Analysis of the end regions of the CERN lead-ion 4-rod RFQ

A 4-rod-like Radio Frequency Quadrupole for the new CERN lead-ion injector will be built at the Laboratori Nazionali di Legnaro, Italy. By means of 3D simulations with MAFIA codes and measurements on a cold model, an asymmetry was found in the gap between the end plate and electrodes, yielding a non-zero longitudinal component E_z on the axis of the RFQ and therefore a potential barrier in the gap. In this paper an explanation is given for this fact and possible solutions are proposed for end cell tuning, which allow to eliminate the undesirable E_x component and also to obtain good inter-electrode voltage distribution along the structure. Results of MAFIA calculations and cold model measurements are shown