Analyzing the effects of post couplers in DTL tuning by the equivalent circuit model

Stabilization of the accelerating field in Drift Tube Linac(DTL) is obtained by inserting Post Couplers(PCs).On the basis of the circuit model equivalent for the DTL with and without asymmetrical PCs, stabilization is deduced quantitatively: let $\delta \omega/\omega_0$ be the relative frequency error, then we discover that the sensitivity of field to perturbation is proportional to $\sqrt{\delta \omega / \omega_0}$ without PCs and to $\delta \omega/\omega_0$ with PCs. Then we adapt the circuit model of symmetrical PCs for the case of asymmetrical PCs. The circuit model shows how the slope of field distribution is changed by rotating the asymmetrical PCs and illustrates that the asymmetrical PCs have the same effect as the symmetrical ones in stabilization.