A Novel Clamping Method for Resistive Magnets

High field resistive magnets may be built by stacking hundreds of so-called Bitter disks to form a coil. Nowadays, these disks have strongly elongated cooling holes arranged in a staggered pattern. Near the outer diameter of the disks one finds larger holes, round or flattened, to accommodate a tie-rod. These tie-rods assist in the assembly of the coils and are usually pretensioned to ensure a net compressive clamping force on the coil. However, at high fields, the axial magnetic forces toward the center of the coil dominate over any pretension in the system, reducing the net resulting clamping force at the end plates to a very low value. This is of particular importance in hybrid magnets where the resistive insert coils experience the background magnetic field of a large superconducting magnet. Several different strategies have been reported to mitigate the effect. We describe a novel clamping method that employs a water-filled, pressurized bellow that exerts a compressive force to the different subcoils of the resistive coil assembly. In contrast to the tie-rods, the force exerted by the bellow will remain almost constant when the magnet is energized and contracts. This new clamping method will be a key element for the mechanical design of the resistive insert coils of the HFML 45 T hybrid magnet.