Pattern Excitation Tests of the Short Model Superconducting Magnet for a Compact Heavy-Ion Synchrotron

A project to develop a compact heavy-ion radiotherapy system is underway at the National Institutes for Quantum Science and Technology (QST). One of the aims of this project is to downsize the synchrotron by applying superconducting technology, and we have been developing a superconducting magnet for this system in collaboration with QST. This superconducting magnet was designed to generate a dipole field of 3.5 T at a rapid cycle of more than 0.64 T/s by applying the conduction cooling method and using 4 K Gifford-McMahon (GM) cryocoolers. Such high-speed excitation generates large AC loss, and this heat generation is one of the major issues in this development. Therefore, we fabricated a short straight model magnet and carried out pattern excitation tests to verify the thermal design. This model magnet has the same cross section as the full scale magnet, but its length is shortened by 1/3. In the pattern excitation test repeated 50 times, the maximum coil temperature was calculated to be 4.03 K and measured to be 3.82 K. The quench occurred at 5.9 K, which is near the temperature of 5.76 K where the load factor is expected to be 100% in overload test. This pattern excitation test results shows that the coil performance was obtained as expected. Also, the consistency between the calculations and the test results confirmed that the conduction cooling method using 4 K GM cryocoolers is applicable to the full scale superconducting magnet for the synchrotron. In this paper, the excitation test results are reported.