Measurement and compensation of horizontal crabbing at the Cornell Electron Storage Ring Test Accelerator

Abstract

In storage rings, horizontal dispersion in the rf cavities introduces horizontal-longitudinal (xz) coupling, contributing to beam tilt in the xz plane. This coupling can be characterized by a “crabbing” dispersion term ζ_{a} that appears in the normal mode decomposition of the 1-turn transfer matrix. ζ_{a} is proportional to the rf cavity voltage and the horizontal dispersion in the cavity. We report experiments at the Cornell Electron Storage Ring Test Accelerator where xz coupling was explored using three lattices with distinct crabbing properties. We characterize the xz coupling for each case by measuring the horizontal projection of the beam with a beam size monitor. The three lattice configurations correspond to (i) 16 mrad xz tilt at the beam size monitor source point, (ii) compensation of the ζ_{a} introduced by one of two pairs of rf cavities with the second, and (iii) zero dispersion in rf cavities, eliminating ζ_{a} entirely. Additionally, intrabeam scattering is evident in our measurements of beam size vs rf voltage.