Measurement of the nonlinear momentum compaction factor in RHIC

During gold beam acceleration in the Relativistic Heavy Ion Collider (RHIC), the transition energy has to be crossed at /spl gamma//sub t/ /spl ap/ 23. Since close to /spl gamma//sub t/ the longitudinal slip factor /spl gamma//sub t//sup -2/ - /spl gamma//sup -2/ becomes very small, the longitudinal momentum compaction factor /spl alpha//sub 1/ becomes significant. Measurements of this factor using longitudinal phase space tomography will be reported.