Bunch length modulation in highly dispersive storage rings

Modulation of the bunch length in an electron storage ring appears with the combination of a high rf voltage derivative and large dispersion in the dipoles, producing drifting of the longitudinal phase plane along the closed orbit. The comparison is done between two different regimes: the one corresponding to high momentum compaction structures, in which the drifting in all dipoles has always the same direction, and the other one corresponding to low momentum compaction in which the drifting changes sign along the ring. Expressions for the longitudinal Twiss functions, energy spread and longitudinal emittance are given. The generalization to multiple rf systems is mentioned