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Suppression of Longitudinal Coupled Bunch Instability by Harmonic Cavity in UVSOR Electron Storage Ring

Akira Mochihashi, Masaki Fujimoto, Jun Hasegawa, Kenji Hayashi, Masahito Hosaka, Masahiro Katoh, Yoshifumi Takashima, Schaa, Volker RW (Ed.), Satogata, Todd (Ed.)
In the UVSOR electron storage ring, which is dedicated for a VUV synchrotron radiation light source, a longitudinal coupled bunch instability (LCBI) is observed in multi-bunch operation. To suppress the LCBI, we routinely operate a third harmonic cavity (HCV) in a passive mode. By properly tuning HCV, the instability is almost completely suppressed. Because of the lower beam energy (750 MeV) and brilliant beam emittance (17.5 nm-rad), the Touschek effect becomes severe in the UVSOR. To
more » ... UVSOR. To guarantee enough beam lifetime, we also apply HCV for lengthening the bunch. The suppression of the instability and increasing the beam lifetime are crucial benefits by HCV for the UVSOR. However, not only the origin of the LCBI but also the Landau damping effect by HCV has not been understood systematically yet. We have noticed that one of the HOMs at HCV itself could cause the LCBI and observed the behavior of the instability, which strongly depends on the beam current. From the experiment we have discussed the cause of the instability with the HOM theory. We have also tried to observe synchrotron tune spread and discussed a competition between the Landau damping and the instability growth.
doi:10.18429/jacow-ipac2018-thpak031 fatcat:xou4ceqspjf23gpvnvam2va5gy