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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

A Tabletop Persistent-Mode, Liquid-Helium-Free, 1.5-T/90-mm MgB2 "Finger" MRI Magnet for Osteoporosis Screening: Two Design Options.

In this paper we present two design options for a tabletop liquid-helium-free, persistent-mode 1.5-T/90-mm MgB2 "finger" MRI magnet for osteoporosis screening. Both designs, one with and the other without an iron yoke, satisfy the following criteria: 1) 1.5-T center field with a 90-mm room-temperature bore for a finger to be placed at the magnet center; 2) spatial field homogeneity of <5 ppm over a 20-mm diameter of spherical volume (DSV); 3) persistent-mode operation with temporal stability of <0.1 ppm/hr; 4) liquid-helium-free operation; 5) 5-gauss fringe field radius of <50 cm from the magnet center; and 6) small and light enough for placement on an exam table. Although the magnet is designed to operate nominally at 10 K, maintained by a cryocooler, it has a 5-K temperature margin to keep its 1.5-T persistent field up to 15 K. The magnet will be immersed in a volume of solid nitrogen (SN2) that provides additional thermal mass when the cryocooler is switched off to provide a vibration-free measurement environment. The SN2 enables the magnet to maintain its persistent field over a period of time sufficient for quiescent measurement, while still limiting the magnet operating temperature to ≤15 K. We discuss first pros and cons of each design, and then further studies of our proposed MgB2 finger MRI magnet.

A Field-Shaking System to Reduce the Screening Current-Induced Field in the 800-MHz HTS Insert of the MIT 1.3-GHz LTS/HTS NMR Magnet: A Small-Model Study.

In this paper, we present experimental results, of a small-model study, from which we plan to develop and apply a full-scale field-shaking system to reduce the screening current-induced field (SCF) in the 800-MHz HTS Insert (H800) of the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) currently under construction-the H800 is composed of 3 nested coils, each a stack of no-insulation (NI) REBCO double-pancakes. In 1.3G, H800 is the chief source of a large error field generated by its own SCF. To study the effectiveness of the field-shaking technique, we used two NI REBCO double-pancakes, one from Coil 2 (HCoil2) and one from Coil 3 (HCoil3) of the 3 H800 coils, and placed them in the bore of a 5-T/300-mm room-temperature bore low-temperature superconducting (LTS) background magnet. The background magnet is used not only to induce the SCF in the double-pancakes but also to reduce it by the field-shaking technique. For each run, we induced the SCF in the double-pancakes at an axial location where the external radial field Br > 0, then for the field-shaking, moved them to another location where the external axial field Bz ≫ BR. Due to the geometry of H800 and L500, top double-pancakes of 3 H800 coils will experience the considerable radial magnetic field perpendicular to the REBCO tape surface. To examine the effect of the field-shaking on the SCF, we tested each NI REBCO DP in the absence or presence of a radial field. In this paper, we report 77-K experimental results and analysis of the effect and a few significant remarks of the field-shaking.

Construction and Test Results of Coils 2 and 3 of a 3-Nested-Coil 800-MHz REBCO Insert for the MIT 1.3-GHz LTS/HTS NMR Magnet.

We present construction and test results of Coils 2 and 3 of a 3-coil 800-MHz REBCO insert (H800) for the MIT 1.3 GHz LTS/HTS NMR magnet currently under construction. Each of three H800 coils (Coils 1-3) is a stack of no-insulation REBCO double pancakes (DPs). The innermost 8.67-T Coil 1 (26 DPs) was completed in 2016; the middle 5.64-T Coil 2 (32 DPs) has been wound, assembled, and tested; and for the outermost 4.44-T Coil 3, its 38 DPs have been wound and preliminary tests were performed to characterize each DP at 77 K. Included for Coil 2 are: 1) 77-K data of critical current, index, and turn-to-turn characteristic resistivity of each DP; 2) stacking order of the 32 DPs optimized to maximize the Coil 2 current margin and minimize its Joule dissipation in the pancake-to-pancake joints; 3) procedure to experimentally determine and apply a room-temperature preload to the DP stack; 4) 77-K and 4.2-K test results after each of 64 pancakes was over-banded with 75-µm-thick stainless steel tape for a radial thickness of 5 mm. Presented for each DP in Coil 3 are 77-K dada of critical current, index, and turn-to-turn characteristic resistivity.

A REBCO Persistent-Current Switch, Immersed in Solid Nitrogen, Operating at Temperatures near 10 K.

We present design and test results for a thermally-activated persistent-current switch (PCS) applied to a double pancake (DP) coil (151 mm ID, 172 mm OD), wound, using the no-insulation (NI) technique, from a 120-m long, 76-µm thick, 6-mm wide REBCO tape. For the experiments reported in this paper, the NI DP assembly was immersed in a volume of solid nitrogen (SN2), cooled to a base temperature of 10 K by conduction to a two-stage cryocooler, and energized at up to 630 A. The DP assembly operated in quasi-persistent mode, with the conductor tails soldered together to form a close-out joint with resistance below 6 nΩ. The measurements confirm PCS activation at heating powers below our 1-W design target, and a field decay time constant in excess of 900 h (i.e 0.1% h-1 field decay rate), limited by the finite resistance of the close-out joint.

Construction and Test Results of Coil 2 of a Three-Coil 800-MHz REBCO Insert for the 1.3-GHz High-Resolution NMR Magnet.

This paper focuses on the construction and test results of Coil 2 that is part of a trio of nested coils composing the REBCO 800 MHz insert. Upon its completion, the REBCO 800 MHz insert will be placed in the bore of a 500 MHz low temperature superconducting (LTS) NMR magnet (L500) to form the MIT 1.3 GHz high-resolution NMR magnet. Coil 2 is a stack of 32 double pancake (DP) coils wound with 6-mm wide REBCO tape using the no-insulation (NI) technique. Each pancake is wound on a stainless steel inner supporting ring to prevent the collapsing of its crossover due to the external pressure exerted by the winding pack. Coil 2 will be constructed in the following sequence: 1) after winding each DP will be individually tested in a bath of liquid nitrogen at atmospheric pressure to determine its current carrying capabilities; 2) DPs will be then assembled as a stack with interconnecting joints, and 3) as in Coil 1, each pancake will be overbanded with a stainless steel tape, this time to a thickness of 5 mm, thickness determined by a stress analysis previously performed. Finally the fully assembled Coil 2 will be tested in liquid nitrogen at 77 K and then in liquid helium at 4.2 K. We present here details of the stress analysis leading to the sizing of the DP inner supporting stainless steel ring and of the overbanding thickness required. Test results include coil index, critical current, charging time constant.