RF Phase Shake and Counterphasing at Phase Transition

The KEK 12-GeV PS suffers from noticeable beam loss just after the phase transition when the beam intensity exceeds 2 × 1012 ppp. Longitudinal instability has been suspected to be the cause of the loss. (We have observed microwave disturbance in the beam as well as very turbulent coherent oscillations and subsequent large emittance blow-up.) Artificial bunch dilution before the transition has been proposed to moderate these dangerous instabilities and to prevent phase transition beam loss. Two RF manipulations have been tested; one is RF phase shake and the other is RF cavity counterphasing. The "phase shake" modulates the RF phase and the "counterphasing" lowers the net accelerating voltage. The phase shake has remarkably reduced the beam loss when the phase modulation frequency is much higher than the synchrotron frequency. Counterphasing has exhibited similar effect. Both operations can decrease the beam loss when they produce bunch flattening before the transition crossing. These methods contributed to the KEK 12-GeV PS record intensity 4 × 1012 ppp (4.4 × 10ll p/bunch).