Model Study on the Reduction of RF Leakage in the TRIUMF Cyclotron

Since the 1 MW RF cavity of the TRIUMF cyclotron is contained completely inside the vacuum chamber, RF leakage from the resonator cavity into the tank vacuum chamber can cause undesirable heating and damage to uncooled structures and diagnostic equipment. The cavity traversed by the beam outside the resonators has been successfully simulated with the two dimensional code SUPERFISH and the quasi-three-dimensional code RFQ3D. Numerical calculations using these codes and precise 1:10 model measurements show that the power coupling between the operating mode of the resonant cavity and the parasitic modes of the beam cavity depends on the voltage distribution along the dee gap, as well as on the frequencies and the field patterns of the parasitic modes. These parameters are quite sensitive to the geometries and imperfections of the centre post, the flux guides an the cantilevered hot arms. Adjustments of these geometries have achieved a significant reduction in the RF leakage in the model.