Using a double-frequency RF system to facilitate on-axis beam   accumulation in a storage ring
  An on-axis injection scheme using a double-frequency RF system in a storage ring with small dynamic aperture is proposed. By altering RF voltages, empty RF buckets can be created which will be used for on-axis injection. After bunches are injected, a reverse RF voltage altering process is performed and the injected bunches can be longitudinally dumped to the main RF buckets. The scheme allows reaping the advantages of the on-axis injection while still performing accumulation. 
B. C. Jiang, Z. T. Zhao, S. Q. Tian, M. Z. Zhang, Q. L. Zhang
physics.acc-ph