A cascade of two ring cyclotrons for the RIKEN RI beam factory are being constructed to provide primary heavy ions, up to uranium ions, with energies exceeding 100MeV/nucleon. The one in the second stage is a large-scale superconducting ring cyclotron with six sector magnets. The concept of the superconducting ring cyclotron design and status of the construction of its full-scale prototype are described. Compact superconducting cyclotrons now operating around the world are also introduced.
The reversibility of the magnetic behaviour of superconductors assured by the Meissner effect allows the application of thermodynamics to analyse the macroscopic properties of superconductors. In this second lecture, the fundamentals of electromagnetism and thermodynamics will be reviewed and applied to the analysis of superconductive phenomenon.
Some strands may carry large current in a multi-strand cable consisting of strands having a highly resistive layer, such as chrome plating, among the strands as a result of non-uniform current distribution. We analytically studied the characteristics of this current after normal transition using a distributed model circuit. The parameters used in this calculation are those of the US-DPC coil, which first exhibited instability due to current imbalance, so-called ramp-rate limitation. Thus, the conductance among strands and inductance of unit length loop and length of the conductor are assumed to be 1-100kS/m, 0.5μH/m and 150m, respectively. The calculation results show that the current of the strand reaching a normal state decays in order of 10-100ms. It was also found that the current transfer from the strand having normal resistance to the other superconducting strands does not take place at the end of the conductor within a few seconds after normal transition when normal resistance appears at the center of the conductor. In this case, the current is transferred among the strands without any effect from the boundary as well as the infinitely long conductor. Consequently, the current increases by return of the current which is transferred to the other strands if normal resistance decreases at the center of the conductor. These results indicate that there is a possibility of conductor quench due to the normal transition of several strands which carry the large current and initially transfer to normal state in a conductor consisting of strands that have highly resistive layers among them.
Superconducting sector magnets are to be the main components of the superconducting ring cyclotron for the RIKEN RI beam factory. A full-scale prototype sector magnet is being developed and constructed. One of the critical points of the design is how to support the huge magnetic force exerted on the superconducting main coil. A cold-pole arrangement is adopted as the solution to this problem. Unbalanced magnetic forces were measured using 1/6-scaled-Cu/Fe magnets. A cryogenic stabilizing method is applied to make the main and trim coils quench-free. The whole structure, magnetic force and its support structure, quench protection, eddy current loss in the cold-pole, and helium cooling system are described.
Superconducting sector magnets are to be the main components of the superconducting ring cyclotron for the RIKEN RI beam factory. The sector magnet has superconducting main and trim coils. A cryogenic stabilization method is applied to both coils in order to make them quench-free. Both superconducting coils and their wires, design of stabilization, quench simulation, and measurement of the stabilized currents using small test coils are described.