Design and test of a 0.4-m long short model of a conduction-cooled superconducting combined function magnet for a compact, rapid-cycling heavy-ion synchrotron

A project developing a compact, rapid-cycling heavy-ion synchrotron that can be accommodated in an ordinary hospital building has been initiated in the National Institutes for Quantum Science and Technology (QST). One of the most effective ways to achieve the compact synchrotron is to enhance the magnetic field by replacing normal conducting magnets with superconducting magnets. Thus, a concept design of a superconducting bending magnet (SCBM) equipped with a main dipole coil and an additional quadrupole coil has been proposed to provide the field of 3.5 T and the field gradient of 1.5 T/m for this advanced heavy-ion synchrotron. The SCBM requires cyclic operation at a ramp rate of 0.64 T/s, and a "dry"system in which the magnet is conduction-cooled from the Gifford-McMahon (GM) cryocoolers via the high conductivity thermal path to facilitate the SCBM use. It is a non-trivial problem that the AC loss generated at such a ramp rate results in a huge temperature rise during the cyclic operation. To validate the thermal characteristics as well as the feasibility of the magnet fabrication, in this paper we develop a 0.4-m long straight short model of the SCBM. The short model is enclosed in a cryostat and conduction-cooled with two GM cryocoolers via pure aluminum thermal paths. The nominal field of 3.5 T and the nominal field gradient of 1.5 T/m are reached after a total of 39 training quenches. We measure the central magnetic field and field gradient with five cryogenic hall sensors at the cryogenic temperature. After that, we operate this short model at the maximum ramp rate of 0.7 T/s with only one-cryocooler operation until the system reaches the quasi-static state to test the thermal stability and measure the AC loss by using the cooling capacity of the GM cryocooler. The paper describes the design and the test results of the short model including the magnetic field measurement, the AC loss measurement and the quench behavior.