Design of HOM Damped Multi-Cell SRF Cavities for CW Operation in High Current Storage Rings

Currently, superconducting RF (SRF) systems for high-current storage rings are generally limited to low-frequency, moderate voltage, and single-cell cavities. For a new class of cavities to be used in longitudinal beam phase-space manipulation, high-voltage third harmonic multi-cell cavities are required, resulting in very challenging impedance considerations and higher-order mode (HOM) powers of the order of several kW per cavity. Thus, the cavity design requires far more attention on the HOM spectrum to be off-resonance with circulating beam harmonics. Special techniques have been developed to analyze the HOM spectrum and damping beyond the standard frequency range, which typically lies at a few GHz, as required by the VSR Demo project. Within the presented work, a four-cell 1.5 GHz cavity is designed including end-groups with multi-waveguide damping for a space-saving design capable of handling over 2.5 kW of HOM power per cavity. These cavities are designed for high-voltage operation with beam currents of at least 300 mA. Prototype systems are now in production. This article provides an overview of the advanced techniques for SRF cavity design and their application to tailoring the HOM spectrum and its application for the VSR Demo project.