Influence of projecting electrodes on shunt impedance and flatness of 4-rod radiofrequency quadrupole cavity

Due to its advantages at low frequency, a 4-rod radiofrequency quadrupole(RFQ) structure has been adopted for the RFQ cavity at the Institute of Modern Physics, Chinese Academy of Sciences, which works at 100 MHz and is used for the direct plasma injection scheme. After the beam dynamics design, the parameters of the stems of the cavity were optimized to obtain the maximum shunt impedance. The capacitance existing in the projecting electrodes will affect the properties of the cavity, therefore, the influence of the projecting electrodes should be taken into account. Their influence on shunt impedance and flatness of the 4-rod RFQ cavity was studied, and the results show that the influence on shunt impedance can be neglected but it can not be neglected on flatness. In addition, the length r