Thermal Stability of Conduction-Cooled HTS Magnets for Rotating Gantry

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Carbon ion cancer therapy is becoming more widespread due to its high curative effects and low burden on patients. In particle cancer therapy, it is desirable that the treatment beam irradiates the tumor from different directions in order to reduce the dose on normal cells. A rotating gantry is a suitable apparatus for meeting this requirement. Since the irradiation device orbits a patient inside the gantry, the beam can irradiate the tumor from any direction without changing the posture of the patient. Rotating gantries are already commonly used for proton cancer therapy. On the other hand, those designed for carbon ion cancer therapy have not yet been adopted because they are too large and heavy for installation in general hospitals. An R&D project to reduce the size of a rotating gantry for carbon ion cancer therapy by applying high-temperature superconducting (HTS) magnets is now in progress. It is difficult to use a coolant for the magnets mounted on the rotating gantry because they are rotating. Therefore, conduction cooling should be employed. On the other hand, the magnetic field generated by the magnets of the rotating gantry should be changed depending on the energy of the carbon ion beam in raster-scanning irradiation, and there is some possibility that the carbon ion beam will collide with the coils. This causes anomalous thermal inputs, such as those due to ac loss and beam loss, resulting in the problem of poor thermal stability of the conduction-cooled HTS coils heated by such thermal inputs. Therefore, it is important to estimate the thermal stability of the conduction-cooled HTS coils correctly. In this paper, a saddle-shaped HTS coil was designed, and the thermal runaway current of the HTS coil was numerically simulated.