  We report on extensive measurements at the Cornell Electron-positron Storage
Ring of electron-cloud-induced betatron tune shifts for trains of positron
bunches at 2.1 and 5.3 GeV with bunch populations ranging between 0.64x10^10
and 9.6x10^10. Measurements using a witness bunch with variable distance from
the end of the train and variable bunch population provide information on cloud
decay and cloud pinching during the bunch passage. We employ Monte Carlo
simulations of the reflection and absorption of synchrotron radiation photons
to determine the pattern of absorption sites around the circumference of the
storage ring. The Geant4 simulation toolkit is used to model the interactions
of the photons with the beampipe wall and determine the production energy and
location distributions of the photoelectrons which seed the electron cloud. An
electron cloud buildup model based on fitted ring-averaged secondary-yield
properties of the vacuum chamber predicts tune shifts in good agreement with
the measurements.
