Nose-cone removed pillbox cavity for high power klystron amplifiers

A simple pillbox cavity without the reentrant nose-cone is proposed as a new type of RF resonator in klystron amplifiers. It is advantageous in high-power applications, because of its lower electric field concentrations. The velocity modulation of an electron beam in the pillbox cavity is analyzed numerically by using the finite-difference method. The beam coupling coefficients are compared with the analytical solutions for a conventional reentrant cavity. It is shown that for a high-voltage electron beam case the pillbox cavity can provide almost as large a coupling coefficient as the conventional reentrant cavity. As an example, the analysis is carried out on a practical S-band pulsed high-power SLAC-5045 klystron. The maximum surface electric field in the output cavity of the pillbox type is shown to be reduced by 50% as compared to the conventional reentrant cavity used in the 5045 klystron. Discussed are several benefits of this nose-cone-less cavity, such as suppression of the multipactor, lower sensitivity of the resonance frequency to dimension errors, and ease of fabrication