Transverse mode coupling instabilities

Abstract

Transverse mode coupling instabilities (TMCI) emerged between 1974 and 1980 as the main limitation of dense bunches in electron synchrotrons and storage rings. A two-particle model allows one to calculate the beam break-up (BBU) instability in linacs. Extending this to synchrotrons shows that the BBU instability is suppressed below a threshold intensity by synchrotron oscillations. The classical theory of head-tail modes, together with the general properties of coupling impedances, is used to show how single bunches become unstable when head-tail modes couple together: this is the TMCI threshold. Above threshold, observations in both proton and electron synchrotrons can be described by BBU theory.