A cryogenic cooling system for a 25-T cryogen-free superconducting magnet (25T-CSM) was developed. The 25T-CSM consisted of 11-T high-Tc superconducting (HTS) coils and 14 T low-Tc superconducting (LTS) coils. The HTS coils, which had about 10-W AC-loss, were cooled to about 10 K by two GM cryocoolers and a helium gas circulation unit. In this system, the gas flow rate had an optimized value and was controlled by a mass flow controller. The LTS coils were cooled to about 4 K by two GM/JT cryocoolers. Each GM/JT cryocooler had two sets of precooling gas lines. These pre-cooling lines were cooled by two single-stage GM cryocoolers and the HTS coil cooling system via a heat exchanger, between the HTS gas circulation line and the LTS cool-down line. As a result of cooling test of the 25T-CSM, the LTS coils and HTS coils were cooled from room temperature to 4.3 K and 4.6 K within 164 h (about seven days), respectively. During the excitation test of the HTS coils, their maximum temperature increased to 7.6 K, which was still sufficiently low.
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