Observation of Beam-Bunch Lengthening in the 1.3-GeV Electron Synchrotron

The beam-bunch lengths in the accelerating stage of the 1.3-GeV electron synchrotron were measured at the operating energies (E0) of 0.6-0.9 GeV. Both of the obtained behaviors: the bunch-length reduction from 1.0 ns (rms width) to 0.35 ns and the position variation on the rf phase, are described well by a tracking-simulation calculation. An intensity-dependent bunch lengthening: 0.19 ns at 0.5 mA and 0.30 ns at 100 mA for a voltage pattern of the rf accelerating field at E0=0.6 GeV, is observed. Neither the tracking calculation nor other known bunch-lengthening mechanisms can explain the data.