Bunch lengthening observed using real-time bunch-length monitor in the TRISTAN AR

A new type of a bunch length monitor, detecting two frequency components of a beam spectrum, has been installed in the TRISTAN Accumulation Ring (AR). Calibration of the monitor was carried out with the calculated natural bunch length at a low beam current. Bunch lengthening was measured as a function of the beam current. When the beam current increased, a sudden blowup of the bunch length was observed together with strong coherent oscillations. It was also observed that the blowup exhibited hysteresis when the beam current decreased, which suggested a strong nonlinear force in the bunch lengthening process.<>