Calculation of Coupling Factor for Double-Period Accelerating Structure

In the design of the linear accelerating structure, the coupling factor between cavities is a crucial parameter. The error of coupling factor accounts for the electric or magnetic field error mainly. To accurately design the coupling iris, the accurate calculation of coupling factor is essential. The numerical simulation is widely used to calculate the coupling factor now. By using MAFIA code, two methods have been applied to calculate the dispersion characteristics of the single-period structure, one method is to simulate the traveling wave mode by the period boundary condition; another method is to simulate the standing wave mode by the electrical boundary condition. In this work, we develop the two methods to calculate the coupling factor of double-period accelerating structure. Compared to experiment, the results for both methods are very similar, and in agreement with measurement within 15% deviation.