This paper presents the design of the resistive insert magnet for the 45 T hybrid magnet under construction at the High Field Magnet Laboratory (HFML) in Nijmegen. We describe the design process and tools used to obtain the coil and disk design, and we describe the mechanical design aspects of the housing for the magnet. We address the end turn issue and introduce an innovative new element to provide sufficient clamping of the coil at all times. Finally, we address possible fault scenarios and the magnetic misalignment forces associated with such faults and their impact on the housing.

The High Magnetic Field Laboratory in Nijmegen has been collaborating with the National High Magnetic Field Laboratory (NHMFL) in Florida on a 45 T hybrid magnet project. The primary scope of the collaboration was the design and manufacture of the hybrid magnet's superconducting cold mass. The 7.5 ton cold mass includes a single 13 T solenoid wound with high J(c) RRP Nb-3 Sn/Cu cable-in-conduit conductor. The coil will he forced flow supercritical helium and operated in parallel with a set of Bitter-disk resistive coils. Coil winding, reaction heat treatment, epoxy impregnation, and cold mass assembly has been completed at the NHMFL. The full cold mass has been delivered to Radboud University and will be assembled with the cryostat and interfaced with the system utilities.

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.

A pair of binary vapor-cooled/high-temperature-superconducting current leads were fabricated at the National High Magnetic Field Laboratory (NHMFL) and tested at the High Field Magnet Laboratory (HFML) at the Radboud University Nijmegen. The leads supply current to the NHMFL series-connected hybrid magnet. Details of the fabrication are discussed, including that of the resistive and high-temperature-superconducting parts and the integration of the two sections. A test assembly was made, including the pair of current leads, a jumper made from NbTi cable-in-conduit, instrumentation, and mounting hardware. The test facility includes a liquid nitrogen supply vessel and a closed-loop supercritical helium supply system, to replicate the cooling used in the magnet assembly. Results from the testing of the leads to the full operating current, and under simulated loss-of-cooling conditions are presented.

To extend its user's facilities, the High Field Magnet Laboratory (HFML-EMFL) at the Radboud University is in the process of building a 45-T hybrid magnet. The magnet system will consist of a 22 MW 32.7 T resistive insert and a 600-mm-bore 12.3 T superconducting outsert magnet, and the design was significantly adjusted after a thorough design revision in 2011. The HFML hybrid magnet will be operated with separate current sources for the superconducting and resistive coils (20 kA at 10 V and 40 kA at up to 550 V, respectively). The outsert coil is a solenoid layer wound with all-Nb 3 Sn/Cu cable-in-conduit conductor (CICC), cooled by a forced flow of supercritical helium and operated at 20 kA. Similar to the series-connected hybrids for the HZB (Berlin, Germany) and the NHMFL (Tallahassee, FL, USA), the HFML outsert coil contains three grades of conductor. All CICC grades are based on high-current density Nb3Sn strand produced by Oxford Superconducting Technology. The CICC production and qualification program has been completed successfully. The coil will be wound at the NHMFL and, after heat treatment and impregnation, sent to Nijmegen for integration into the cryostat. In this paper, the design choices and current status of the program are presented.