Abstract
The purpose of this study is to develop the NbTi superconducting magnet system directly cooled by a 4-K two-stage G-M (Gifford-McMahon) refrigerator. The performance test has been done by the system which had the following characteristics. 1) The coil has been made by multifilamentary NbTi conductor of 0.249mm in diameter. The coil constant is 0.233T/A at the center field. Therefore a high magnetic field can be generated by a small excitation current. 2) The Bismuth oxide superconductor has been used as a portion of current lead to decrease the Joule-heating along the lead. 3) The 2nd-regenerator material of Er3Ni has been used for the G-M refrigerator to give a very low operating temperature. The cool-down time from room temperature to 3.9K was about 6 hours. The NbTi magnet was operated at 3.9K. The central magnetic field of 7.3T and the maximum field of 7.6T was generated at a quench current of 31.1A. This data agrees well with the critical state of the NbTi conductor. The continuous operation at 30.8A has been obtained reproducibly.
