Measurement of coherent synchrotron frequencies under conditions close to the Robinson limit at the Aichi Synchrotron Radiation Center

Past measurements* of coherent synchrotron frequencies at the Photon Factory storage ring revealed that the behavior of measured coherent frequencies could not be well explained with standard 4th-order characteristic equation under conditions close to the Robinson limit. To investigate whether similar phenomenon occurs in other storage rings, we measured the coherent synchrotron frequencies at a 1.2-GeV electron storage ring of Aichi Synchrotron Radiation Center as a function of the cavity voltage and the beam current. At beam currents higher than about 200 mA, we observed double peaks, one with a frequency higher than the incoherent synchrotron frequency and one with a lower frequency, that can correspond to two independent solutions of the 4th-order characteristic equation. Our preliminary analysis indicated that the frequencies of lower-frequency peak did not agree well with those predicted by the characteristic equation. We also observed that under a condition very close to the Robinson limit, the beam exhibited strong longitudinal coherent self-excited oscillation without beam dump. We present these measurement results and updated analysis.