Kelly, M.P. JACoW Geneva, Switzerland Superconducting cavities are attractive for light source storage rings both as main rf cavities and as harmonic cavities due to their compact size and relatively straightforward handling of higher order modes which are a serious technical issue in nearly all new ampere-class machines. A 1.4 GHz superconducting bunch lengthening cavity for the Advanced Photon Source Upgrade project and ancillary systems including a high-power cw variable coupler and beamline higher order mode damper have all been prototyped and tested. Speaker - Sang-hoon Kim