Stabilization of high intensity beam by higher harmonic cavity in electron synchrotron

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A 1.2 GeV Stretcher-Booster (STB) ring at Laboratory of Nuclear Science(LNS), Tohoku University, has been mainly operated for experiments of nuclear physics. The STB is not the light source ring but also has storage ring-like function. One of the many issues limiting performance of the STB ring is supposed to be strong longitudinal coupled-bunch instability. Actually very strong synchrotron oscillation signal have been observed around 3.6 GHz with a threshold current of about 10 mA. In order to suppress the instability, applicability of a third-harmonic Landau cavity has been studied.For the full energy operation, analytic modeling and simulations were first performed. The results show that if three harmonic cavities are used, near-optimal bunch-lengthening may be obtained at the maximum ring current of 100 mA. For injection energy of 200 MeV, almost same analysis were performed and showed that increased energy spread from the instability or lost macro-particles occurred for nearly all currents and tuning angles. Although the simulation study indicates that more cavities are required to suppress the instability, one harmonic cavity was manufactured and tested in the STB as the first step of the study. The very strong synchrotron oscillation signal due to the coupled-bunch instability was drastically damped by the harmonic cavity at a specific tuning angle. However, it is not considered that this suppression is due to the usual Landau damping, since a significant voltage is never excited in the harmonic cavity because of the large tuning angle. A tracking simulation using rigid-bunch model was performed to understand this situation. The result showed that a higher order mode of harmonic cavity can suppress the instability. For the lower energy operation, the effect of harmonic cavity might be also interesting, but no obvious improvement has been seen yet. Toward the future development, a study of the harmonic cavity in acceleration process will be also progressed.