Analysis and design considerations of coupler tuning based on improved equivalent circuit and 3D-EM simulations

Power coupler is critical for the successful development of high-intensity linacs. As a main task in its design, pulsed heating affects the beam brightness, even might damage the structures on high-repetition rates. Although shaping the incident RF-pulse can reduce such risk by speeding up the field filling time, a complicated RF sub-system is inevitable. While another mechanism of reducing the filling time, which is increasing the coupling factor, presents its advantages. In this context, based on the analysis of temperature rise for a coupled cavity, the resonant frequency variations have been qualitatively investigated by improving the equivalent circuit via adopting a mutual inductance, the results have been verified by electro-magnetic (EM) codes. Moreover, by taking a waveguide-coupler with an inserted-post for instance, further EM simulations have demonstrated that the tunable coupler can achieve a balance among a proper coupling range, a small frequency offset and an acceptable field-asymmetry. The proposed analysis methods can be extended to provide preliminary guides in designing adjustable RF couplers.