Review of coupled betatron motion parametrizations and applications to strongly coupled lattices

Abstract

The coupling of transverse motion is a natural occurrence in particle accelerators, either in the form of a residual coupling arising from imperfections or originating by design from strong systematic coupling fields. While the first can be treated perturbatively, the latter requires a robust approach adapted to strongly coupled optics and a parametrization of the linear optics must be performed to explore beam dynamics in such peculiar lattices. This paper reviews the main concepts commonly put forth to describe coupled optics and clarifies the proposed parametrization formalisms. The links between the generalized Twiss parameters used by the different approaches are formally proven, and their physical interpretations are highlighted. The analytical methods have been implemented in a reference Python package and connected with a ray-tracing code to explore strongly coupled lattices featuring complex 3D fields. Multiple examples are discussed in detail to highlight the key physical interpretations of the parametrizations and characteristics of the lattices.