Higher-Order Modes in the BNL Energy Recovery Linac: Measurement and Waveguide Coupler Design

Abstract

As in many resonators, higher-order modes (HOMs) occur in particle accelerator radio frequency (RF) cavities. The excitation of these harmonics causes additional heat loads in cryogenic systems and can result in problematic single bunch and multi-bunch effects. It is therefore critical to understand HOM prevalence and structure, and devise a method of extracting their power from the accelerator cavity. Research on these topics is ongoing in the Collider-Accelerator Dept. at Brookhaven National Laboratory (BNL), where the construction of an Energy Recovery Linac is underway. An HOM coupler implementing a dual-ridge waveguide has been designed for use on the next-generation ERL cavity. A full description of the design from its early stages is given, along with simulated analysis of its performance. Also, to better understand resonances occurring in the existing ERL, a measurement routine was developed to characterize and identify HOMs using a bead-pulling technique on a copper prototype cavity. Initial results of these measurements show a good correspondence with the harmonics predicted by the simulation code CST Microwave Studio.