Helium Transfer Line With Conduction-Cooled Nb-Ti Superconducting Wires For Comet Muon Transport Solenoid

Cryogenic system of the superconducting magnet for COMET (COherent Muon to Electron Transition) experiments has been constructed in the J-PARC since FY2013. The cryogenic system can be roughly divided into superconducting magnet, refrigerator system and inter-connect system. The refrigeration system is composed of cold box and warm screw compressor system which are located on the surface. On the contrary, magnet system will be installed in the underground. Therefore the inter-connect system is mainly composed of current lead box and transfer line, which are regarded as a system for connecting between ground facility and the underground superconducting system. Since the superconducting magnet is cooled by two-phase flow helium at around 4.5 K, the transfer line has an adiabatic structure with shield line of around 50 K. In addition, conduction cooled superconducting cables are laid in parallel with the two-phase flow cooling line. In this study, simulation on the pressure drop of gas-liquid two-phase flow of helium is introduced to determine the cooling pipe diameters in the transfer line. Then the detail structures of the transfer line and cooling scheme of the superconducting cables optimized by several experiments will be presented.