Measurement of Beam Bunch Length in the 1.3-GeV Electron Synchrotron

We observed the bunch structure of the circulating beam in the 1.3-GeV Electron Synchrotron (ES) for the first time. The ES operated at a repetition rate of 21 Hz and an rf frequency of 138 MHz. The bunch length variation during the rf acceleration was determined by measuring time distributions of photons produced by the synchrotron radiation. The observed lengths show a rapid decrease from 1.83 to 0.72 ns (FWHM) in the 2-5 ms time interval after the start of acceleration (energy range 0.1-0.25 GeV), and they are almost constant, 0.6-0.7 ns, by the end of acceleration (0.25-1.1 GeV). The behavior is well reproduced by a calculation taking into account the effects of the adiabatic damping, radiation damping, and radiation excitation.