Higher order mode damping in kaon factory RF cavities

Proposed designs for kaon factory accelerators require that the RF cavities support beam currents of the order of several amperes. The beam current has Fourier components at all multiples of the RF frequency. Empty RF buckets produce additional components at all multiples of the revolution frequency. If a Fourier component of the beam coincides with the resonant frequency of a higher-order mode of the cavity, which is inevitable if the cavity has a large frequency swing, significant excitation of this mode can occur. The induced voltage may then excite coupled bunch mode instabilities. Effective means are required to damp higher modes without significantly affecting the fundamental mode. A mode damping scheme based on coupled transmission lines has been investigated and is reported. Theoretical analysis and measurement indicate that this scheme is effective for damping higher-order TEM (transverse electromagnetic) modes without significantly affecting the fundamental mode