Lawrence Berkeley National Laboratory

The nonlinear fields in insertion devices can have a significant impact on the beam dynamics in storage rings. Various tools have been developed which allow the dynamical effects of a wiggler to be predicted, based on a detailed model of the magnetic field. The wigglers in the KEK-ATF have recently been commissioned, and provide an opportunity for benchmarking some of the analysis tools. We report on initial studies of the nonlinear effects of the KEK-ATF wigglers, based on studies of the change in betatron tunes with change in orbit through the wigglers, and compare the results with the predictions based on detailed field models.