Self-Protection Characteristic Comparison between No-Insulation, Metal-as-Insulation, and Surface-Shunted-Metal-as-Insulation REBCO coils.

The metal tape co-winding or a metal-as-insulation (MI) winding method is an excellent way to improve the mechanical properties and reduce the average current density, thereby decreasing the stress in high-field REBCO magnet without completely losing the benefits of the no-insulation (NI) winding method. However, the MI winding increases the resistance between turns, which is known as characteristic resistance. The increased characteristic resistance can reduce the bypass current during abnormal transition situation, such as quench, which may not be desirable from a magnet protection point of view. To take advantage of both the MI and NI winding, one possible solution to reduce characteristic resistance of the MI winding coils is to add a shunt on top of the winding surface of the coil. We call this method surface-shunted-metal-as-insulation (SSMI). In this presentation, we compare the characteristic resistances and their correlated self-protecting characteristics between NI, MI, and SSMI. We present the test results of single pancake coils which wound using different winding methods (NI, MI, and SSMI) with same winding pressure of 20 N. In particular, we investigated how the SSMI method affects the characteristic resistance.

First-Cut Design of a Benchtop Cryogen-Free 23.5-T/25-mm Magnet for 1-GHz Microcoil NMR.

As a preliminary work, we have completed a 12.5-mm-cold-bore high-temperature superconducting (HTS) REBCO magnet prototype and successfully operated it up to 25 T at 10 K cooled by a cryocooler only, without liquid helium. In this paper we present the first-cut design of a cryogen-free all-REBCO 23.5-T/25-mm-warm-bore magnet having a high homogeneity of <0.1 ppm over a 1-cm diameter of spherical volume for a benchtop 1-GHz microcoil NMR spectroscopy. We also investigate a shielding design to reduce a 5-gauss fringe field radius to ≤1.5 m. This benchtop magnet will incorporate all the innovative design and operation concepts validated by the prototype magnet: 1) all-HTS composition and operation at above 4.2 K; 2) no-insulation winding technique with an extra shunting that makes this high-field REBCO magnet compact, mechanically robust, and self-protecting; 3) a single coil formation that leads, compared with the traditional multi-nested high-field NMR magnet, to simpler and more affordable manufacturing processes; 4) operational temperature-controlled screening-current reduction method which reduces peak stresses within the REBCO coil and field errors; and 5) cryogenic design for conduction-cooling operation.

Conceptual Design of a Portable, Solid-Nitrogen-Cooled 0.5-T/560-mm Point-of-Care MRI Magnet.

We describe the conceptual design of a portable, liquid-helium-free, all-REBCO, 0.5-T/560-mm point-of-care magnetic resonance imaging (MRI) magnet. It is free from an external power supply and a refrigeration system during operation. In our portable MRI magnet, we use a detachable "cryocirculator" that circulates, in a closed circuit, cold working fluid, and most importantly for portability, it can be readily coupled to or decoupled from the magnet, in contrast, a conventional cryocooler is mechanically attached to the magnet. Another unique feature of our system is a volume of solid nitrogen (SN2) in the cold chamber that adds enough thermal mass to the magnet in the 30-36-K operating temperature range, enabling it to maintain its field over a period of, for this system, ≥10 hours, plenty enough for this portable MRI system, uncoupled from its cryocirculator, to perform its mission before it needs recooling.

Review of progress and challenges of key mechanical issues in high-field superconducting magnets.

The development of modern science and technology requires high magnetic fields exceeding 25T. Second-generation high-temperature superconducting wires, i.e. REBCO (REBa2Cu3O7-x, RE refers to Y, Gd, Dy, Eu and other rare-earth elements) coated conductors (CCs), have become the first choice for high-field magnet construction because of their high irreversible magnetic field. The mechanical stresses caused by manufacturing, thermal mismatch and Lorenz forces closely influence electromagnetic performance during operation for REBCO CCs. In addition, the recently studied screen currents have effects on the mechanical characteristics of high-field REBCO magnets. In this review, the experimental and main theoretical works on critical current degradation, delamination and fatigue, and shear investigations on REBCO CCs, are reviewed at first. Then, research progress on the screening-current effect in the development of high-field superconducting magnets is introduced. Finally, the key mechanical problems facing the future development of high-field magnets based on REBCO CCs are prospected.

Sudden-Discharging Quench Dynamics in a No-Insulation Superconducting Coil.

It is generally agreed that no-insulation (NI) high-temperature superconducting (HTS) magnets do not quench because of the turn-to-turn energy-releasing bypass unique to NI. However, these magnets, especially with high operating current and low ambient thermal capacity, still occur unexpected quenches when the current through the magnets suddenly drops to zero (i.e., the sudden-discharging quench). Here, we report this kind of quench, which is different from that widely-reported quench happening during charging (i.e., the energizing quench). Here, a demonstrative coil with 655-turns, 350 A operating current, and 4 K conduction cooling, is used to prove this sudden-discharging quench, and a simulation model is built to reveal the quench dynamics. Results show the turn-to-turn heat triggers the initial partial quench in the inner coil turns and then the induced overcurrent spreads out the quench like an avalanche to the outer coil turns.

On fault-mode phenomenon in no-insulation superconducting magnets: A preventive approach.

Here, we present experimental and analytical results of a preventive approach applied to a fault-mode phenomenon caused by electrodes or power-source failure in a no-insulation (NI) high-temperature superconducting REBa2Cu3O7-x (REBCO, RE = rare earth) magnet. It is generally agreed that the NI magnets, at least those of laboratory scale, are self-protected from overheating and, therefore, from quenching, chiefly because of turn-to-turn current bypassing unique to NI. However, these NI magnets do experience unexpected quenches, e.g., when the current through the magnet suddenly drops due to the aforementioned fault-mode phenomenon. Here, we report this phenomenon of a sudden-discharging-triggered quench of an NI REBCO coil, conduction-cooled, and operated at 4.2 K. We also present our preventive approach for this phenomenon that relies on an appropriately designed resistor shunted across the coil terminals. With this shunt resistor, a quench was prevented by suppressing the quench initiating turn-to-turn heat and induced overcurrent within the NI winding, and the coil current decayed safely.

Partial-Insulation HTS Magnet for Reduction of Quench-Induced Peak Currents.

The No-insulation-like (NI) coil's turn-to-turn current paths prevent local heating by forcing the current to bypass into nearby turns when a hot spot appears in a coil. However, the changing direction of the current by bypassing will change the magnetic flux, which generates unwanted induced currents in the adjacent coils in a multiply-stacked HTS magnet. This induced current can temporarily exceed the designed maximum currents in the NI coils, damaging the magnet. A partial-insulation (PI) coil, in which a single or multiple insulated, with a polyimide-like material or a thin ceramic film, is inserted between windings to hinder the current paths, can reduce the peak induced currents in the NI HTS coil's current paths. In this paper, we present the results of a simulation study on the peak-induced current upon a quench of the PI HTS magnet with a double pancake. The study shows that the peak-induced current varies with the number of insulated turns. We also discuss the induced current turn-by-turn simulation. According to the simulation result, the PI effectively reduces overall induced current, especially insulation applied every two turns.

A Cryogen-Free 25-T REBCO Magnet with the Extreme-No-Insulation Winding Technique.

We present the operation result of a cryogen-free 23.5 T/φ12.5 mm-cold-bore magnet prototype composed of a stack of 12 no-insulation (NI) REBCO single pancake coils-ten middle coils of 6-mm wide and two end coils of 8-mm wide tape-forming 6 double pancake (DP) coils with inner joints. Each coil was wound with the tape having only 1-µm-thick copper layer on each side to overcome the conductor thickness uniformity issue and enhance the mechanical strength within the winding, and then, additional electrical shunting by thin layers of solder was applied on the top and bottom surfaces of each DP coil for effective cooling and quench protection-called extreme-NI winding technique. With this small prototype magnet towards a benchtop 1-GHz NMR, we validate our coil design that include conductor performance, screening-current-induced field and stresses, and conduction-cooling cryogenics. Included in the paper are: 1) conductor issues and our counterproposal in winding; 2) screening-current reduction method; 3) design and manufacture summary of the magnet; and 4) operating test results of the magnet up to 25 Tesla.

Construction and test result of an all-REBCO conduction-cooled 23.5 T magnet prototype towards a benchtop 1 GHz NMR spectroscopy.

A compact benchtop high-field REBCO NMR is one of the most promising HTS applications. An all-REBCO, conduction-cooled magnet is a very attractive design option for demonstrating the unique potential of REBCO for forefront magnets. In this research, we have successfully constructed and tested a prototype all-REBCO, conduction-cooled, 23.5 T magnet operating at 10 K. We have applied the concept of an extreme No-Insulation (NI) winding technique, coupled with a solder-shunting procedure to improve magnet performance. We have also used a temperature-controlled charging sequence (TCCS) to reduce the screening current. The magnet was energized to 23.6 T at 14 K; it was further operated to 25 T at 10 K for nearly 60 hours.

An MgB2 Superconducting Joint with its Own Heat-Treatment Schedule.

We suggested an MgB2 joint process with its own heat-treatment schedule to apply it for our 1.5-T MgB2 "finger" MRI magnet. In fabricating the MgB2 magnet, the optimal heat-treatment schedule to attain a reproducible and high critical current is different in a joint and a coil. To solve this problem, we introduced an additional heating system, which is composed of a cartridge heater and a thermocouple connected with a copper block, into a box-type furnace. Then, we carried out heat-treatments with exclusively increasing the joint-part temperature above the Mg melting point of 645 °C-the joint was actually heated up to 700 °C. We evaluated a critical current and a crystal structure of the obtained MgB2 joint. From experimental results, we found that the joint heated with the own heat-treatment schedule, which is 700 °C for 1 h + 600 °C for 11 h, showed a good I c of over 450 A at 15K under self-field. The joint resistance was estimated by the coil operation for 18 days, and it was expected to be less than 10-12 Ω.

Hot-Spot Modeling of REBCO NI Pancake Coil: Analytical and Experimental Approaches.

The No-Insulation (NI) winding provides intrinsic bypassing current paths that enable self-protection from overheating. The self-protection of the NI coil is one of the most promising protection techniques for the high field high-temperature superconductor (HTS) magnet applications. Since the additional paths are valid for an HTS magnet with a thinner matrix, the self-protection mechanism is applicable even for the higher current density magnet with reduced matrix thickness inside the HTS tape. However, reducing the matrix can cause damage to the magnet by producing excessive heat during the quench. This research introduces a new modeling method to investigate the hot-spot characteristics in the REBCO NI pancake coil. The model is also validated with a sample NI HTS coil experiment result. Radial direction Normal Zone Propagation (NZP) velocity of the sample coil is estimated based on the suggested model. The calculated radial direction NZP velocity is applied to calculate the center field drop of the NI HTS coil, and the result is well-matched with the experiment result. We also introduce one example of the model applications. The maximum current density that will not exceed a given reference temperature in the adiabatic cooling condition is estimated using the model.

Design Overview of the MIT 1.3-GHz LTS/HTS NMR Magnet with a New REBCO Insert.

We present a design overview of the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) with a newly designed 835-MHz REBCO insert (H835) as a replacement for the 800-MHz REBCO insert (H800) that was damaged when it quenched during operation in 2018. The new H835 is designed to contribute 19.6 T in a background field of 10.93 T by an LTS NMR magnet that normally rated at 11.74 T (500 MHz): combined, 1.3G generates a total field of 30.53 T corresponding to a proton resonance frequency of 1.3 GHz. H835 is designed to operate stably while meeting 1.3G design constraints. We have also designed H835 to protect it from permanent damage in an improbable event like a quench. Key design features are: 1) a single-coil formation, composed of 38 stacked metal-co-wound no-insulation and 2 stacked no-insulation double-pancake coils, all with mechanically improved cross-over sections; 2) enhanced thermal stability; and 3) reduced current margin with a detect-and-heat method. This paper includes: 1) electromagnetic and mechanical design of H835; 2) cryogenics overview; 3) quench protection strategy; and 3) discussion on the next steps to successfully complete 1.3G.

Prototype REBCO Z1 and Z2 shim coils for ultra high-field.

We present promising results of novel high-temperature superconducting (HTS) shim coil prototypes that circumvent the size and strength limitation of our earlier innovative HTS shim concept based on 46-mm wide REBCO tape. The HTS shim coil is placed inside the HTS magnet, mainly for ultra-high-field (> 1 GHz or 23.5 T) NMR magnets, and thus unaffected from the windings' diamagnetic wall effects. One full-scale version will be applied to clean up Z1 and Z2 harmonic errors in the MIT 1.3-GHz high-resolution NMR magnet composed of an 835-MHz HTS insert, while another version for an MIT 1-GHz microcoil NMR magnet whose small-scale model we are currently building. The prototype sets were wound with a 2-pile, 1.03-mm wide, 0.30-mm thick REBCO conductor. Operated at 77 K, the Z1 shim set generated a 1st harmonic field strength of 179 kHz/cm at 70 A, while the Z2 shim set, composed of two pairs, Z21 and Z22, generated the 2nd harmonic field of 141 kHz/cm2 at 50 A. Together with discussion on technical challenges for this REBCO shim coil concept, we demonstrate its feasibility for the next generation of ultra-high-field (UHF) HTS NMR magnets.

Quench Analyses of the MIT 1.3-GHz LTS/HTS NMR Magnet.

The MIT 1.3-GHz LTS/HTS NMR magnet is currently under development. The unique features of this magnet include a 3-nested formation for an 800-MHz REBCO insert (H800) and the no-insulation (NI) winding technique for H800 coils. Because when it is driven to the normal state, an NI REBCO magnet will respond electromagnetically, thermally, and mechanically that may result in permanent magnet damage, analysis of a quenching magnet is a key aspect of HTS magnet protection. We have developed a partial element equivalent circuit method coupled to a thermal and stress finite element method to analyze electromagnetic and mechanical responses of a nested-coil REBCO magnet each a stack of NI pancake coils. Using this method, quench simulations of the MIT 1.3-GHz LTS (L500)/HTS (H800) NMR magnet (1.3G), we have evaluated currents, strains, and torques of H800 Coil 1 to Coil 3 and L500, and center fields of 1.3G, L500, and H800. Our analyses show H800 is vulnerable to mechanical damage.

Experimental and Numerical Studies on a Method to Mitigate Screening Current-Induced Field for No-Insulation REBCO Coils.

We present experimental and numerical studies on a method to mitigate screening current-induced field (SCF) for NI REBCO coil. The SCF is the major field error to incorporate a REBCO insert for a high field LTS/HTS magnet. The field-shaking technique is going to be used to mitigate the SCF of 800-MHz REBCO insert magnet (H800) for MIT 1.3-GHz LTS/HTS NMR magnet (1.3 G). The field-shaking using 500-MHz LTS background magnet generates the SCF in H800, due to huge self and mutual inductances of them. In this paper, we tested the effect of the induced current in the NI REBCO coil on the field-shaking technique to mitigate the SCF. The amount of the induced current was decided by the NI REBCO coil status; the open- or closed-loop coil. We performed the three cases of experimental tests and analyzed them. From the test results, we may conclude that we need to limit the ramp rate of L500 during the field-shaking, to minimize the induced current in the HTS insert which consists of the NI REBCO coil.

Assembly and Test of a 3-Nested-Coil 800-MHz REBCO Insert (H800) for the MIT 1.3 GHz LTS/HTS NMR Magnet.

We present assembly and test results of a 3-nested-coil 800-MHz (18.8 T) REBCO insert (H800) for the MIT 1.3 GHz LTS/HTS NMR magnet currently under completion. Each of the three H800 coils is a stack of no-insulation (NI) REBCO double-pancake coils (DPs). The innermost 8.7-T Coil 1 (26 DPs) was completed by mid-2016; the middle 5.6-T Coil 2 (32 DPs) was complet-ed in mid-2017; while the outermost 4.5-T Coil 3 (38 DPs) was completed in early 2018. Coils 1, 2 & 3 were assembled together in early 2018 as a 3-nested-coil, the H800, and tested, first in liquid nitrogen to a power supply current of 20 A, followed by testing in liquid helium to a power supply current of 251.3 A, the H800's design operating current. After roughly five minutes settling time at 251.3 A, the H800 quenched. In this paper we examine probable sources of quench initiation and simulate ensuing quench behavior. Remedial efforts to minimize the tendency towards quenching in the H800 are presented and discussed.

MIT 1.3-GHz LTS/HTS NMR Magnet: Post Quench Analysis and New 800-MHz Insert Design.

We present post-quench analyses of the MIT 800-MHz REBCO insert magnet (H800), unexpectedly quenched during operation in March 2018, and design study of a new 800-MHz HTS insert (H800N). The as-wound H800 was supposed to contribute 18.7 T and, with an LTS background magnet (L500), produce 30.5 T corresponding to a proton resonance frequency of 1.3 GHz. The H800 was operated at 4.2 K in liquid helium and, about 5 minutes after the power supply reached a target operating current of 251.3 A, it experienced a quench. Because the damage in the H800 was more widespread than it first appeared, we decided to design and build a new insert magnet, H800N. In designing H800N, we try to eliminate unanticipated flaws in our H800 design. H800N is to be more stable not to quench and more reliably survive against quench without permanent damage by: 1) adopting a single solenoid structure composed of 40 stacked double pancake coils with improved cross-over sections; 2) enhancing thermal stability; and 3) reducing excessive current margin for quench protection.

Design of a Tabletop Liquid-Helium-Free 23.5-T Magnet Prototype towards 1-GHz Microcoil NMR.

We present a design study of a liquid-helium (LHe)-free 23.5-T, ϕ25-mm RT-bore REBCO magnet for high-resolution 1-GHz microcoil nuclear magnetic resonance (NMR) spectroscopy. A microcoil NMR magnet is compact and thus its cost will be less by nearly an order of magnitude than that of the standard NMR magnet, and placeable on a bench, thereby resulting in a large saving in space. In addition, LHe-free operation enables the user to be independent from a cooling source in short supply. This paper includes: 1) magnet design and conductor requirement specification; 2) conceptual design of a full-scale tabletop LHe-free 1-GHz NMR magnet; and 3) design of a 10-K operating REBCO 23.5-T magnet prototype with a ϕ20-mm cold-bore. This small-size magnet prototype will be built and tested by 2020 for validation of performance and manufacturing challenges such as splices between coils. The paper concludes with discussion of stray-field shielding methods and a screening-current-inducing field (SCF) effect.

HTS Shim Coils Energized by a Flux Pump for the MIT 1.3-GHz LTS/HTS NMR magnet: Design, Construction, and Results of a Proof-of-Concept Prototype.

In this paper we present design, construction, and preliminary results of a proof-of-concept prototype of high-temperature superconductor (HTS) shim coils operated at 77 K and energized, for the first time among all shim coils, by a flux pump, here called digital flux injector (DFI). Although the prototype shims were wound with 2-mm wide REBCO tape, and DFI with Bi2223 and REBCO tapes, the HTS Z1 and Z2 shims to be installed in the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) currently under construction and operated at 4.2 K will be wound with reinforced Bi2212 wire and DFI with Nb3Sn tape. The paper concludes with two sets of Bi2212 Z1 and Z2 shims for 1.3G.