EXPERIMENTAL STUDY OF HORIZONTAL-LONGITUDINAL ∗

In storage rings, the presence of horizontal dispersion in the RF cavities introduces x-z coupling. The result is that the beam is skewed in the horizontal-longitudinal plane. The skew angle is proportional to the V15 term of the 6 × 6 coupling matrix which is proportional to the RF cavity volt- age and the horizontal dispersion in the cavity. Here we report experiments at CesrTA where x-z coupling was ex- plored using three distinct lattice configurations with dif- ferent V15 coupling terms by measuring the horizontal pro- jection of the beam with a beam size monitor as the RF voltage is varied. The first lattice has about 1 md isper- sion in the RF cavities, resulting in a V15 term at the beam size monitor source point corresponding to 16 mrad x-z tilt. In the second, the V15 generated in one pair of cavities is compensated at the second pair by adjusting the hori- zontal betatron phase advance between the cavity pairs. In the third, the optics are adjusted so that the RF cavity re- gion is dispersion-free, eliminating the coupling entirely. Additionally, intra-beam scattering is evident in our mea- surements of beam size vs. RF voltage.