Thermal Design and Test Results of the Superconducting Magnet for a Compact Heavy-Ion Synchrotron

A project to develop a next-generation small facility for heavy-ion radiotherapy called Quantum Scalpel is underway at the National Institutes for Quantum Science and Technology (QST), having started in 2016. One of the aims of this project is to downsize the synchrotron by applying superconducting technology. With a view to accomplishing this, we have been developing a superconducting magnet for a compact heavy-ion synchrotron in collaboration with QST. This superconducting magnet can generate a dipole field of 3.5 T at an operating current of 265 A. It adopts conduction cooling with GM cryocoolers and is designed to be able to raise the magnetic field from 0.3 T to 3.5 T in 5 seconds. Such high-speed excitation causes large AC loss in the superconducting coil. In this paper, the thermal design result, including countermeasures for this AC loss, is described. In addition, a short model with the same cross section as the designed coil was fabricated and an excitation test was carried out. As a result of the test, it was confirmed that the magnet can generate the designed maximum field of 3.5 T in 5 seconds without quench. And it was also confirmed that there was a difference of about 0.5 K between the measurement results and the thermal calculation results of the pattern test. The design of a full-scale magnet is almost completed, and the results of the present work shows that the conduction-cooling method using 4K GM cryocoolers is applicable to the superconducting magnet for the synchrotron.