In this paper, we apply a surface-shunting method to prevent quenches in no-insulation (NI) REBCO magnets triggered by external failures of magnet current leads or power suppliers (i.e., fault mode). In a high-field magnet system, an NI coil may still be at risk during the mentioned quench events even if the whole magnet is well-designed, non-defective, and properly operated. The mechanism of this fault-mode quench initiation and propagation still remains unclear, complicating the development of reliable quench protection. Here, we present this mechanism to demonstrate a corresponding practical quench-preventive approach named surface shunting, which utilizes a low-temperature solder attached to the top and bottom of pancake coils. We validate the effectiveness of this approach by comparing the electromagnetic, thermal, and mechanical behaviors in the fault mode with and without the shunt. We conclude that the surface shunt suppresses the fault-mode quench initiation and propagation by redirecting the original turn-to-turn current and induced overcurrent out of the NI winding. We anticipate this work can provide a solution to improve the operational safety of high-field HTS NI magnets against quench and potential damage during fault modes.

Brain imaging MRI comprises a significant proportion of MRI scans, but the requirement for including the shoulders in the magnet bore means there is not a significant size reduction in the magnet compared to whole-body magnets. Here we present a new design approach for brain imaging MRI magnets targeting ±20 kHz variation over the imaging volume rather than the more usual ±200 Hz making use of novel high-bandwidth MRI pulse sequences and distortion correction. Using this design approach, we designed and manufactured a 1.5 T class ReBCO cryogen-free magnet. The magnet is dome-like in form, completely excludes the shoulders and is <400 mm long. The magnet was wound using no-insulation style coils with a conductive epoxy encapsulant where the contact resistance of the coils was controlled so the emergency shut-down time of the magnet was less than 30 s. Despite acceptable coil testing results ahead of manufacture, during testing of the magnet, several of the epoxy coils showed signs of damage limiting stable performance to <55 A compared to the designed 160 A. These coils were replaced with insulated paraffin encapsulated coils. Subsequently the magnet was re-ramped and was stable at 81 A, generating 0.71 T as several other coils had sustained damage not visible in the first magnet iteration. The magnet has been passive shimmed to ±20 kHz variation over the imaging volume and integrated into an MRI scanner. The stability of the magnet has been evaluated and found to be acceptable for MRI.

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.

Bulk samples of magnesium diboride (MgB) doped with 0.5 wt% of the rare earth oxides (REOs) NdO and DyO (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB doped with 0.5 wt% DyO (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature ( ), magnetic and resistive measurements of upper critical field ( ) and irreversibility field ( ), as well as magnetic and transport measurements of critical current densities versus applied field ( () and (), respectively). It was found that although the products of REO doping did not substitute into the MgB lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density ( ) versus reduced field ( = / ) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the () of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures () was still further increased by a DyO additive-induced increase in of about 1 T at all temperatures up to 20 K (and beyond). It is noted that DyO increases and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB conductors.

MgB superconducting wires made using a Mg infiltration method have reached a higher performance than either or mixed powder based routes. Indeed, very high layer coupled with whole-strand (critical current per total strand cross section) exceeding 10 A cm at 4.2 K, 10 T have been found for monocore MgB wires. However, previous multicore infiltration route wires have not reached their potential for due to partially reacted and non-uniform MgB layers. This study shows that 18-core MgB AIMI wires processed using a low temperature route can attain higher and more uniform values due to a more uniform MgB reaction layer. The formation of fully reacted, uniform MgB layers is attributed to the switch from a liquid-solid to a vapor-solid reaction route.

Overpressure (OP) processing of wind-and-react BiSrCaCuO (2212) round wire compresses the wire to almost full density, decreasing its diameter by about 4 % without change in wire length and substantially raising its . However, such shrinkage can degrade coil winding pack density and magnetic field homogeneity. To address this issue, we here present an overpressure predensification (OP-PD) heat treatment process performed before melting the 2212, which greatly reduces wire diameter shrinkage during the full OP heat treatment (OP-HT). We found that about 80 % of the total wire diameter shrinkage occurs during the 50 atm OP-PD before melting. We successfully wound such pre-densified 1.2 mm diameter wires onto coil mandrels as small as 10 mm diameter for Ag-Mg-sheathed wire and 5 mm for Ag-sheathed wire, even though such small diameters impose plastic strains up to 12% on the conductor. A further ~20% shrinkage occurred during a standard OP-HT. No 2212 leakage was observed for coil diameters as small as 20 mm for Ag-Mg-sheathed wire and 10 mm for Ag-sheathed wire, and no degradation was observed on straight samples and 30 mm diameter coils.

The development of coils that can survive a quench is crucial for demonstrating the viability of MgB-based main magnet coils used in MRI systems. Here we have studied the performance and quench properties of a large (outer diameter: 901 mm; winding pack: 44 mm thick × 50.6 mm high) conduction-cooled, react-and-wind (R&W), MgB superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents ( ), and distinguished from the minimum energy needed to generate a normal zone (MGE). During these measurements, normal zone propagation velocities (NZPV) were also determined using multiple voltage taps placed around the heater zone. The conduction cooled coil obtained a critical current ( ) of 186 A at 15 K. As the operating currents ( ) varied from 80 A to 175 A, MQE ranged from 152 J to 10 J, and NZPV increased from 1.3 to 5.5 cm/s. Two kinds of heater were involved in this study: (1) a localized heater ("test heater") used to initiate the quench, and (2) a larger "protection heater" used to protect the coil by distributing the normal zone after a quench was detected. The protection heater was placed on the outside surface of the coil winding. The test heater was also placed on the outside surface of the coil at a small opening made in the protection heater. As part of this work, we also developed and tested an active protection scheme for the coil. Such active protection schemes are of great interest for MgB-based MRIs because they permit exploitation of the relatively large MQE values of MgB to enable the use of higher values which in turn lead to competitive MgB MRI designs. Finally, the ability to use a quench detection voltage to fire a protection heater as part of an active protection scheme was also demonstrated.

Conceptual designs of 1.5 and 3.0 T full-body magnetic resonance imaging (MRI) magnets using conduction cooled MgB superconductor are presented. The sizes, locations, and number of turns in the eight coil bundles are determined using optimization methods that minimize the amount of superconducting wire and produce magnetic fields with an inhomogeneity of less than 10 ppm over a 45 cm diameter spherical volume. MgB superconducting wire is assessed in terms of the transport, thermal, and mechanical properties for these magnet designs. Careful calculations of the normal zone propagation velocity and minimum quench energies provide support for the necessity of active quench protection instead of passive protection for medium temperature superconductors such as MgB. A new 'active' protection scheme for medium based MRI magnets is presented and simulations demonstrate that the magnet can be protected. Recent progress on persistent joints for multifilamentary MgB wire is presented. Finite difference calculations of the quench propagation and temperature rise during a quench conclude that active intervention is needed to reduce the temperature rise in the coil bundles and prevent damage to the superconductor. Comprehensive multiphysics and multiscale analytical and finite element analysis of the mechanical stress and strain in the MgB wire and epoxy for these designs are presented for the first time. From mechanical and thermal analysis of our designs we conclude there would be no damage to such a magnet during the manufacturing or operating stages, and that the magnet would survive various quench scenarios. This comprehensive set of magnet design considerations and analyses demonstrate the overall viability of 1.5 and 3.0 T MgB magnet designs.

We performed a feasibility study on a high-strength Bi Pb SrCaCuO (Bi-2223) tape conductor for high-field solenoid applications. The investigated conductor, DI-BSCCO Type HT-XX, is a pre-production version of Type HT-NX, which has recently become available from Sumitomo Electric Industries (SEI). It is based on their DI-BSCCO Type H tape, but laminated with a high-strength Ni-alloy. We used stress-strain characterizations, single- and double-bend tests, easy- and hard-way bent coil-turns at various radii, straight and helical samples in up to 31.2 T background field, and small 20-turn coils in up to 17 T background field to systematically determine the electro-mechanical limits in magnet-relevant conditions. In longitudinal tensile tests at 77 K, we found critical stress- and strain-levels of 516 MPa and 0.57%, respectively. In three decidedly different experiments we detected an amplification of the allowable strain with a combination of pure bending and Lorentz loading to ≥ 0.92% (calculated elastically at the outer tape edge). This significant strain level, and the fact that it is multi-filamentary conductor and available in the reacted and insulated state, makes DI-BSCCO HT-NX highly suitable for very high-field solenoids, for which high current densities and therefore high loads are required to retain manageable magnet dimensions.

This paper presents construction details and test results of a persistent-mode 0.5-T MgB magnet developed at the Francis Bitter Magnet Lab, MIT. The magnet, of 276-mm inner diameter and 290-mm outer diameter, consisted of a stack of 8 solenoidal coils with a total height of 460 mm. Each coil was wound with monofilament MgB wire, equipped with a persistent-current switch and terminated with a superconducting joint, forming an individual superconducting loop. Resistive solder joints connected the 8 coils in series. The magnet, after being integrated into a testing system, immersed in solid nitrogen, was operated in a temperature range of 10-13 K. A two-stage cryocooler was deployed to cool a radiation shield and the cold mass that included mainly ~60 kg of solid nitrogen and the magnet. The solid nitrogen was capable of providing a uniform and stable cryogenic environment to the magnet. The magnet sustained a 0.47-T magnetic field at its center persistently in a range of 10-13 K. The current in each coil was inversely calculated from the measured field profile to determine the performance of each coil in persistent-mode operation. Persistent-current switches were successfully operated in solid nitrogen for ramping the magnet. They were also designed to absorb magnetic energy in a protection mechanism; its effectiveness was evaluated in an induced quench.

Superconducting joints are one of the key components needed to make Ag-alloy clad BiSrCaCuO (Bi-2212) superconducting round wire (RW) successful for high-field, high-homogeneity magnet applications, especially for nuclear magnetic resonance (NMR) magnets in which persistent current mode (PCM) operation is highly desired. In this study, a procedure for fabricating superconducting joints between Bi-2212 round wires during coil reaction was developed. Melting temperatures of Bi-2212 powder with different amounts of Ag addition were investigated by differential thermal analysis (DTA) so as to provide information for selecting the proper joint matrix. Test joints of 1.3 mm dia. wires heat treated in 1 bar flowing oxygen using the typical partial melt Bi-2212 heat treatment (HT) had transport critical currents of ~900 A at 4.2 K and self-field, decreasing to ~480 A at 14 T evaluated at 0.1 μV/cm at 4.2 K. Compared to the of the open-ended short conductor samples with identical 1 bar HT, the values of the superconducting joint are ~20% smaller than that of conductor samples measured in parallel field but ~20% larger than conductor samples measured in perpendicular field. Microstructures examined by scanning electron microscopy (SEM) clearly showed the formation of a superconducting Bi-2212 interface between the two Bi-2212 round wires. Furthermore, a Bi-2212 RW closed-loop solenoid with a superconducting joint heat treated in 1 bar flowing oxygen showed an estimated joint resistance below 5×10 Ω based on its field decay rate. This value is sufficiently low to demonstrate the potential for persistent operation of large inductance Bi-2212 coils.

Magnetic Resonance Imaging (MRI), a powerful medical diagnostic tool, is the largest commercial application of superconductivity. The superconducting magnet is the largest and most expensive component of an MRI system. The magnet configuration is determined by competing requirements including optimized functional performance, patient comfort, ease of siting in a hospital environment, minimum acquisition and lifecycle cost including service. In this paper, we analyze conductor requirements for commercial MRI magnets beyond traditional NbTi conductors, while avoiding links to a particular magnet configuration or design decisions. Potential conductor candidates include MgB, ReBCO and BSCCO options. The analysis shows that no MRI-ready non-NbTi conductor is commercially available at the moment. For some conductors, MRI specifications will be difficult to achieve in principle. For others, cost is a key barrier. In some cases, the prospects for developing an MRI-ready conductor are more favorable, but significant developments are still needed. The key needs include the development of, or significant improvements in: (a) conductors specifically designed for MRI applications, with form-fit-and-function readily integratable into the present MRI magnet technology with minimum modifications. Preferably, similar conductors should be available from multiple vendors; (b) conductors with improved quench characteristics, i.e. the ability to carry significant current without damage while in the resistive state; (c) insulation which is compatible with manufacturing and refrigeration technologies; (d) dramatic increases in production and long-length quality control, including large-volume conductor manufacturing technology. In-situ MgB is, perhaps, the closest to meeting commercial and technical requirements to become suitable for commercial MRI. Conductor technology is an important, but not the only, issue in introduction of HTS / MgB conductor into commercial MRI magnets. These new conductors, even when they meet the above requirements, will likely require numerous modifications and developments in the associated magnet technology.

Overpressure (OP) processing increases the critical current density ( ) of BiSrCaCuO (2212) round wires by shrinking the surrounding Ag matrix around the 2212 filaments, driving them close to full density and greatly increasing the 2212 grain connectivity. Indeed densification is vital for attaining the highest . Here, we investigate the time and temperature dependence of the wire densification. We find that the wire diameter decreases by 3.8 ± 0.3 % after full heat treatment at 50 atm and 100 atm OP. At 50 atm OP pressure, the filaments start densifying above 700 °C and reach a 3.30 ± 0.07 % smaller diameter after 2 h at 820 °C, which is below the melting point of 2212 powder. The densification is homogeneous and does not change the filament shape before melting. The growth of non-superconducting phases is observed at 820 °C, suggesting that time should be minimized at high temperature prior to melting the 2212 powder. Study of an open-ended 2.2 m long wire sample shows that full densification and the high OP ( varies by about 3.1 times over the 2.2 m long wire) is reached about 1 m from the open ends, thus showing that coil-length wires can be protected from leaky seals by adding at least 1 m of sacrificial wire at each end.

This paper presents a warm bore ferromagnetic shimming design for a high resolution NMR magnet based on spherical harmonic coefficient reduction techniques. The passive ferromagnetic shimming along with the active shimming is a critically important step to improve magnetic field homogeneity for an NMR Magnet. Here, the technique is applied to an NMR magnet already designed and built at the MIT's Francis Bitter Magnet Lab. Based on the actual magnetic field measurement data, a total of twenty-two low order spherical harmonic coefficients is derived. Another set of spherical harmonic coefficients was calculated for iron pieces attached to a 54 mm diameter and 72 mm high tube. To improve the homogeneity of the magnet, a multiple objective linear programming method was applied to minimize unwanted spherical harmonic coefficients. A ferromagnetic shimming set with seventy-four iron pieces was presented. Analytical comparisons are made for the expected magnetic field after Ferromagnetic shimming. The theoretically reconstructed magnetic field plot after ferromagnetic shimming has shown that the magnetic field homogeneity was significantly improved.

Long lengths of metal/MgB composite conductors with high critical current density (J), fabricated by the power-in-tube (PIT) process, have recently become commercially available. Owing to its electromagnetic performance in the 20 K - 30 K range and relatively low cost, MgB may be attractive for a variety of applications. One of the key issues for magnet design is stability and quench protection, so the behavior of MgB wires and magnets must be understood before large systems can emerge. In this work, the stability and quench behavior of several conduction-cooled MgB wires are studied. Measurements of the minimum quench energy and normal zone propagation velocity are performed on short samples in a background magnetic field up to 3 T and on coils in self-field and the results are explained in terms of variations in the conductor architecture, electrical transport behavior, operating conditions (transport current and background magnetic field) and experimental setup (short sample vs small coil). Furthermore, one coil is quenched repeatedly with increasing hot-spot temperature until J is decreased. It is found that degradation during quenching correlates directly with temperature and not with peak voltage; a safe operating temperature limit of 260 K at the surface is identified.

This letter presents a flux pumping method and the results gained when it was used to magnetize a range of different YBCO coils. The pumping device consists of an iron magnetic circuit with eight copper coils which apply a traveling magnetic field to the superconductor. The copper poles are arranged vertically with an air gap length of 1 mm and the iron cores are made of laminated electric steel plates to minimize eddy-current losses. We have used this arrangement to investigate the best possible pumping result when parameters such as frequency, amplitude and waveform are varied. We have successfully pumped current into the superconducting coil up to a value of 90% of and achieved a resultant magnetic field of 1.5 T.

In this article I present our experience at the Magnet Technology Division of the MIT Francis Bitter Magnet Laboratory on liquid-helium (LHe)-free, persistent-mode MgB MRI magnets. Before reporting on our MgB magnets, I first summarize the basic work that we began in the late 1990s to develop LHe-free, high-temperature superconductor (HTS) magnets cooled in solid cryogen-I begin by discussing the enabling feature, particularly of solid nitrogen (SN2), for HTS magnets. The next topic is our first LHe-free, SN2-HTS magnet, for which we chose Bi2223 because in the late 1990s Bi2223 was the only HTS available to build an HTS magnet. I then move on to two MgB magnets, I and II, developed after discovery of MgB in 2000. The SN2-MgB Magnet II-0.5-T/240-mm, SN2-cooled, and operated in persistent mode-was completed in January 2016. The final major topic in this article is a tabletop LHe-free, persistent-mode 1.5-T/70-mm SN2-MgB "finger" MRI magnet for osteoporosis screening-we expect to begin this project in 2017. Before concluding this article, I present my current view on challenges and prospects for MgB MRI magnets.

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.

In Part 2 of these articles, an extensive analysis of pinning-force curves and raw scaling data was used to derive the Extrapolative Scaling Expression (ESE). This is a parameterization of the Unified Scaling Law (USL) that has the extrapolation capability of fundamental unified scaling, coupled with the application ease of a simple fitting equation. Here in Part 3, the accuracy of the ESE relation to interpolate and extrapolate limited critical-current data to obtain complete datasets is evaluated and compared with present fitting equations. Accuracy is analyzed in terms of root mean square (RMS) error and fractional deviation statistics. Highlights from 92 test cases are condensed and summarized, covering most fitting protocols and proposed parameterizations of the USL. The results show that ESE reliably extrapolates critical currents at fields , temperatures , and strains that are remarkably different from the fitted minimum dataset. Depending on whether the conductor is moderate- or high- , effective RMS extrapolation errors for ESE are in the range 2-5 A at 12 T, which approaches the measurement error (1-2%). The for extrapolating full characteristics is also determined from raw scaling data. It consists of one set of data at a fixed temperature (e.g., liquid helium temperature), and one set of data at a fixed strain (e.g., zero applied strain). Error analysis of extrapolations from the minimum dataset with different fitting equations shows that ESE reduces the percentage errors at individual data points at high fields, temperatures, and compressive strains down to 1/10th to 1/40th the size of those for extrapolations with present fitting equations. Depending on the conductor, percentage fitting errors for are also reduced to as little as 1/15th the size. The extrapolation accuracy of the ESE relation offers the prospect of straightforward implementation of the USL in several new areas: (l) A five-fold reduction in the measurement space for unified temperature-strain apparatuses through extrapolation of minimum datasets; (2) Combination of data from temperature and strain apparatuses, which provides flexibility and productive use of more limited data; and (3) Full conductor characterization from as little as a single curve when a few core parameters have been measured in a similar conductor. Default scaling parameter values are also given, based on analysis of a wide range of practical NbSn conductors.

We present a kinetic-inductance-based superconducting memory element with non-destructive readout, femtojoule read and write energies, both read and write shunts, which is writeable with pulses shorter than 400 ps. The element utilizes both a high-kinetic-inductance layer made from tungsten silicide as well as a low-kinetic-inductance layer made from niobium. By using tungsten silicide-which has a long (20 ns) thermal time constant-and measuring bit error rates from 10 MHz to 1 GHz, we were able to verify that the thin-film elements could be operated at a data rate at least as fast as the material thermal time constant with a bit error ratio less than 10. We also analyze the margins of the device, and outline the characteristics by which a more efficient device may be designed.