Investigation on the Effects of Assembly Gaps in the Resonant Cavity of Klystrons

During the test of an S-band multibeam klystron (MBK) resonant cavity, an interesting experimental phenomenon was observed: the measured ohmic quality factor ( ${Q}_{{0}}$ ) of the cavity was surprisingly low (around 200, design value of 5090). This prompted an in-depth investigation, revealing that the low ${Q}_{{0}}$ was due to the presence of a small gap on the perimeter of the resonant cavity, which resulted from an inadvertent mechanical design. The topology of this assembly gap was similar to an extremely thin coaxial line. To further investigate the impact of such assembly gap, theoretical and numerical models were developed to quantitatively analyze the effect of concentric/eccentric assembly gap on the cavity ${Q}_{{0}}$ , field asymmetry, the shunt impedance, mode competition, etc. The analysis indicates that the primary factor affecting ${Q}_{{0}}$ is the length of the small gap. The concentricity and width mainly influence the symmetry and shunt impedance, respectively. The analysis showed good agreement with the experimental results. Based on the above findings, a new approach was proposed to conveniently reduce the ${Q}_{{0}}$ of the resonant cavity via employing an appropriately long, relatively narrow gap (1/1000 of its radii, to ensure concentricity even in the worst scenario), thereby facilitating the development of broadband klystrons.