Improvement of the strongQstrong-factor measurement in RF cavities.pdfVIP

BNL-98926-2012-IR C-A/AP/#472 November 2012 Improvement of the Q-factor measurement in RF cavities Wencan Xu, S. Belomestnykh, I. Ben-Zvi, H. Hahn Collider-Accelerator Department Brookhaven National Laboratory Upton, NY 11973 Notice: This document has been authorized by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The United States Government retains a non- exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this document, or allow others to do so, for United States Government purposes. Improvement of the Q-factor measurement in RF cavities Wencan Xu#,1, S. Belomestnykh1,2, I. Ben-Zvi1,2, H. Hahn1 1) Collider-Accelerator Department, Brookhaven National Lab, Upton, NY 11973, USA 2) Physics Astronomy Department, Stony Brook University, Stony Brook, NY 11794, USA Abstract The Q values of Higher-order-modes (HOMs) in RF cavities are measured at room temperature with the 3 dB bandwidth reading by a network analyzer. The resonant curve distortion is created by the resonance splitting due to the ellipticity caused by manufacture tolerance and RF ports. Therefore, the measured Q values are usually lower than the simulated or theoretical Q values. In some cases, even only one mode’s Q can be measured with the 3 dB method. There may be two reasons for this happening. One is that only one mode was excited and the neighbor splitmode was close to 90º polarized; the other reason is that the resonant curve of one mode was di