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 ${\ensuremath{\zeta}}_{a}$ that appears in the normal mode decomposition of the 1-turn transfer matrix. ${\ensuremath{\zeta}}_{a}$ 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 ${\ensuremath{\zeta}}_{a}$ introduced by one of two pairs of rf cavities with the second, and (iii) zero dispersion in rf cavities, eliminating ${\ensuremath{\zeta}}_{a}$ entirely. Additionally, intrabeam scattering is evident in our measurements of beam size vs rf voltage.