Measurement of octupole induced decoherence at CESR

The Landau damping of the transverse oscillations of a relativistic bunch can be enhanced by creating an amplitude-dependent betatron tune spread in the bunch. This tune spread can be created by introducing octupole magnets into the lattice. In order to further the understanding of this mechanism, the authors conducted experiments in the Cornell Electron Storage Ring (CESR) where the tune spread of the beam has been measured via measurements of the damping of the centroid motion of the beam after the beam has been kicked. The turn-by-turn transient response of the transverse motion to excitation by injection kickers was measured as a function of octupole current and excitation amplitude. The experimental data are shown to be in good agreement with particle tracking results and the theory of octupole-induced decoherence.<>