Prediction and suppression of two-point 1st order multipacting

A method is developed to facilitate the prediction of the power levels for which two-point multipacting occurs and their corresponding locations for coaxial power couplers. The method is cross-checked with particle-in-cell 3D simulations and RF testing of cyromodules at the Facility for Rare Isotope Beams (FRIB) [1], where multipacting barriers were found during RF conditioning of Half-Wave Resonators (HWR) and the high RF power input couplers. Measurements with other types of coaxial power couplers also validate the method of prediction. A feasible geometrical optimization for the FRIB coaxial coupler is presented, which mitigates the multipacting according to this method. In the appendix, we also show that cyclotron model is suitable for two-point 1st order multipacting in an elliptical cavity, Quarter Wave Resonators (QWR) and HWR.