High-average power broadband 18-beam klystron circuit and collector designs

Summary form only given. The circuit and collector designs for a high-average power S-band multiple-beam klystron are presented. The klystron will be powered by the recently completed 41.6 A, 42 kV, 18-beam, high-repetition rate electron gun. A compact circuit yielding 680-kW peak power and 13% 1-dB bandwidth has been designed. Design of a high-average power compatible collector for the klystron is also presented. Key issues such as collector power loading and particle reflections due to time-dependent virtual cathode formation set up by the spent (bunched) beam in the collector are addressed. The circuit design, performed with the particle-in-cell code MAGIC-3D, is comprised of seven cavities in a total length of ~23 cm. All two-gap cavities operate in the mode with the same gap-to-gap coupling approach as used in our previous broadband 8-beam design. The filter-loaded two-gap output cavity permits broadband and efficient RF power coupling from the bunched electron beam. In addition, the interaction impedance across the band can be tailored to compensate for any frequency-dependence of the beam gain current. This flexibility was employed to produce the output power frequency response with 1-dB flatness across the 13% bandwidth. The high-average power compatible collector design was performed using the 3-D gun and collector code MICHELLE and the magnet code MAXWELL-3D. The collector shape and magnetic field profile were designed in tandem to properly spread out the beamlets to ensure that the collector power loading is within the limit of current cooling technology. Of particular import is the optimization of the entrance to the collector to minimize low-energy electron reflections caused by a potential virtual cathode induced by the time-dependent nature of the spent beam upon entering the collector.