Thermal characteristics of a helium-free superconducting magnet system for a fast-ramping heavy-ion synchrotron

A compact synchrotron has been proposed for the next generation heavy-ion therapy system at the National Institutes for Quantum Science and Technology (QST). In this synchrotron, four 90-degree superconducting bending magnets designed on the basis of the superconducting magnet for the rotating-gantry currently operated at QST are adopted to achieve a compact size. These NbTi superconducting bending magnets are conduction-cooled by several Gifford-McMahon (GM) cryocoolers and are designed to generate both the dipole field of 3.5 T and the quadrupole field of 1.5 T/m to operate at a ramp rate of 0.7 T/s. It is a challenge to remove the heat generated from AC losses at such a fast ramp rate, and keep the superconducting magnets from quenching during the continuous operation without using liquid helium. In this paper, we studied the feasibility for realizing a fastramping synchrotron by using a helium-free superconducting magnet system for heavy-ion therapy. Using the numerical models, we estimated the AC losses including eddy current loss in mechanical structures, iron loss and loss in superconductor to be about 212 J in a cycle duration of 20 s for each 90-degree superconducting bending magnet, and it was possible to eliminate that amount of AC losses by using GM cryocoolers. To validate the transient temperature rising during the continuous operation, a dynamic simulation was performed using the estimated AC losses as input, and the peak temperature at thermal equilibrium was evaluated to be lower than 5 K for a practical operation scenario of heavy-ion therapy irradiation.