Estimation Method for Self-Impedance's Real Part of Multigap Output Cavity of Klystrons Using Group Delay

A new estimation method by group delay for the real part of the self-impedance of multigap (two or more gaps) output cavity of klystrons is presented. Although these electromagnetic data can be easily obtained by using electromagnetic simulation (e.g., MWS or HFSS), we still need a cold test to confirm and adjust the values because in actual manufacturing, there are several steps that can slightly change the sizes of cavities and the parameters of high frequency. The traditional impedance measurement method for output cavity needs three steps, i.e., open, short, and perturbation, and the problems caused by a multigap cavity in the measurement of middle gaps (excluding the gaps at two sides) will meet difficulties when setting perturbation and short states. Beginning with microwave network theory and aiming at three-gap output cavity, this paper finally establishes the relationship between the group delay and the real part of the self-impedance of the output cavity. Then, the new method is no longer limited by the number of gaps through eliminating the perturbation and the short states of the traditional method. Using the new and traditional methods, a double-gap output cavity of an S-band klystron has been separately measured. Moreover, the feasibility of the method has also been verified by comparing their results. Finally, the relevant results of the five-gap output cavity of an X-band sheet-beam klystron are given.