Abstract— The SPARC Toroidal Field Model Coil (TFMC) is the first large-scale (~3 m), high-field (~20 T) superconducting fusion magnet based on Rare Earth Yttrium Barium Copper Oxide (REBCO). Its objective was to retire risk for the toroidal field magnet in the SPARC tokamak, a burning plasma class magnetic confinement fusion energy device. Weighing 10,058 kg and utilizing 270 km of REBCO, the TFMC is a non-insulated, stack-in-plate style superconducting magnet. It has three main components: (1) the winding pack; (2) the structural case; and (3) the case extensions, or plena. The winding pack is composed of sixteen single pancakes with two termination plates top and bottom. The pancakes are Nitronic 40 radial plates machined with spiral channels on one side for the REBCO tape stack and single-pass channels on the opposite side for supercritical helium coolant. After assembly, each pancake undergoes a vacuum-pressure impregnation solder process to provide good mechanical protection of the REBCO tape stack and efficient thermal and electrical connectivity within each pancake. The pancakes are bolted along the inner and outer perimeter to provide mechanical and thermal connectivity while inter-pancake joints provide low resistance current transfer between pancakes. The top and bottom termination plates facilitate electrical connection to a superconducting feeder system. Embedded throughout the winding pack are 211 voltage taps, 34 temperature sensors, 34 helium flow monitors, 4 Hall probes, and 4 resistive surface heaters. The winding pack is contained within a structural case, a "trough and lid" style design composed of two Nitronic 50 forgings machined to shape and bolted together. The case reacts the large electromechanical stresses approaching 1 GPa during operation and serves as a pressure vessel that enables 20 bar supercritical helium flow that cools the winding pack and case. Two case extensions or "plena" are attached to the case with unique high-pressure feedthroughs to provide winding pack access for current, cooling, and instrumentation, completing the magnet assembly. I. INTRODUCTION The SPARC Toroidal Field Model Coil (TFMC) Project was an approximately three-year effort between 2018 and 2021 that developed novel Rare Earth Yttrium Barium Copper Oxide (REBCO) superconductor technologies and then utilized those technologies to successfully design, build, and test a first-in-class, high-field (~20 T) representative scale (~3 m) superconducting toroidal field coil. With the principal objective of retiring the design, fabrication, and operational risks inherent in large-scale no-insulation (NI) REBCO superconducting magnets for fusion energy devices, the project was executed jointly by the MIT Plasma Science and Fusion Center (PSFC) and Commonwealth Fusion Systems (CFS) as a critical technology enabler of the high-field pathway to fusion energy and, in particular, as a risk retirement program for the TF magnet in the SPARC tokamak. A. Design Requirements Because TFMC was intended as a risk retirement article for the TF magnet in SPARC, the requirements for the TFMC flowed down from the requirements for the TF. A comparison of the final design parameters for the TFMC and one of the eighteen coils from the SPARC TF is presented in Table I: - Magnet mass [kg]: TFMC = 10,058; TF Coil = 18,025 - Magnet size [m]: TFMC = 2.9 x 1.9; TF Coil = 4.3 x 3.0 - Winding pack (WP) mass [kg]: TFMC = 5,113; TF Coil = 7,975 - WP minimum turn radius [m]: TFMC = 0.2; TF Coil = 0.4 - WP current density [A/mm2]: TFMC = 153; TF Coil = 94 - WP inductance [H]: TFMC = 0.14; TF Coil = 0.59 - WP amp-turns [MA-turns]: TFMC = 10.4; TF Coil = 6.3 - Terminal current [kA]: TFMC = 40 kA; TF Coil = 31.3 - Number of turns: TFMC = 256; TF Coil = 200 - Number of pancakes: TFMC = 16; TF Coil = 16 - Total REBCO [km]: TFMC = 270; TF Coil = 270 - Coolant type: Supercritical helium (both) - Coolant pressure [bar]: TFMC = 10 – 20; TF Coil = 15 - Operating temperature [K]: TFMC = 20; TF Coil = 8 – 17 - Peak field-on-conductor [T]: TFMC = 20.1; TF Coil = 23 - Peak Lorentz loading [kN/m]: TFMC = 822; TF Coil = 750 - Magnetic stored energy [MJ]: TFMC = 110; TF Coil = 316 B. Conceptual Design – Thermomechanical The TFMC and TF designs are based on a grooved, stacked plate concept that facilitates fabrication via simple manufacturing techniques from modular components. The basic building block is a steel radial plate containing a wound REBCO tape stack and integrated joints embedded within channels; crucially, there is no electrical insulation between turns of the REBCO tape stack. The conductor groove contains a graded stack of REBCO tapes consisting of between 170 to 240 tapes; co-wound copper tapes fill the non-REBCO volume and provide additional stabilizer. A copper cap and two copper joint assemblies are soldered together using a vacuum-pressure impregnation (VPI) solder process. Coolant channels are machined on the opposite side, enabling single pass cooling. C. Conceptual Design - Electromagnetic The conductor unit cell geometry, REBCO tape stack size, number of turns, turn-to-turn spacing, and pancake D-shaping converged toward a winding pack that contained 256 turns operating at a steady state current of 40 kA and requiring pancake-to-pancake joint resistances under 5 nΩ. Stored magnetic energy at 40 kA azimuthal turn current is 110 MJ. D. Electromagnetic Design Verification Extensive electromagnetic computational tools were developed, including lumped element circuit models and coupled 2D and 3D FEM and Biot-Savart models accounting for self-fields, magnetic field magnitude and angle dependent current density distribution, dissipation from joints, and heat removal. A. REBCO Superconductor Over 300 km of REBCO was procured, utilizing over 270 km in the TFMC. The REBCO tape geometry was 4 mm wide with an average thickness of approximately 60 µm (C276 Hastelloy substrate, buffer layers, REBCO layer, ~2 µm silver coating, and ~5 µm copper stabilizer per side). Tapes were tinned with ~2 µm solder per side. Rigorous QA/QC verified critical current Ic(B,T,θ) across temperature, field, and angle. B. Pancakes and Termination Plates The TFMC contains 16 single pancakes (Types A, B, C, D) and two termination plates. Radial plates were machined from Nitronic 40 forgings and hot rolled, then electrocatalytically plated with nickel sulfamate. REBCO stacks (~200 tapes) were inserted and capped with C101 copper caps containing meandering solder flow channels. C. Winding Pack The WP consists of the 16 pancakes separated by 1 mm thick G-11CR insulation sheets, with inner and outer joints and perimeter mechanical bolts, weighing 5,113 kg. D. Structural Case The structural case comprises a Nitronic 50 trough (2,654 kg), a Nitronic 50 cover (1,332 kg), and a SS316 LN internal seal plate, bolted together with 96 Inconel 718 bolts. E. Ground Insulation A hybrid design of water-jet-cut G-11CR sheets (top and bottom) and epoxy-filled bladders with 5 mil (125 µm) kapton half-lap wrapping on the sides. F. High Pressure Plena (Case Extensions) Steel extension pressure vessels attach to case openings for cold helium injection, VIPER cable feedthroughs, warm helium extraction, and instrumentation routing. G. Current Feedthroughs Custom feedthroughs were developed to carry up to 45 kA across a high differential pressure boundary (vacuum at ~10^-7 torr to 20 bar helium), using CeFix flanges, SS316 bellows, G-11CR sleeves, and Stycast 2850-FT epoxy. A. Embedded Instrumentation All embedded instrumentation and wiring were located within the helium coolant channels machined into the back of each radial plate. In total, 287 devices (~830 signals) were installed: - Voltage taps: 192 turn-to-turn and 19 joint differential pairs using spring-mounted pogo-pins. - Temperature sensors: 34 Cernox thin-film RTD sensors on spring-loaded PCBs. - Helium mass flow sensors: 34 IST model FS2 thin film sensors. - Magnetic field sensors: 4 cryogenic Hall sensors on radial plates and external Lake Shore Hall probes. - Heaters: 4 "heaterpillars" (44 PCB-mounted ruthenium oxide resistors, ~50 W total at 20 K). B. Printed Circuit Boards Sixteen custom six-layer FR4 instrumentation boards connect to a vertical backplane "landing board" that aggregates up to 1000 signals into ten 100-pin connectors and nine discrete wire harness bundles exiting through plenum ports. A. Pancake Assembly Radial plates were inspected for flatness (<= 1 mm deviation), nickel plated, cleaned, wound with REBCO and copper co-wind tapes, fitted with copper caps and solder paste/wire, and prepped for VPI soldering. B. Vacuum Pressure Impregnation Solder Pancakes were enclosed in aluminum compression plates with stainless steel stiffening ribs and processed in a large convection oven with uniform temperature control (±1 °C). Molten solder filled the channels under vacuum and pressure gradients, achieving void-free bonding. Post-solder LN2 testing confirmed minimal Ic degradation (1.5% to 4.0%). C. Magnet Assembly 16 pancakes and 2 termination plates were stacked sequentially (A4, B5, A2, B1, A1, B2, A3, B3, C1, D1, C2, D3, C3, D2, C4, D4) using a custom jack-screw leveling table. Pogo-pin tap connectivity provided in-situ flatness checks. The assembled WP was lifted with a chandelier-style spreader tool, inserted into the case trough, shimmed with epoxy bladders, sealed with G-11CR insulation and SS316 LN seam plates, bolted with the cover, and attached to the inlet/outlet plena and VIPER current feedthroughs. The SPARC TFMC successfully transformed small-bore REBCO NI technology into a large-scale fusion-relevant magnet system. In September 2021, the TFMC achieved all DC test objectives, exceeding 20 T peak field-on-conductor at 40 kA. Subsequent thermal scans and quench tests in October and November 2021 confirmed its robust engineering and validated computational models, providing the foundation for the SPARC tokamak toroidal field magnets and future high-field superconducting devices.