Higher order modes in the SRS 500 MHz accelerating cavities

As an important part of the beam instability studies on the SRS (Synchrotron Radiation Source), the higher-order modes in the accelerating cavities have been investigated. A two-dimensional model of the cavity has been used to determine higher-order modes up to 4 GHz using the URMEL-T code. The lower-frequency modes (with larger impedances) were then investigated on a three-dimensional model using the MAFIA codes. These predictions have helped in locating and exciting the strongest modes in tests on a spare SRS cavity, and perturbation techniques have been used to identify and measure these modes. Comparisons between computed results and perturbation measurements are made as well as comparisons with results from other laboratories using similar cavities. Computer calculations of the higher-order modes in the 500-MHz accelerating cavities has provided useful data on the impedance presented by these modes. Three-dimensional modeling is particularly useful and provides a great deal of information. However, aperture coupled cavities such as those used on the SRS provide broadband coupling into the waveguide, thus reducing the impedance of many higher-order modes. The use of low-pass absorptive filters in the feeder waveguide is seen to further reduce the Q values of many modes, indicating the value of such filters in higher-order mode suppression for aperture coupled cavities