Novel rf design of bimodal normal-conducting active rf cavity for synchrotron radiation light sources

Double-rf systems have become a standard solution in advanced storage ring light sources for beam acceleration and bunch lengthening. In this context, a compact single-cell normal-conducting bimodal rf cavity is proposed, in which the TM_{010} accelerating mode and the TM_{020} harmonic mode operate simultaneously within a single cavity. To suppress harmonic power leakage through the fundamental-mode coupler, a low-pass filter is introduced. In addition, tunable band-stop filters are employed to provide selective isolation of the operating modes and to enable efficient extraction and damping of parasitic higher-order modes. Based on this concept, a complete rf design of a single-cell bimodal cavity with a fundamental frequency of 500 MHz and a corresponding third-harmonic frequency of 1.5 GHz is presented, and key issues including frequency tuning, shunt impedance optimization, and higher-order-mode damping are systematically investigated. The results show that, under the condition of frequency matching between the two operating modes, their frequencies and coupling strengths can be adjusted with a high degree of independence. With higher-order modes effectively suppressed, the shunt impedances of the TM_{010} and TM_{020} modes reach 4.74 and 1.2 MΩ, respectively, corresponding to reductions of only 0.35% and 1.25% compared with the undamped case. These results validate a general design approach toward compact bimodal cavities for storage-ring double-rf systems. The mechanical design has been completed, and fabrication is currently in progress.