Electromagnetic design of a transit-gap-loaded resonant cavity for compact 324 MHz klystrons

A transit-gap-loaded re-entrant cavity with a four-bridge annular metallic element is investigated for compact 324-MHz klystrons. With the cavity envelope and element unchanged, relocating the element from the drift-tube region to the transit gap lowers the simulated resonant frequency from 405.5 to 325.4 MHz (19.8%), while the effective characteristic impedance M2R/Q changes from 82.46 to 82.69 Ω. At 325.4 MHz, the four-bridge-loaded and double-bridge complementary electric split-ring resonator (CeSRR)-loaded cavities differ by at most 1.68% in electromagnetic parameters. Both reduce envelope volume by 39% relative to the frequency-matched unloaded cavity. Slater's first-order relation qualitatively explains the loading-position trend, and mode-specific parallel-RLC circuits are derived. A six-cavity design yields 3.65 MW output power, 61.45% electron efficiency, and 49.09 dB gain in a three-dimensional particle-in-cell simulation at 110 kV and 54 A.