LLRF controls in SuperKEKB Phase-1 commissioning
First beam commissioning of SuperKEKB (Phase-1), which is an asymmetry double ring collider of 7-GeV electron and 4-GeV positron beams, which had started from February, has been successfully accomplished at the end of June 2016, and the desired beam current for Phase-1 was achieved in both rings. This paper summarize the operation results related to low level RF (LLRF) control issues during the Phase-1 commissioning, including the system tuning, the coupled bunch instability and the bunch gap transient effect. RF system of SuperKEKB consists of about thirty klystron stations in both rings. Newly developed LLRF control systems were applied to the nine stations among the thirty for Phase-1. The RF reference signal distribution system has been also upgraded for SuperKEKB. These new systems worked well without serious problem and they contributed to smooth progress of the commissioning. The old existing systems, which had been used in the KEKB operation, were still reused for the most stations, and they also worked as soundly as performed in the KEKB operation.
A Three cavity system which suppresses the coupled bunch instability associated with the accelerating mode
If the beam loading on a large electron or positron storage ring is extremely heavy, the accelerating mode of a normal-conducting cavity gives rise to a coupled-bunch synchrotron instability. This instability can be cured by increasing the stored energy of a cavity system, for example by attaching a storage cavity to the accelerating cavity. However, three problems should be solved in order to put the system into practical use. First, the stored energy in the storage cavityshould be made significantly higher than that in the accelerating cavity. Second, the parasitic modes inherent to a coupled-cavity system causes another instability. Third, the accelerating field is unstable against beam loading if one uses a two-cavity system comprising only accelerating and storage cavities. In order to solve these problems simultaneously we propose the use of a three-cavity system with a coupling cavity in between the accelerating and storage cavities. In particular, a coupler installed to the coupling cavitycan efficiently damp both the 0 and 1r modes, while having a minimal effect on the accelerating 1r/2 mode. Furthermore, the excitation effect of one of the 0 and 1r modes on coupled-bunchoscillation is approximatelycancelledbythe damping effect ofthe othermode. A theoretical analysis using the coupled-resonator model shows that the system is quite promising.
DEVELOPMENT OF TIMING DISTRIBUTION SYSTEM WITH FEMTO-SECOND STABILITY
A timing distribution system with femto-second stability has been developed for the RF synchronization of accelerator and the laser synchronization of the pumpprobe experiments. The system uses a phase stabilized optical fiber(PSOF) and an active fiber length stabilization. The PSOF has 5ps/km/degC of the temperature coefficient. The active fiber length stabilization uses the phase detection of the round-trip sinusoidal wave and the deviation is compensated with the fiber stretcher. In this paper, we present the test results of the signal transmission using a 900m long PSOF for 2.856GHz and 1.3GHz RF signals. The stabilities of 2.856GHz and 1.3GHz signal transmission were 0.04degree(40fs) and 0.06degree(130fs), respectively.