Commonwealth Fusion Systems (CFS) completed the design, construction, assembly, and full field dual magnet factory acceptance testing of two identical compact (< 2 ton), high-field (20 T on tape, 17 T in warm bore) HTS REBCO magnets for a magnetic mirror in an axial fusion device. The CFS magnets will serve as the high field end coils for the ARPA-E funded project, “An HTS Axisymmetric Magnetic Mirror on a Faster Path to Lower Cost Fusion Energy.” CFS is a subrecipient of the ARPA-E BETHE Fusion Grant with the University of Wisconsin-Madison.The CFS magnets are DC magnets which will operate in the presence of other magnetic coils. The magnets use partial insulation pancake design. They are conduction cooled by cryocoolers to a nominal design operating temperature of 20 K. Field-induced mechanical stresses in the winding pack are managed such that the tape is constrained within allowable stress and strain during magnet operation. The winding pack is mechanically supported and thermally isolated in the cryostat by a set of high strength, low thermal conductivity supports. The cold mass supports are designed to sustain over 60 tons of axial loading during peak operating conditions. Results of the magnet tests and commissioning are presented below.

This paper presents design, fabrication, and operational results for a novel pair of binary Current Leads (CLs). The CLs were purpose built for the SPARC Toroidal Field Model Coil (TFMC) Test Facility at the MIT Plasma Science and Fusion Center (PSFC). The CLs operate stably at current ramp rates up to 50 kA/s and steady-state currents of 50 kA from ambient temperature power supplies to the facility's 20 K test environment. The CLs have three main sections: an upper copper heat exchanger (HEX) cooled with gas nitrogen (GN2) which connects to room temperature power supply (PS); a central liquid nitrogen (LN2) boiling chamber (BC) which vents into the HEX; and a lower Rare Earth Yttrium Barium Copper Oxide (REBCO) Section. The CLs have three distinguishing features: the BCs high surface area maintains <1 K temperature difference between their surfaces and the nucleate boiling LN2 within; each REBCO section is composed of six parallel "petals" that were individually qualified prior to installation; indium seals were used to simultaneously provide electrical continuity and hermetic sealing which enabled fabrication without need for braze or electron beam welding qualification processes. Additionally, the CLs can reduce the LN2 boiling pressure and thus temperature which significantly improves the temperature margins within the REBCO Section. The CLs were designed and built within 1.5 years and used successfully to deliver 40.5 kA of current to the SPARC TFMC. Since then they have been thermally cycled over 10 times and operated at both 1 atm and 0.65 atm.

A new superconducting magnet test facility was created at the MIT Plasma Science and Fusion Center (PSFC) for the SPARC Toroidal Field Model Coil (TFMC) program. The facility was designed and constructed in parallel with the TFMC between 2019 and 2021, with capabilities and design approaches tailored to the needs of this project and its timeline. The major components of the facility include a new cryostat (outer dimensions, 5.3 m×3.7 m×1.5 m) with open bore; a novel cooling system circulating supercritical helium in a closed-loop to provide ∼600 W cooling power at ∼20 bar-a, ∼20 K; a 50 kA, ±10 V power supply with supporting nitrogen-cooled HTS binary current leads operating at record currents, as well as VIPER-cable HTS cold bus; and a new instrumentation and PLCbased control system handling ∼650 input and output signals distributed between the facility and the test article. Substantial legacy infrastructure inherited from the PSFC's Alcator C-Mod tokamak program, including liquid nitrogen facilities and 10 MW of AC power, was instrumental in the rapid deployment of these new systems. Immediately after initial commissioning, the facility was used successfully to test the SPARC TFMC, operating the magnet in a campaign achieving 20 T on the coil, as well as a second campaign performing quench testing. The facility has since undergone several upgrades and has been used in campaigns of other test articles, and it is expected that the facility will remain a resource for the community for the foreseeable future to develop fusion magnets and related technology.

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.

From June 2019 to July 2021, the MIT Plasma Science and Fusion Center, in collaboration with Commonwealth Fusions Systems, designed, built, and commissioned a test facility at the Massachusetts Institute of Technology to evaluate the performance of a rare-earth-yttrium-barium-copper-oxide-based, 2.9-m tall, 1.9-m wide Toroidal Field Model Coil (TFMC) for the SPARC tokamak. This article presents the facility's supercritical helium (SHe) circulation system design and measured performance. The facility employed a forced-flow SHe circulation loop cooled by cryocoolers to provide a nominal cooling power of 600 W at 20 K and up to 70 g/s SHe flow to the TFMC at an absolute pressure of 20 bar. The reliance on cryocoolers as the facility's cooling source was an ideal arrangement. Procurement costs were modest, acquisition time was reasonable, and seating requirements were minimal. A steady improvement in cryocooler design provided a simple-to-use system with sufficient cooling capacity for our needs. Extensive, closed-loop analyses were performed both to support this procurement and to finalize the overall design of the SHe cooling circuit. The SHe system worked reliably, permitting flexible operation of the TFMC test facility under all working conditions.

This paper outlines the preliminary design of an experimental compact (<2 tons), high-field (∼20 T on tape, ∼17 T at center bore), and conduction-cooled HTS REBCO magnet. An identical pair of these magnets will be manufactured for a magnetic mirror axial fusion device. The magnet consists of eight single pancakes in series, each of which is an interchangeable, dry-wound, partial-insulated winding. Once charged, the magnet is operated with a constant current and will be used in the presence of other magnets. Field-induced mechanical stresses in the winding pack are managed using a novel technique of ‘partitions’ keyed into structural plates. The system is conduction-cooled by cryocoolers to operate at 20 K. The winding pack is mechanically supported and thermally isolated in the cryostat by high-strength, low-thermal conductivity brackets designed to sustain over 60 tons of axial loading during peak device operating conditions.

The Massachusetts Institute of Technology has been collaborating with the Laboratori Nazionali del Sud, Istituto Nazionale di Fisica Nucleare (INFN), in Catania, Sicily, on the conceptual design of a replacement magnet for the existing LNS cyclotron used by INFN. The existing magnet was built in the early 1980s. Future nuclear physics experiments require an upgrade of the superconducting cyclotron to increase the intensity of beams by a factor of 10-100. To achieve this goal, the extraction channel through the superconducting magnet needs to be larger than the present one in both the radial and axial directions. It is for these reasons that a new superconducting magnet fitting the new requirements must be built to replace the present one. Magnetic analyses succeeded in defining a coil set satisfying the specified field form factors. Several design options were considered, including a cryostable liquid-helium-pool-cooled design, as well as several epoxy impregnated (potted) designs. The potted and helium-pooled magnet design was developed at the conceptual level. The proposed design is viable and will be used as the baseline for the next stages of the design work.

Turn-key superconducting magnet systems are increasingly conduction-cooled by cryogenerators. Gifford-McMahon systems are reliable and cost effective, but require annual maintenance. A usual method of servicing is replacing the cold head of the cryocooler. It requires a complicated design with a vacuum chamber separate from the main vacuum of the cryostat, as well as detachable thermal contacts, which add to the thermal resistance of the cooling heat path and reduce the reliability of the system. We present a rapid warm-up scheme to bring the cold head body, which remains rigidly affixed to the cold mass, to room temperature, while the cold mass remains at cryogenic temperature. Electric heaters thermally attached to the cold head stations are used to warm them up, which permits conventional cold head maintenance with no danger of contaminating the inside of the cold head body. This scheme increases the efficiency of the cooling system, facilitates annual maintenance of the cold head and returning the magnet to operation in a short time.

The 12 GeV upgrade at Jefferson Lab includes plans for a new solenoid that will replace the existing solenoid made with the Large Aperture Superconducting Solenoid (LASS) coils from Stanford Linear Accelerator Center (SLAC). The conceptual design for the replacement solenoid presented here includes the magnetic design, winding arrangement, conductor selection, quench detection, and protection and cooling scheme. The magnetic design implements three separate coils to provide a 3.8 T field parallel to the beam direction and addresses the fringe field requirements of the facility while integrating into the existing iron yoke. The conductor consists of Superconducting Super Collider (SSC) cable (Nb-Ti) soldered into a copper channel stabilizer. The conductor is layer wound the hard way onto individual internal mandrels. The quench protection system implements a dump resistor and switch. Results from a 3-D quench code are provided for quench initiating at different locations. The magnet is conduction cooled by natural circulation of two-phase helium through cooling tubes mounted at the Outside Diameter (OD) of the cold mass.

A liquid nitrogen (LN2) is usually used to keep the high-temperature superconducting (HTS) cable low temperature. A pump is utilized to circulate LN2 inside the cryopipes. In order to minimize heat leakage, a thermal siphon circulation scheme can be realized instead. Here, we discuss the effectiveness of thermal siphon with counter-flow circulation loop composed of cryogen flow channel and inner cable channel. The main feature of the system is the existence of essential parasitic heat exchange between upwards and downwards flows. Feasibility of the proposed scheme for cable up to 500 m in length has been investigated numerically. Calculated profiles of temperature and pressure show small differences of T and p in the inner and the outer flows at the same elevation, which allows not worrying about mechanical stability of the cable. In the case under consideration the thermal insulating properties of a conventional electrical insulating material (polypropylene laminated paper, PPLP) appear to be sufficient. Two interesting effects were disclosed due to analysis of subcooling of LN2. In case of highly inclined siphon subcooling causes significant increase of temperature maximum that can breakup of superconductivity. In case of slightly inclined siphon high heat flux from outer flow to inner flow causes condensation of nitrogen gas in outer channel. It leads to circulation loss. Results of numerical analyses indicate that counter-flow thermosiphon cooling system is a promising way to increase performance of short-length power transmission (PT) lines, but conventional subcooling technique should be applied carefully.

In recent years, the technologies of manufacturing of high-temperature superconducting (FITS) tapes have achieved the critical current of 100 A. An era of industrial application of HTS power supply cables is coming. The liquid nitrogen (LN2) is usually used to keep the HTS cable at low temperature. LN2 must circulate inside the cryopipes, and the pump is used. However, the pump power is one of heat loads for the cryogenic system, and if it is high the effectiveness of the SC cable system decreases. In order to resolve this problem, a thermal siphon can be applied to circulate LN2. One of the possible system configurations have been analyzed recently by Radovinsky and Zhukovsky [1] along the basic idea proposed by S. Yamaguchi as collaboration between Chubu University and MIT. Here, we discuss the effectiveness of thermal siphon with a counter-flow circulation loop composed of a cryogen flow channel and an inner cable channel. Such configuration simplifies apparatus significantly, especially for short-distance applications.

The Muon-to-Electron Conversion experiment (MECO) was planed to be installed in the Alternating Gradient Synchrotron (AGS) facility at Brookhaven National Laboratory (BNL). Four large superconducting solenoid magnets in MECO are supported by a kilowatt helium cryoplant. This report describes the MECO cryogenic system including cooling methods for each magnet and the helium refrigerator configuration.

The Levitated Dipole Experiment (LDX) is an innovative facility to study plasma confinement in a dipole magnetic field, created by a superconducting solenoid (floating coil), which is magnetically levitated in the center of a 5 m diameter by 3 m tall vacuum chamber. The floating coil (F-coil) consists of a Nb3Sn magnet installed inside a strong vessel filled with high-pressure helium gas at room temperature. It is surrounded by a fiberglass-lead composite radiation shield and by a toroidal vacuum shell. The cryostat design provides the ability to operate the magnet for several hours of warming while suspended in the middle of the vacuum chamber without electric and cryogenic connections to the coil. For this reason the magnet is charged/discharged inductively in a lower part of the vacuum chamber. The retractable cryogenic transfer lines serve to cool down the magnet to 4.5 K before it is lifted to the operating position. The F-coil can be recooled multiple times while maintaining its field and current. This paper describes the thermal performance of the F-coil.

The levitated dipole experiment (LDX) explores the physics of high-temperature plasmas confined by a dipole magnetic field. Stable high-beta plasma has been created and confined by the magnetic field of a superconducting coil. Discharges containing trapped electrons form when microwaves cause strong perpendicular heating at cyclotron resonance. To eliminate the losses to the supports, the magnetic dipole (a superconducting solenoid) will be magnetically levitated for several hours. The dipole magnetic field is generated by a Nb3Sn floating coil (F-coil), a maximum field of 5.3 T, operating for up to 2 h. A NbTi charging coil (C-coil) surrounds a portion of the vacuum chamber and induces the current in the floating coil. After the F-coil is lifted to the center of the chamber, the levitation coil (L-coil), made from high-temperature superconductor, magnetically supports it. In the first year of operation, the device has been operated in a supported mode of operation while experience has been gained in the cryogenic performance of the F-coil and the integration of the F-coil and C-coil. Current work focuses on the integration of the F and L coils in preparation for first levitation tests. (c) 2006 Published by Elsevier B.V.

Because of safety concerns on ozone accumulation in liquid nitrogen vessels under high nuclear radiation, the cooling of thermal radiation shield and heat conduction interception of supporting struts in three magnets of MECO will adopt cold helium gas as cooling medium instead of liquid nitrogen. This work is to investigate the feasibility of producing high mass flow at low temperature for the thermal shields and heat interception cooling by modifying the so called “standard machine” that is available in helium refrigerator market. This report discusses the solutions on issues such as (1) technology feasibility of commercial helium refrigerators with large mass flow output at low intermediate temperature; (2) the cooling scheme for thermal radiation shield; (3) heat interception design for heavy supporting struts.