Coupling and Decoupling in Storage Rings

Abstract

The accidental or intentional introduction of solenoids and skew quadrupoles into a storage ring couples the transverse motion unless the lattice is trimmed. Coupling can cause, for example, irreversible emittance blowup at injection (protons), resonance excitation, flat beam blowup in collision (electrons), and can destructively modify the beam-beam equations of motion. Apart from these effects themselves not many diagnostics are available to guide compensation attempts. This paper first develops a matrix description of coupling, then analyzes the compensation of a lattice insertion, and goes on to show how random errors can be handled close to a coupling resonance. The global decoupling of random errors in the SPS, achieved by the observation of eigenfrequencies on a spectrum analyzer, is described. Rough estimates show that global compensation usually makes further (local) compensation unnecessary.