Study of uneven fills to cure the coupled-bunch instability in SRRC

Abstract

The performance of the 1.5-GeV storage ring light source TLS in SRRC has been limited by a longitudinal coupled-bunch beam instability. To improve the performance of the TLS, the beam instability has to be suppressed. One possible way considered for the TLS to suppress its coupled-bunch instability uses uneven filling patterns according to the theory of Prabhakar (2000). By knowing the harmful high-order-modes (HOMs), a special filling pattern can be designed to utilize either mode coupling or Landau damping to cure the beam instability. In TLS the HOMs are contributed from the Doris RF cavity installed in the storage ring. The HOMs of a 3-D Doris cavity was numerically analyzed. The filling patterns with equal bunch current according to theory had been calculated to cure the most harmful HOM. A longitudinal particle tracking program was used to simulate the coupled-bunch beam instability with both the uniform filling and the specially designed filling. A filling pattern with unequal bunch current was also studied. The results of the simulation were discussed and compared to the theory.