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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.

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.

A compact benchtop high-field REBCO nuclear magnetic resonance is one of the most promising high-temperature superconductor 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 winding technique, coupled with a solder-shunting procedure to improve magnet performance. We have also used a temperature-controlled charging sequence 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 h.

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.

Screening currents in REBCO conductors, induced by time-varying magnetic fields, not only affect the field quality of HTS coils but also cause strain gradients along REBCO tape width that may overstress REBCO conductors used in NMR and other high-field magnets. In this paper, we present results of an experimental and analytical study on screening-current-induced strain gradients, performed with small REBCO pancake coils. Because we believe that screening current effect is reduced in multifilament conductor, we have studied 2 test coils, φ150 mm, one wound with monofilament and the other with 3-striate/4-filament REBCO tapes. A 5-T/ 300-mm room-temperature bore magnet was used not only to excite a strong screening current but also apply the nonuniform Lorentz force to each coil at 4.2 K. Our experiment and analysis have quantitatively demonstrated that we can effectively suppress the screening-current effect on strain gradient, not surprisingly, by using striated multifilament REBCO conductor.

We have finalized the design of a full-scale tabletop 1.5-T/90-mm MgB $_2$ finger MRI magnet system for osteoporosis screening based on our preliminary test results of small coils and superconducting joints. The magnet will operate in persistent mode at 10 K with an additional 5 K temperature margin. The magnet design which includes six main coils and an iron shield satisfies the required specification of a field intensity of 1.5 T, homogeneity of $\leq$ 5 ppm over a 20-mm diameter of spherical volume, and a fringe field of $\leq$ 5 gauss at 0.5 m in radius from the magnet center. An active protection method using external heaters will be applied to prevent a local hot spot in the MgB $_2$ windings from being overheated when quench occurs. Active shield transverse and axial gradient coils for this tabletop osteoporosis MRI, having primary and shield coil pairs, are designed to minimize stray fields that can induce eddy currents on nearby metal surface and thus imaging artifacts. This paper covers design and analysis of: 1) the main coils and iron shield; 2) coil former; 3) quench protection; and 4) active shield gradient coils. We also discuss design changes of the cryostat and equipment plan for the overall system. The magnet system will be completed and then, equipped with other MRI hardware components including an in-house-made gradient coil assembly and RF coils for demonstration of 1.5-T finger MRI in 2020.

In this paper we present quench analysis results of a Low-Temperature Superconducting (LTS) quadrupole triplet magnet system, a part of the In-flight Fragment (IF) separator of a heavy ion linear accelerator complex, named RAON, currently being constructed by the Institute of Basic Science (IBS). This magnet system is composed of three quadrupole magnets: a triplet, surrounded by iron yokes and embedding hexapole/octupole LTS coils for field correction. The magnet will be operated at 4.2 K in liquid helium. For reliable and safe operation of this complex superconducting system, quench and protection analysis with possible failure scenarios must be performed. In this paper, we first discuss probable quench scenarios and then present results of the quench propagation analysis on: 1) coil currents and voltages by multi-coil model circuit analysis; and 2) simulated temperature distribution inside each coil. Our quench analysis results show that the maximum voltage and temperature in each coil are below safety limits, 2000 V and 150 K, respectively, and confirm that this quadruple triplet magnet system is self-protecting.

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 completed in mid-2017; while the outermost 4.5-T Coil 3 (38 DPs) was completed in early 2018. Coils 1, 2, and 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 toward quenching in the H800 are presented and discussed.

The MIT 1.3-GHz LTS/high-temperature superconducting (HTS) nuclear magnetic resonance (NMR) magnet is currently under development. The unique features of this magnet include a three-nested formation for an 800-MHz REBa2Cu3Ox (RE = Y, Gd) (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 Coils 1 to 3 and L500, and center fields of 1.3G, L500, and H800. Our analyses show H800 is vulnerable to mechanical damage.

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.

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.

We suggested an MgB(2) joint process with its own heat-treatment schedule to apply it for our 1.5-T MgB(2) “finger” MRI magnet. In fabricating the MgB(2) 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 MgB(2) 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.

In this paper, we report preliminary experimental results on protection study of two REBCO pancake coils based on the metal-as-insulation (MI) winding technique, a variant of the no-insulation (NI) winding technique, in which a metallic tape is cowound. Against the more-proven NI technique, our results demonstrate that the MI technique, too, is quite viable for HTS pancake coils with the following features: 1) nearly self-protecting;2) significantly smaller charging-delay time constant; and 3) better control of coil parameters. It also permits stable operation at 97% of a quench current. We present 77-K results of NI and MI pancakes: first, comparing the advantages and drawbacks of the two winding techniques and, second, dealing with stability and quench parameters. Finally, using a simple circuit model, we quantitatively show that metallic tape thickness has little detrimental effect on the self-protecting feature of the MI pancakes.

We present results of full-current testing at 4.2 K of a z-axis 0.866-T solenoid and an x-axis 1.225-T dipole coil that comprise a 1.5-T/75-mm room temperature bore magic-angle-spinning nuclear magnetic resonance magnet developed at the MIT Francis Bitter Magnet Laboratory. Also included in the paper are results of the magnet performance when the magnet assembly is immersed, to enhance its thermal mass, in solid nitrogen, and operated in the temperature range of 4.2-4.3 K.

Temporal “enhancement” of trapped fields was observed in the central region of a compact NMR magnet comprising a stack of 2800 YBCO “square” annuli (YP2800), field-cooled at 4.2 K. This paper presents an analytical model to simulate the trapped field enhancement in YP2800. First, based on an inverse calculation technique, the current distributions in the 560 5-plate modules of YP2800 were computed from the measured trapped field distribution. Then, YP2800 was modeled as a set of “three magnetically-coupled subcoils”: the “bottom” coil (C(B), 140 modules); the “middle” coil (C(M), 280 modules); and the “top” coil (C(T), 140 modules). With the index resistance of each coil included, the circuit model shows that the average current in C(M) “slowly” increases, induced by “fast” current decays in C(B) and C(T). As a result, the center field in YP2800, dominated by the C(M) currents, increases in time. The simulation agrees reasonably well with the measurement, which validates the analytical model.

This paper presents a study on the effects of winding tension on the characteristic resistance of a no-insulation (NI) coil. Two ReBCO NI test pancake coils, having the same winding i.d. (60 mm), o.d. (67.6 mm), and number of turns (60), were sequentially prepared in a way that the first test coil was wound with a winding tension of 12-N, tested, and then rewound with a new winding tension of 20-N for the same tests. In each test, the test coil was energized at a target current, the power supply was “suddenly” disconnected, and then the temporal decay of the coil center field was measured, from which the time constant of the test coil and the consequent characteristic resistance were obtained. To check the reproducibility of experimental data, each test was repeated four times and each time the test coil was unwound and rewound with a given winding tension. The experimental results were analyzed with equivalent circuit analyses. Correlation between the winding tension and the characteristic resistance was discussed in detail.

This paper presents experimental and analytical studies on the time-varying behavior of an NI (no-insulation) high-temperature superconductor pancake coil, alone or magnetically coupled to an external coil. An NI coil and another insulated coil (as an external), both of identical winding i.d. and number of turns, were fabricated. Another external coil used in this study was a 300-mm/5-T low-temperature superconductor magnet. An equivalent circuit model is proposed to simulate the NI coil, and the external coil, under time-varying conditions. Good agreement between experiment and simulation shows that the proposed equivalent circuit model is valid to characterize the time-varying electromagnetic behavior of an NI coil, alone or magnetically coupled to an external coil.

In this paper, we present two design options for a tabletop liquid-helium-free, persistent-mode 1.5-T/90-mm MgB 2 “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

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 high-temperature superconducting (HTS) Insert (H800) of the MIT 1.3-GHz low-temperature superconducting (LTS)/HTS nuclear magnetic resonance magnet (1.3 G) currently under construction—the H800 is composed of three nested coils, each a stack of no-insulation (NI) REBCO double pancakes (DP). In 1.3 G, 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 DPs, one from Coil 2 (HCoil2) and one from Coil 3 (HCoil3) of the three H800 coils, and placed them in the bore of a 5-T/300-mm room-temperature bore LTS background magnet. The background magnet is used not only to induce the SCF in the DPs, but also to reduce it by the field-shaking technique. For each run, we induced the SCF in the DPs at an axial location where the external radial field BR > 0, and then, for the field shaking, moved them to another location where the external axial field $Bz \gg B_{R}$ . Due to the geometry of H800 and L500, top DPs of three 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.

We present the 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- and 4.2-K test results after each of 64 pancakes was over-banded with a 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.

In this paper, we report preliminary results of our on-going effort to develop a superconducting persistent-current switch (PCS) for REBCO pancake coils that will be operated in liquid helium. In the first part of this paper, we briefly describe experimental results of our PCS operated in the temperature range 77–57 K, i.e., liquid- and solid-nitrogen environments. The rest we devote to a new PCS heater design in which we target a heating power of $ 1 W in liquid helium.

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.

A 1.3-GHz/54-mm LTS/HTS NMR magnet, assembled with a three-coil (Coils 1-3) 800-MHz HTS insert in a 500-MHz LTS NMR magnet, is under construction. The innermost HTS insert Coil 1 has a stack of 26 no-insulation (NI) double pancake (DP) coils wound of 6-mm-wide and 75-μm-thick REBCO tapes. In order to keep the hoop strains on REBCO tape

A high-resolution 1.3-GHz/54-mm low-temperature superconducting/high-temperature superconducting (HTS) nuclear magnetic resonance magnet (1.3 G) is currently being built at Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology. One of its key components is an 800-MHz HTS insert (H800) comprising three nested coils. Each coil is a stack of double-pancake coils wound with 6-mm-wide 75-μm-thick REBCO tape. For this H800 generating its self-field of 18.6 T and being exposed to a total field as high as 30.5 T, overbanding each pancake coil is necessary to keep the conductor strain at < 0.6%. Although electromagnetic and mechanical details of the H800 had been considered during its design stage, a parametric study on the overband radial build considering winding tension effect should further confirm the results of our previous analysis. Thus, in this paper, based on Maxwell's equations and the equilibrium equations for mechanical deformation, we examine stress levels that the H800 experiences as H800 undergoes winding-energizing sequences during operation at 1.3 GHz. We also discuss the effects of overband radial build and winding tension on conductor stress in each coil. Finally, based on this analysis, we may further optimize the stainless-steel overbanding and winding tension on each H800 coil.

This paper presents a high-resolution magnetic field mapping system in development that is capable of collecting spatial magnetic field data for NMR magnets. An NMR probe was designed and built with a resonant frequency of 5.73 MHz. The measured Q-factor of the NMR probe is ∼191 with a half-power bandwidth in the range of 5.72–5.75 MHz. An RF continuous-wave technique with magnetic field modulation was utilized to detect the power dispersion of water molecules. The zero-crossing frequency of the NMR dispersion signal corresponds to the magnetic field at the center of the water sample. An embedded system was developed to sweep the frequency and record the reflected RF power simultaneously. A numerically controlled digital oscillator is able to provide a precise frequency step as small as 0.02 Hz, which is equivalent to 4.7 e-7 mT for hydrogen atoms. An RF preamplifier was built to supply up to 4 W of RF power to a bidirectional coupler. The coupler supplies RF power to the NMR probe and channels reflect the RF power back to the detection circuit, which detects the reflected RF power from the NMR probe during the frequency sweep. The homogeneity of an NMR magnet can be determined by magnetic field data.

A no-insulation (NI) technique has been applied to wind and test a NI HTS (YBCO) double-pancake coil at 4.2 K. Having little detrimental effect on field-current relationship, the absence of turn-to-turn insulation enabled the test coil to survive a quench at a coil current density of 1.58 kA/mm(2). The NI HTS coil is compact and self-protecting, two features suitable for large high-field magnets. To investigate beneficial impacts of the NI technique on >1 GHz LTS/HTS NMR magnets, we have designed six new NI HTS inserts for our ongoing 1.3 GHz LTS/HTS NMR magnet, which require less costly LTS background magnets than the original insulated HTS insert. A net result will be a significant reduction in the overall cost of an LTS/HTS NMR magnet, at 1.3 GHz and above.

This paper presents a passive shimming design approach for a magic-angle-spinning (MAS) NMR magnet. In order to achieve a 1.5-T magic-angle field in NMR samples, we created two independent orthogonal magnetic vector fields by two separate coils: the dipole and solenoid. These two coils create a combined 1.5-T magnetic field vector directed at the magic angle (54.74° from the spinning axis). Additionally, the stringent magnetic field homogeneity requirement of the MAS magnet is the same as that of a solenoidal NMR magnet. The challenge for the magic-angle passive shimming design is to correct both the dipole and solenoid magnetic field spherical harmonics with one set of iron pieces, the so-called ferromagnetic shimming. Furthermore, the magnetization of the iron pieces is produced by both the dipole and solenoid coils. In our design approach, a matrix of 2 mm by 5 mm iron pieces with different thicknesses was attached to a thin-walled tube, 90-mm diameter and 40-mm high. Two sets of spherical harmonic coefficients were calculated for both the dipole and solenoid coil windings. By using the multiple-objective linear programming optimization technique and coordinate transformations, we have designed a passive shimming set that can theoretically reduce 22 lower-order spherical harmonics and improve the homogeneity of our MAS NMR magnet.

A critical component of the 1.3-GHz nuclear magnetic resonance magnet (1.3 G) program, currently ongoing at the Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center, Massachusetts Institute of Technology, and now approaching its final stage, is the all high-temperature superconductor 800-MHz insert (H800). The insert consists of three nested double-pancake (DP) coils fabricated with 6-mm-wide REBCO conductor. Coil 1, the innermost coil of H800, has already been fabricated and tested at 77 and 4.2 K. In addition, one third of the DPs for Coil 2 have been wound and each DP individually fully tested. Work described here includes details of Coil 1 fabrication: DP winding, DP testing, assembling, joint performance, overbanding, and coil testing; winding details of DPs for Coil 2 and their testing are also included.

The screening current induced in rare-earth barium copper oxide (REBCO) tape generates an unwanted irregular magnetic field. The screening current-induced field (SCIF) is a challenging issue for MRI, NMR, and accelerators magnet composed of REBCO coils. A few FEM-based simulation methods have been proposed to estimate the SCIF; however, they require a long computation time. Recently, we have proposed a simple SCIF computation method based on the self and mutual inductances of REBCO pancake coils and screening current radial paths on the top and bottom of pancake coils. The accuracy of the proposed method is not excellent; however, the computation time is quite short. In this paper, we report an equivalent circuit model that includes the self and mutual inductances of a REBCO pancake coil and screening current radial path. Moreover, with this proposed method, we can compute the SCIF of no-insulation (NI) REBCO pancake coils, which is not the case with the previously proposed FEM-based simulation method. The proposed method has been validated by experimentation. The proposed method is available online.

We present results—cool-down, energization, and persistent-mode operation—of a solid-nitrogen (SN2)-cooled, magnesium diboride (MgB2) small-scale test coil. The test coil, immersed in a volume of solid nitrogen at 6 K, successfully operated in persistent-mode at 108 A for a period of 5 d. Although designated a ‘persistent-mode’ coil, its center field was measured to decay at a rate of −1, which is still considered low enough to meet the temporal stability requirement of −1, for most magnetic resonance imaging magnets. This decay rate translates to a calculated circuit resistance of –12 Ω, which is mainly from one MgB2-MgB2 joint in the circuit. However, when the coil temperature increased from 6 to 16 K, the field had dropped by 0.33%: we believe this was caused by the change of magnetization in the MgB2 superconductor, which in turn decreased a screening-current field (SCF) at the magnet center. We performed a finite element analysis with a simplified numerical model based on H formulation to verify whether magnetization-induced SCF is responsible for this 0.33% drop. Indeed, the model shows that the change of magnetization, i.e. screening-current reduction and current density redistribution, happens during temperature-cycle-induced Jc (T) variation, and thus affects the center magnetic field. However, the Jc (T) variation in the 2nd cycle had little effect on MgB2 magnetization and thus had negligible magnetic field change.

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.

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.

A high-resolution 1.3-GHz/54-mm low-temperature superconducting/high-temperature superconducting (HTS) nuclear magnetic resonance magnet (1.3 G) is currently in the final stage at the Massachusetts Institute of Technology Francis Bitter Magnet Laboratory. Its key component is a three-coil (Coils 1–3) 800-MHz HTS insert comprising 96 no-insulation (NI) double-pancake coils, each wound with a 6-mm-wide GdBCO tape. In this paper, after describing the overall 1.3-G system, we present innovative design features incorporated in 1.3 G: 1) an NI winding technique applied to Coils 1–3 and its adverse effect in the form of charging time delay; 2) persistent-mode HTS shims; 3) a “shaking” magnet; and 4) preliminary results of Coil 1 operated at 4.2 K.

This paper deals with the mechanical strain issue in a high-temperature superconducting (HTS) insert for a GHz–class (> 23.5 T) LTS/HTS NMR magnet. We present results, experimental and analytical, of hoop strains in a double-pancake (DP) test coil, wound with 6-mm wide YBCO coated conductor (CC) and equipped with strain gauges at their innermost and outermost turns. To keep the YBCO CC to within a 95% Ic retention, the conductor tensile strain must be limited to 0.6%. To satisfy this strain limit in our test DP coil, we wrapped 0.08-mm thick, 6-mm wide stainless steel strip over its outermost turn of an 4.8-mm overband radial build deemed sufficient by our stress analysis based on force equilibrium and generalized Hooke’s law with plane stress approximation. A control test DP coil, actually the same test DP coil, without overbanding, was run under the same experimental condition. In each case the test DP coil was energized up to 350 A at 4.2 K in a background field of 4 T. We report the experiment and analysis, with discussion on the merit of overbanding as a means to limit hoop strain in high-field HTS inserts.

Two types of shaking coils are focused on reducing screening currents induced in solenoid coils wound with high-temperature superconducting (HTS) tapes. One is a pair of copper shaking coils coaxially located inside and outside the HTS coil to apply an ac magnetic field in the axial direction. The other is an HTS shaking coil with notch located only outside the HTS coil to minimize the radial components of local ac fields applied to windings of the HTS coil as small as possible. It is found that the copper shaking coils yield the allowable amount of power dissipation in liquid helium. The effectiveness of the HTS shaking coil to reduce screening-current-induced fields generated by another magnetized HTS coil is also experimentally validated in liquid nitrogen using a commercially available coated conductor with narrow width.

In this paper, we present results, experimental and numerical, of the electromagnetic interaction forces between pairs of racetrack coils under time-varying conditions. Three turn-to-turn insulation designs were applied to wind three racetrack coils with GdBCO coated conductor: 1) no insulation (NI); 2) partial insulation (PI) of a polyimide layer every eight turns; and 3) insulation (INS) of a polyimide layer between each, i.e., NI, PI, and INS racetracks. Two racetrack pairs, namely, NI-INS and PI-INS, were tested for their interaction forces, measured with load cell under current-ramping conditions in a bath of liquid nitrogen at 77 K. Good experimental and simulation results validate our equivalent circuit model to compute interaction forces of PI-INS racetrack pair. Overcurrent test of NI and PI coils, where each racetrack coil was charged above critical current (I c ), was also performed to compare coil stability. This result implies that, although the PI winding technique improves the dynamic response, stability will be somewhat compromised.

Stability margin of a high-temperature superconducting (HTS) coil is two or three orders of magnitude greater than that of a low-temperature superconducting coil. In recent years, many papers have reported test results of turn-to-turn no-insulation (NI) HTS coils having extremely enhanced thermal stability, such that burnout never occurs in an NI coil, even at an operating current exceeding 2.5 times the critical current. Thus, The main goal of this paper is to clarify transient electromagnetic and thermal behaviors and mechanism of the high thermal stability in an NI REBCO coil. A partial element equivalent circuit (PEEC) model is proposed for the numerical simulation of an NI REBCO coil, which considers a local electrical contact resistance between turns, an I–V characteristic of an REBCO tape, and local self and mutual inductances of the NI REBCO coil. Using the PEEC model, we investigate the influence of the turn-to-turn contact resistance on the transient behavior of the NI REBCO coil during sudden discharging. We also perform thermal conduction analyses with the PEEC model to clarify the transient behavior of an NI REBCO coil during an overcurrent operation.

As a sequel to our previous report on the key concept of the multiwidth (MW) no-insulation (NI) winding technique that applies the NI technique to a magnet assembly of double-pancake (DP) coils wound with varying tape widths (thus MW), this paper presents construction and test results of a 7-T/68-mm cold-bore MW-NI GdBCO magnet. A total of 13 DP coils were fabricated: five central DP coils were wound with 4.1-mm wide tape, while each of the remaining four pairs of DP coils, axially symmetric to the midplane, was wound, from center to end, with 5.1-, 6.1-, 7.1-, and 8.1-mm wide tapes. Each DP coil was tested in a bath of liquid nitrogen (LN2) at 77 K, to obtain its critical current and characteristic resistance. After stacking, the 13-DP magnet was tested in LN2 at 77 K. The key focuses of this paper are: 1) winding and testing of MW-NI DP coils; 2) DP–DP joints; 3) charging response of the magnet; 4) spatial field distribution; and 5) self-protecting feature in overcurrent operation.

In this paper, we report final operation results of our compact annulus NMR magnet, named YP2800, with a homemade micro-NMR probe in a bath of liquid helium at 4.2 K. YP2800 comprises of a stack of 2800 YBCO "plate annuli," 0.08 mm thick, either 46 mm or 40 mm square, each having a 26-mm hole machined at the center. By the field-cooling technique, YP2800 was energized at 130 MHz (3.05 T); an overall peak-to-peak homogeneity of 487 ppm within |z| < 5 mm was measured at a moment when a field drift of 11 ppm/h was reached in three days after field cooling. Due to a small (9.2 mm) bore size, no commercial probes could fit into the bore; an 8.5-mm micro-NMR probe was designed and constructed. Following a general description of YP2800 and design construction details of the micro probe, this paper presents NMR signals captured by the probe for a dimethyl sulfoxide sample of ϕ 4.4 and 5 mm long at a base frequency of 130 MHz with a half-peak width of 60 kHz; the corresponding frequency impurity of 461 ppm is chiefly due to a spatial field error, i.e., 487 ppm in the target space.

The second generation high temperature superconductor, specifically REBCO, has become a new research focus in the development of a new generation of high-field (>25 T) magnets. One of the main challenges in the application of the magnets is the current screening problem. Previous research shows that for magnetized superconducting stacks and bulks the application of an AC field in plane with the circulating current will lead to demagnetization due to vortex shaking, which provides a possible solution to remove the shielding current. This paper provides an in-depth study, both experimentally and numerically, to unveil the vortex shaking mechanism of REBCO stacks. A new experiment was carried out to measure the demagnetization rate of REBCO stacks exposed to an in-plane AC magnetic field. Meanwhile, 2D finite element models, based on the E–J power law, are developed for simulating the vortex shaking effect of the AC magnetic field. Qualitative agreement was obtained between the experimental and the simulation results. Our results show that the applied in-plane magnetic field leads to a sudden decay of trapped magnetic field in the first half shaking cycle, which is caused by the magnetic field dependence of critical current. Furthermore, the decline of demagnetization rate with the increase of tape number is mainly due to the cross-magnetic field being screened by the top and bottom stacks during the shaking process, which leads to lower demagnetization rate of inner layers. We also demonstrate that the frequency of the applied AC magnetic field has little impact on the demagnetization process. Our modeling tool and findings perfect the vortex shaking theory and provide helpful guidance for eliminating screening current in the new generation REBCO magnets.

We are currently working on a program to complete a 1.5-T/75-mm RT (room temperature) bore MAS (magic-angle-spinning) NMR (nuclear magnetic resonance) magnet. The MAS magnet comprises a z-axis 0.866-T solenoid and an x-axis 1.225-T dipole coil. The combination of the fields creates a 1.5-T field pointed at 54.74 degrees (magic angle) from the rotation (z) axis. During the 2nd year of this 3-year Phase I program, both coils have been wound and testing has begun. Some preliminary field mapping has been performed, and the design of the MAS magnet assembly has been completed. During the final year, the magnet assembly will be integrated into the cryogenic structure and tested at ~5.5 K in a solid nitrogen environment. Each coil will be energized separately, and the magnetic field will be mapped accurately. We expect a bare magnet uniformity of 100 ppm over a 10-mm diameter, 20-mm-long cylindrical volume. Then, using the field data, the uniformity will be improved to < 0.1.ppm with a combination of ferroshims and cryoshims. Final field measurement will be performed as the cryostat-magnet system is spun manually at ~0.1 Hz.

The superconducting dipole magnets wound with HTS wires have been developed for a magic-angle-spinning NMR. The magnetic field of the dipole magnets is tilted from z-axis. Since the highly homogeneous magnetic field is required, the magnetic field of the dipole magnets is necessarily compensated by passive and/or active shimming. However, the compensation of all the components has not been reported. Usually, due to the axial symmetry of a solenoid magnet, only z component of the magnetic field is homogenized. In this paper, all the x, y, and z components of the magnetic field generated by the superconducitng dipole magnet are compensated because of the tilt from the z-axis.

A no-insulation winding technique for an NbTi superconducting coil is expected to improve its dynamic stability during charging or discharging. Experimental results, recently reported, demonstrated the charging stability enhancement of an NI coil. In an NI winding, an amount of current can bypass to adjacent wires to avoid the generation of a local hot spot and, as a result, the dynamic stability is improved. In spite of the potential technical merits of the NI technique, the charging responses of an NI coil, including the non-uniform current distribution under a time-varying condition, have not been investigated analytically yet. In this paper, we propose a simulation method using an equivalent circuit equation for an NI coil with its inductance and the contact turn-to-turn resistances taken into consideration. The simulation results were analyzed in three aspects: terminal voltage, loss energy, and stored magnetic energy.

This paper presents experimental and analytical results of electromagnetic forces between no-insulation (NI) and insulation (INS) coils under time-varying conditions for NI coils that may be used in wind power generator. Three test pancake coils, one NI and the others INS, of the identical winding i.d., bore size, and number of turns, are wound with GdBCO coated tape. To determine an electric circuit model of the NI coil, the coil was charged-and-discharged at different ramping rates. The data were used to compute a characteristic resistance of a parallel inductor/resistor circuit model. In force measurement, two sets of pancake pairs were tested, NI-INS and INS-INS, the later for comparison. The coils were tested in a bath of liquid nitrogen at 77 K. The dynamic force response of the NI-INS pair was compared with that of INS-INS. The analysis based on the circuit model was tested with the experimental results. Analysis agrees well with experiment, validating our technique to analyze the time-varying forces between the NI-INS pair.

This paper deals with the latest design of an all High Temperature Superconductor (HTS) insert for our 1.3 GHz NMR program. Due to a loss of our original 600 MHz HTS insert, the entire program was revised, and upon further examination of the options available for the HTS insert, which included not only conductor material properties, but also new winding technologies recently developed here at the FBML, it has finally been decided to build an 800 MHz (H800) insert purely based in SuperPower YBCO conductor. The new H800 will still be comprised of nested stacks of double-pancake coils and will run in the background of a 500 MHz LTS NMR magnet already available at FBML. We present here electromagnetic and mechanical details of the H800 design, winding technique employed, and testing of individual coils.