Design of a Large 5 T Superconducting Magnet for Polarized Target for JLAB Hall B 12 GeV Upgrade Clas12 Detector

Hall B at Jefferson Laboratory (JLAB) will need a 5 T, 78 cm bore polarized target magnet with a field uniformity of ΔB/B 0 -4 in a useful cylinder of the dimensions 0.04 m long × 0.02 m in diameter. The large magnet is designed with a superconducting coil that provides the solenoid with nearly perfect self shielding in order to reduce the fringe field at nearby photo multiplier tubes (PMTs) to less than 3.5 mT. Because the solenoid is also very close to the Clas12 Torus, the nearly perfect shielding provided by the self shielded solenoid greatly reduces force, field, and torque interactions with the six-coil Torus magnet. The solenoid coil consists of 18 coil modules which are made of coils mounted in aluminum plate discs. Each coil module consists of dual double pancake coils with main coils and shield coil partitioned into separate winding cavities in the aluminum plate discs to distribute and reduce radial hoop load and radial coil forces. Each coil module is effectively an enclosed aluminum box and this serves to partition the axial load and thus reduces coil axial forces. Since overall coil forces within each coil module are reduced, this will greatly reduce the number coil training quenches. This is a very important consideration for this solenoid coil because the coil cooling is adiabatic, using in-direct conduction cooling by 4.5 K supercritical helium, which will provide only a relatively small temperature stability margin. Super critical helium is used as per JLAB specification. Detail design of the coil structure, coil assembly, cold mass, and cryogenic control will be presented. The magnet protection system shall be capable of the following features: (1) quench and fault detection, (2) fast discharge of the magnet, (3) limit fault voltages to safe values, (4) monitor interlock signals to prevent unsafe operation, and (5) provide control logic necessary for safe operation of the solenoid. The instrument systems shall be capable of the following features: (1) monitors and display temperatures within the solenoid magnet, (2) measure loads or stress on the magnet suspen sion, (3) monitor voltages within the solenoid magnet and charging bus, (4) monitor pressures and (5) use data logging system to save all sensor data. These systems will be described and a quench analysis presented.