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Transverse sawtooth instability observed in photon factory advanced ring

This paper reports a curious instability observed at the Photon Factory Advanced Ring for pulse X-rays (PF-AR). In a low bunch current, the horizontal betatron oscillation induced by an injection error was suppressed well by the beam feedback system. However, when the bunch current was high, the horizontal oscillation grew over several milliseconds and damped again like a sawtooth, after a injection process was finished. To investigate the mechanism of the sawtooth instability, we measured the horizontal beam size turn by turn as well as the dipole oscillation when the parameters (octupole magnets, the beam feedback and RF voltage) were changed. It was found that the transverse feedback for damping the dipole oscillation was suspicious for the instability, when the dipole oscillation and increase of the beam size coexisted