Correction of transverse coupling in a storage ring

At the Cornell Electron Storage Ring (CESR), a technique has been developed for local measurement and correction of the transverse betatron coupling. This is compared to the standard global coupling correction that minimizes the closest approach of the two normal mode frequencies as observed on a spectrum analyzer. In the proposed local coupling technique, the coupling of the transverse betatron modes is measured and corrected at about 90 different points in the machine. The measurements consist of experimental values for the normalized aspect ratio of the beam, the square root of the vertical to the horizontal emittance. DIMAT, a single-beam simulation program, has been modified to calculate coupling as a function of skew quadrupole strengths. A least squares fitting routine finds values of skew quadrupole strengths which reproduce the measured coupling data. These results are used to change the strengths of about 12 of the skew quadrupoles in CESR and so reduce the coupling. After two iterations, the normalized aspect ratio can be reduced to about 0.015