Development of a 15 T 52 mm Bore Cryocooled Superconducting Magnet

Cryocooled superconducting magnets are becoming popular due to their simple operation as compared to conventional liquid helium cooled magnets. However, for higher fields such as those greater than 13 T, the cryocooled superconducting magnet has not yet become popular, because it is difficult to design and manufacture such a magnet since the critical current is markedly reduced at higher fields and higher temperatures. We have developed a 15 T cryocooled superconducting magnet, which will be among the highest field magnets in operation cryocooled using a GM cryocooler. The magnet was designed using the sophisticated technology of a cryocooled superconducting magnet. The magnet has a 52 mm room temperature bore with overall dimensions of 820 mm in diameter and 680 mm in height. This magnet is designed so as to enable both simple operation and installation. Therefore, the magnet is cooled with a single 1 W GM cryocooler. Additionally, all coils are connected in series and charged with a single power supply. In order to optimize the magnet size and cooling power of the cryocooler, the highest operating temperature is set to 5.2 K. The magnet consists of one Nb3Sn coil with five conductor grades and one NbTi coil with five conductor grades. The total amount of Nb3Sn conductor is 43 kg and that of NbTi conductor is 63 kg, generating 7 T and 8 T, respectively, at 121 A. The magnet was cooled down to 4.5 K within 80 h using a thermal switch cooling accelerator. It was then charged to 15 T in 30 min after three training quenches and again charged slowly to 16.1 T without quenching. During the charging at a rate of 30 min/15 T, the temperature of the magnet reached 5.1 K, which was the same as the calculated value. While pausing at 15 T, the cryocooler was stopped to induce intentional quenching. It quenched after 2 min at 5.3 K. The magnet is fully protected with a cold diode circuit.