Operational Experience of the Superconducting Ring Cyclotron at RI Beam Factory (RIBF) and Upgrade Plans for RIBF Accelerators

The RIKEN Radioactive Isotope Beam Factory (RIBF) has successfully operated since the first beam was produced at the end of 2006 (more than six years). The aim of this project is to access the unexplored regions on the nuclear chart, those far from β-stability. Continuous effort over the past six years has improved the performance of the RIBF accelerator complex. The world's first superconducting ring cyclotron (SRC) is the final booster in the RIBF accelerator complex; it is able to accelerate heavy ions of any element to a speed of approximately 70% of the speed of light. The ring cyclotron consists of six major superconducting sector magnets, each with a maximum field of 3.8 T. The total stored energy is 240 MJ. The overall size of the cyclotron is 19 m in diameter and 8 m in height with a weight of 8300 tons. Reliable standby power supply systems and accurate control systems are necessary for long-term, stable operation of the cryogenic cooling system. For these purposes, a 6.5-MW co-generation system was installed, and the control system for the cryogenic cooling system was improved on the basis of operational experience. Despite our efforts to achieve long-term, stable operation, we have had some problems such as failure of the He compressors and refrigerators and coil discharge. We learned that any small component could easily disrupt the entire cooling system. The two biggest problems were liquid He leakage thorough a crack in a feedthrough into the thermal insulation vacuum on November 8th, 2005 and oil contamination in the He refrigerator found in February of 2008. Overcoming such big problems has resulted in the present, stable operation of the cryogenic cooling system of the SRC. One option for upgrading the RIBF accelerator includes the construction of another SRC (SRC2) to skip the charge strippers (which decrease the beam intensity by approximately one fifth). Our study shows that the design of the sector magnets of the SRC2 is not much more difficult than that of the first SRC.