Study of beam property on a storage ring operated with negative momentum compaction factor

To investigate variations of the bunch length and the energy spread of the electron beam on a storage ring, which are very important beam properties for storage ring free electron lasers, we have measured longitudinal electron distributions and spectra of spontaneous radiation from an optical klystron on the UVSOR storage ring operated with both signs of negative and positive momentum compaction factors. Significant differences in the bunch lengthening and the increase of the energy spread with the beam current have been found between two operations. In a case of the negative momentum compaction, a threshold beam current for the longitudinal microwave instability was observed to be very low, meanwhile “the bunch shortening” was observed.