Cavity perturbation measurements of the shunt impedance of a tuned resonator with higher-order-mode damping

• Main Author: Joffe, David Noah
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/5453

The performance of a prototype higher-order-mode (HOM) damper for the proposed KAON Factory Booster Ring was determined by measuring the shunt impedance of a variable frequency tuned resonator with higher-order-mode damping at both the fundamental and higher resonant frequencies using the cavity perturbation technique. This technique consists of pulling a spherical dielectric bead across the cavity gap and recording the change in the frequency of the cavity resonance at each point along the path. Using the frequency shift values, the electrical properties of the bead, and the quality factor and frequency of the non-perturbed cavity resonance, one can find the relative electric field strengths along the path across the cavity gap, and these may be integrated to give the shunt impedance of the structure. An automated system for performing cavity perturbation measurements was constructed with a single computer both controlling the pulling of the bead and recording the properties of the cavity resonance. This system was tested for accuracy on a cavity with no damper attached, and the effects of different cavity configurations were examined. The impedances of two closely related designs of prototype higher-order-mode dampers were measured using the automated system. The first damper consisted of a five-element high-pass filter made up of three washer-like structures encircling the cavity axis, and the second damper consisted of a seven-element filter similar to the first but with the final two elements being made up of an additional washer and pipe structure (the horn) which were added in order to shield the structure from the beam. The two structures were found to have nearly identical properties in terms of their shunt impedances and abilities to damp higher-order-modes in the cavity. Both damper designs were found to lower the shunt impedance of the cavity gap by less than 10% at the fundamental frequency, and by more than 99% for all higher-order-modes up to the 700 MHz range. Above 700 MHz the dampers were found to lower the shunt impedance of TEM modes by about 90%, and of the first TE mode by 50%. In all cases the shunt impedances of the system at frequencies above the fundamental were below 500 Ω, well below the 1 kΩ maximum allowable in order to maintain a stable beam [1, 2].