Modification and Reconstruction of the SKS Superconducting Magnet

The Superconducting Kaon Spectrometer (SKS) magnet, which was in operation at the K6 beamline in the North Counter Hall of the KEK 12-GeV proton synchrotron from 1991 to 2005, was modified and moved to the newly constructed K1.8 beamline in the Hadron Hall at the Japan Proton Accelerator Research Complex (J-PARC). We reused the original coil assembly which includes coils and vessels, with a Gifford-MacMahon (GM) cryocooler for shield cooling. We used three Gifford-MacMahon/Joule-Thomson (GM/JT) cryocoolers instead of the previous 300-W helium refrigerator. The cooling power of a GM/JT cryocooler is 3.5 W at 4.5 K and 50 Hz. We also installed a new GM cryocooler to cool the GM/JT insert ports. Conventional copper current leads were replaced with Bi2223 high-temperature-superconductor (HTC) leads. We also required a new GM cryocooler to cool the HTC current leads. In total, we installed three GM/JT and three GM cryocoolers. We cooled the reconstructed SKS superconducting magnet at the K1.8 beamline from room temperature to 4.3 K over 1.5 months, applying LN2 and LHe directly. We confirmed that the three GM/JT cryocooles maintained the LHe level in the steady state. Helium gas pressure within the helium vessel was 113 kPa absolute and the condenser temperatures of the three GM/JT cryocoolers were 4.35 K, 4.46 K, and 4.35 K, respectively. We also measured the rise in temperature of the HTC leads when the magnet was continuously excited at a current of 400 A. The temperature of the HTC leads rose to 41 K and attained saturation after 5 hours. This was sufficiently safe for the Bi2223 HTC leads. Furthermore, we confirmed that two GM/JT cryocoolers could maintain the steady state; however, for safety reasons, we decided to use three GM/JT cryocoolers in the case of continuous excitation at more than 350 A.