The Interaction Between a Beam and a Layer of Microwave-Absorbing Material

Walter H. Hartung, Ph.D.

Cornell University 1996

Abstract

A phased luminosity upgrade of the CESR electron-positron storage ring is in progress. The upgrade program calls for the installation of superconducting radio-frequency (RF) cavities with strongly damped higher-order modes (HOMs). The cavity is designed to allow all HOMs to propagate into the beam pipe, where they are damped by a layer of microwave-absorbing ferrite. RF measurements with a copper cavity and loads made of a nickel-zinc ferrite indicate that the design gives adequate HOM damping. Because the absorbing layer is on the beam pipe, there is a direct parasitic interaction with the beam. To quantify this parasitic interaction, the complex permeability and complex permittivity of the ferrite were measured as a function of frequency. These results were used to predict the beam coupling impedance of the ferrite loads via numerical and analytic techniques. The predictive methods were checked using the pulse-on-a-wire method for measuring the coupling impedance. Beam stability predictions based on the calculated impedance indicate that there should be no beam instabilities due to the direct interaction between the beam and the ferrite layer for a 183-bunch beam with a total current of 2 A per beam. The average power dissipation in the ferrite, including both direct and indirect power transfer from the beam, is predicted to be 82 W/cm²with 2 A per beam. To test the predictions, beam measurements were done in CESR on a ferrite load of magnified coupling impedance. The beam-induced power dissipation in the ferrite layer was within a factor of 2 of the predicted value. It was difficult to see an effect on the beam due to the presence of the ferrite, a result that is consistent with our present understanding.

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