Bunch lengthening study for CESR III

The RF system of the Cornell Electron Storage Ring (CESR) is being upgraded in four stages from a copper cavity configuration to a superconducting cavity configuration. The copper cavities, consisting of a total of 20 cells, are being replaced by 4 single cell superconducting cavities. The reduction in total number of cells will allow increasing the total current to one Ampere, and the higher total field allows a shorter bunch length of 1.3 cm. While this charge will provide the necessary accelerating voltage and will lower the loss parameter of CESR, it will also decrease the total capacitive wake potential of the storage ring which is expected to cause more rapid increase in the bunch length with the current. Measurements from two intermediary configurations are analyzed, one with 10 copper cells and 2 superconducting cells (stage 2), and one with only 3 superconducting cells (stage 3). The bunch length as a function of current was calculated from the measured bunch spectrum at bunch lengths approaching 1.3 cm. Projection of bunch lengthening in the future and its impact on luminosity are estimated. (author)