Longitudinal parameters measurement at BEPC II storage rings

A new upgrade to BEPC II has been deployed to improve the luminosity and maximum energy. After the upgrade, the maximum beam energy will reach to 2.8 GeV, and the optimized luminosity at 2.35 GeV will be tripled through squeezing the bunch length and the vertical beta function at the interaction point beta(y)*. Therefore, one more RF cavity will be added to each ring to increase the RF voltage to 3.3 MV. In order to study the feasibility of this upgrade and the longitudinal beam characteristics after the upgrade, some beam-based experiments were carried out at the electron and positron rings of BEPC II. Bunch length and its variation with RF gap voltage are measured, shows that it can be squeezed by approximately 30% at 3.3 MV compared to the normal 1.65 MV operation voltage. Bunch synchronous phases at different RF voltages are also obtained by fitting the variation of bunch centroid with RF voltage at low current. The longitudinal broadband coupling impedance is measured with streak camera, real part impedance is obtained from synchronous phase shift with current, and the imaginary part from bunch lengthening with current. Energy loss per turn at 2.474 GeV is 363.2 +/- 14.5 keV by measuring synchrotron tune varies with RF voltages.