optics simulations of the gun design demonstrate excellent beam transport characteristics with a final beam-to-tunnel fill factor less than 40%.