I. Introduction
Particle radiotherapy treats cancer by irradiating tumors with accelerated ions, and it has been shown to have a high curative effect and can also reduce the burden on patients. For these reasons, particle radiotherapy is becoming more widespread, and the total number of patients treated has been increasing. However, to accelerate ions, an accelerator system such as a synchrotron is required. Such systems are too large to install in general hospitals, and therefore, it is necessary to reduce the size of particle radiotherapy systems. One way to do this is to reduce the bending radius by increasing the magnetic field applied to the charged ions. Superconducting technology is suitable for this purpose, and LTS magnets designed to reduce the size of a rotating gantry, which is part of the irradiation equipment, have been developed in collaboration with NIRS [1]. To achieve further size reductions, high-Tc superconducting (HTS) magnets with higher magnetic fields are promising. And various research groups are working on development of HTS accelerator magnets [2]–[15].