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The development of coils that can survive a quench is crucial for demonstrating the viability of MgB2-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, MgB2 superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents (Iop), 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 (Ic) of 186 A at 15 K. As the operating currents (Iop) varied from 80 to 175 A, MQE ranged from 152 to 10 J, and NZPV increased from 1.3 to 5.5 cm s−1. 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 MgB2-based MRIs because they permit exploitation of the relatively large MQE values of MgB2 to enable the use of higher Je values which in turn lead to competitive MgB2 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. [ABSTRACT FROM AUTHOR]

A react-and-wind MgB2 coil segment for a conduction-cooled magnetic resonance imaging (MRI) machine has been fabricated and tested. The coil was developed as part of a collaborative effort on a conduction-cooled, MgB2-based, whole-body MRI image guided radiation therapy device. This study focuses on the fabrication, winding, instrumentation, cooling, and initial critical current (Ic) testing of this near-full-size MgB2 segment coil. The coil was 0.9 m in diameter; the winding pack, 44.0 mm wide × 50.6 mm high, used 1.7 km of an 18 filament MRI-style conductor with Nb chemical barriers, Cu interfilamentary matrices, and an outer monel sheath. The conductor was insulated and reacted before winding onto a stainless steel former. The coil was instrumented with Cernox and E-type thermocouple temperature sensors, strain sensors, and voltage taps. The conduction-cooled coil was mounted in a cryostat capable of accepting coils of up to 0.9 m in diameter and 0.5 m in height. Critical current measurements were performed as a function of temperature during a controlled heating of the coil. The operational target was I = 200 A at 13 K. The full magnet was designed to produce 0.75 T in the imaging area (at I = 200 A), with a maximum field of 1.93 T in the winding. The single segment coil measured here exceeded this operation specification, with an Ic of 280 A at 15 K and a maximum field 1.93 T in the winding. The coil was modeled using a finite element method, and a load line plot showed that 100% of short sample was reached at 21.5 K and above. These measurements demonstrate the viability of conduction-cooled MgB2 background coils as replacements for liquid helium cooled NbTi background coils in future MRI devices. [ABSTRACT FROM AUTHOR]

The generation and propagation of quench induced by a local heat disturbance or by overcurrents in MgB2 Rutherford cables have been studied experimentally. The analysed cable is composed of 12 strands of monocore MgB2/Nb/Cu10Ni wire and has a transposition length of about 27 mm. Measurements of intra- and inter-strand voltages have been performed to analyse the superconducting-to-normal transition behaviour of these cables during quench. In case of external hot-spots, two different time-dynamic regimes have been observed, a slow stage for the formation of the minimum propagation zone (MPZ), and a fast dynamics once the quench is triggered and propagates to the rest of the cable. Significant local variations of the quench propagation velocity across the strands around the MPZ have been observed, but with average quench propagation velocities closely correlated with the predictions given by one-dimensional-geometry models. For quench induced by overcurrents (i.e. with applied currents higher than the critical current) the nucleation of many normal zones distributed within the cable, which overlap during quench propagation, gives a distinctive and faster quench dynamics. [ABSTRACT FROM AUTHOR]

Two types of Rutherford cables made of two strand layers of commercial MgB2 wires manufactured by Hyper Tech Research, Inc. have been made. Flat rectangular cables consisting of 12 single-core MgB2/Nb/Cu10Ni, or 6-filaments MgB2/Nb/Cu strands, both of diameter 390 mewm, were assembled using a back-twist cabling machine with transposition length of 20 mm. In order to analyze impact of the cable compaction on critical currents, cables were two-axially rolled, each by a single step reduction of 3.5%−29.7% to thickness range of 0.775−0.62 mm. It was found that by increasing the packing factor (PF) of cable above 0.79, the critical current begins to increase. It is improved nearly two times up to the PF limit 0.89. Compaction over the PF limit introduced cable degradation and decrease of critical current. Bending tests applied to cables showed that critical current degradation starts below the bending diameter 120 mm for 6-filaments Cu sheath and 70 mm for single-core Cu10Ni sheath cable. Tensile tests showed similar irreversible strain values for the both types of cables. Rutherford cables assembled of single-core strands are promising for low field (2.7−4 T) applications where low bending diameters are required. [ABSTRACT FROM AUTHOR]

In this paper we present the results of critical current (Ic) measurements of MgB2 wires made with two different set-ups of the four-point probe method: current sweep type—constant magnetic field and increasing current, and field sweep type—constant current and rapidly increasing magnetic field. Results from magnet field sweep type measurements can be interpreted by a new physical concept—a jump of the electric field in low magnetic fields. This physical concept can be correlated with damages in the Nb-barrier existing in the MgB2 wire and be employed as a detection scheme. The damage in Nb barrier reduces critical current density (Jc) and complicates the study on critical temperature (Tc), upper critical field (Bc2), irreversible magnetic field (Birr), pinning force (Fp), and pinning centers in superconducting MgB2 wires. Our proposed method to detect damages in Nb barrier would benefit efforts in development and applications of MgB2 wires. [ABSTRACT FROM AUTHOR]

Multifilamentary MgB2 wires were annealed under the high pressure of low-friction, solid state media such as BN and graphite powders. The idea was to replace the hot isostatic pressing in argon medium, which is beneficial to wires transport properties but is difficult for application on an industrial scale. The experimental results show that annealing in BN or graphite under 200 MPa leads to enhancement of the critical current, similar to that observed earlier for high pressure argon annealing. An unwanted effect of the process is the flattening of the round wires, which after the treatment yields an elliptical cross section with b to a ratio of around 85 ± 5%. Comparison of critical currents and microstructure of samples synthesized with different parameters are presented. [ABSTRACT FROM AUTHOR]

Unreacted MgB2 wires were made at Hyper Tech Research, USA by a continuous tube forming and filling method using mixtures of Mg and B with and without SiC powder additions. All of the wires underwent hot isostatic pressure (HIP) treatment at the Institute of High Pressure. The first part of the wire was annealed at a pressure of 1 GPa, and the second part was annealed at 0.1 MPa. In this work, we show the influence of high pressure on critical current density (Jc), pinning force (Fp), critical temperature (Tc), irreversible magnetic fields (Birr) and the Fp scaling and microstructure of MgB2 wires. The results obtained indicate that after annealing at high pressure, the MgB2 wires show increases of Jc and Fp in high magnetic fields (8 T–12 T); in SiC doped MgB2 wires, Fpmax shifts to higher magnetic fields. We also compared the Jc of the doped and undoped MgB2 wires (without HIP and with HIP). The scanning electron microscope (SEM) results show that HIP increases the density of MgB2 material and improves its uniformity. [ABSTRACT FROM AUTHOR]

A 12.5 kVA, MgB2 superconductor transformer was designed as five alternating stacked coils between the primary and the secondary windings. Hyper Tech Research Inc. (Hyper Tech) manufactured multifilament MgB2 superconductor wire by using a patented process called the continuous tube filling and forming (CTFF) process. The CTFF MgB2 conductor was insulated with yttrium-stabilized zirconia (YSZ) using the chemical solution coating process. An all-copper former was designed and fabricated with an OD of 71 mm, ID of 42 mm and heights of 12 mm and 8 mm for primary and secondary coils, respectively. Two primary and three secondary coils were fabricated using the sol-gel insulated MgB2 wire using the W&R coil technology. One of the primary coils was tested at 4.2 K which showed better than 250 A engineering critical current. These five coils were then stacked as in the design in alternating fashion and connected and instrumented for testing. The transformer was cooled in He gas first then in liquid helium. The tests showed that the individual coils, the primary and the secondary windings performed better than the design currents. Also the open-circuit tests of the transformer showed that 1:1 transfer was achieved. This article was originally submitted for the special issue 'Selected papers from the International Conference on Superconductivity and Magnetism (ICSM2008) (Side, Turkey, 25-29 August 2008)', Superconductor Science and Technology, volume 22, issue 3. [ABSTRACT FROM AUTHOR]

The use of a low annealing temperature during the production of coils made from superconducting materials is very important because it reduces the production costs. In this study, the morphology, transport critical-current density ( J c ), irreversible magnetic field ( B irr ), and critical temperature ( T c ) of straight wires and small 2% C-doped MgB 2 coils were investigated. The coils were made using the wind-and-react (W&R) method and annealed at various temperatures from 610 °C to 650 °C for 2-12 h. Critical-current measurements were made for both the coils and straight wires at the temperatures of 4.2 K, 20 K, 25 K, and 30 K. During our research study, we determined the process window that provides the best critical parameters of the coils (annealing at a temperature of 650 °C for 6 h). Moreover, we observed that small coils made with unreacted MgB 2 wire and then annealed had morphology and critical parameters similar to those of straight 2% C-doped MgB 2 wires. Moreover, small-diameter bending of 20 mm and 10 mm did not lead to transverse cracks, which can cause a large reduction in J c in the coils. This indicates that the processes of optimization of thermal treatment parameters can be carried out on straight MgB 2 wires for MgB 2 superconducting coils.

Superconducting magnets used for Magnetic Resonance Imaging (MRI) scanners need to keep temperature gradients minimized in order to retain thermal and operating current margin. We have used 3D finite element analysis (FEA) simulation in COMSOL Multiphysics software that includes both conductive heat transfer and radiative heating to calculate the temperature distribution both along the winding direction and across the cross-section of an MRI segment coil at its equilibrium operating temperature. We have also modelled the evolution of the thermal properties during cool-down from ambient temperature. The heat capacity and thermal conductivity of the magnet winding were computed for use within this simulation. The heat capacity as a function of temperature was calculated using a rule of mixtures. This procedure was also used for the thermal conductivity along the direction of the wire. However, the thermal conductivity within the composite cross section ( x - and y -directions) was computed using a 2D FEA model. Based on this, a time-dependent, 3D coil model was built to calculate the coil temperature throughout the winding during cool-down in our test cryostat system. The model included a heat leak component to the coil current contacts via conduction through the current leads as well as a radiative component from the surfaces of the cryostat. A key result was that a maximum coil Δ T max = 5.1 K (=maximum temperature within the winding -minimum temperature in the winding) was seen and a coil I c margin of 12.75 A was predicted at steady state, with our first current lead design. A second set of more optimized current leads significantly lowered the Δ T max within the coil at the steady state. The coil I c margin has been analyzed for different current lead designs., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

We show that the structure of multifilament MgB 2 wires made by the powder-in-tube (PIT) method can be texturized by annealing the structure under high isostatic pressure. Our results show that we obtained continuous fibers with a uniform diameter of 250 nm in all 36 filaments, a small grain size of approximately 50 nm and a high density of the superconducting material. These results contribute to a significant improvement in the critical current density in high magnetic fields, e.g., 100 A/mm 2 at 14 T and 4.2 K.

Annealing undoped MgB 2 wires under high isostatic pressure (HIP) increases transport critical current density ( J tc ) by 10% at 4.2 K in range magnetic fields from 4 T to 12 T and significantly increases J tc by 25% in range magnetic fields from 2 T to 4 T and does not increase J tc above 4 T at 20 K. Further research shows that a large amount of 10% SiC admixture and thermal treatment under a high isostatic pressure of 1 GPa significantly increases the J tc by 40% at 4.2 K in magnetic fields above 6 T and reduces J tc by one order at 20 K in MgB 2 wires. Additionally, our research showed that heat treatment under high isostatic pressure is more evident in wires with smaller diameters, as it greatly increases the density of MgB 2 material and the number of connections between grains compared to MgB 2 wires with larger diameters, but only during the Mg solid-state reaction. In addition, our study indicates that smaller wire diameters and high isostatic pressure do not lead to a higher density of MgB 2 material and more connections between grains during the liquid-state Mg reaction.

Bulk samples of magnesium diboride (MgB 2 ) doped with 0.5 wt% of the rare earth oxides (REOs) Nd 2 O 3 and Dy 2 O 3 (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB 2 doped with 0.5 wt% Dy 2 O 3 (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 ( T c ), magnetic and resistive measurements of upper critical field ( B c2 ) and irreversibility field ( B irr ), as well as magnetic and transport measurements of critical current densities versus applied field ( J cm ( B ) and J c ( B ), respectively). It was found that although the products of REO doping did not substitute into the MgB 2 lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density ( F p ) versus reduced field ( b = B / B irr ) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F p ( b ) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F p ( b ) was still further increased by a Dy 2 O 3 additive-induced increase in B irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy 2 O 3 increases B irr 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 2 conductors.

MgB 2 superconducting wires and bulks with nano-La 2 O 3 addition have been studied. A series of MgB 2 superconducting bulk samples with nano-La 2 O 3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc 2 and unchanged B irr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB 6 in the nano-La 2 O 3 doped MgB 2 bulk samples and decreased MgB 2 grain size in nano-La 2 O 3 doped bulks. Monocore powder-in-tube (PIT) MgB 2 wires without and with 5 wt% nano-La 2 O 3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M-H), transport critical current density ( 2 O 3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M-H), transport critical current density ( J c ) was measured at 4.2 K for the nano-La 2 O 3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density J c (B) of P-05 at 4.2 K and 8 T (6.0 ×10 4 A/cm 2 ) was twice that of the control wire. The critical magnetic fields ( Bc 2 and B irr ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05's transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc 2 or B irr .

The development of coils that can survive a quench is crucial for demonstrating the viability of MgB 2 -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 2 superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents ( I op ), 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 ( I c ) of 186 A at 15 K. As the operating currents ( I op ) 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 2 -based MRIs because they permit exploitation of the relatively large MQE values of MgB 2 to enable the use of higher J e values which in turn lead to competitive MgB 2 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 2 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 2 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 2 . A new 'active' protection scheme for medium T c based MRI magnets is presented and simulations demonstrate that the magnet can be protected. Recent progress on persistent joints for multifilamentary MgB 2 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 2 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 2 magnet designs.

This study is a contribution to the development of technology for an MgB 2 -based, cryogen-free, superconducting magnet for an MRI system. Specifically, we aim to demonstrate that a react and wind coil can be made using high performance in-situ route MgB 2 conductor, and that the conductor could be operated in conduction mode with low levels of temperature gradient. In this work, an MgB 2 conductor was used for the winding of a sub-size, MRI-like coil segment. The MgB 2 coil was wound on a 457 mm ID 101 OFE copper former using a react-and-wind approach. The total length of conductor used was 330 m. The coil was epoxy impregnated and then instrumented for low temperature testing. After the initial cool down (conduction cooling) the coil I c was measured as a function of temperature (15-30 K), and an I c of 200 A at 15 K was measured.

The influences of strand twisting and bending (applied at room temperature) on the critical current densities, J c , and n -values of MgB 2 multifilamentary strands were evaluated at 4.2 K as function of applied field strength, B . Three types of MgB 2 strand were evaluated: (i) advanced internal magnesium infiltration (AIMI)-processed strands with 18 filaments (AIMI-18), (ii) powder-in-tube (PIT) strands processed using a continuous tube forming and filling (CTFF) technique with 36 filaments (PIT-36) and (iii) CTFF processed PIT strands with 54 filaments (PIT-54). Transport measurements of J c (B) and n -value at 4.2 K in fields of up to 10 T were made on: (i) PIT-54 after it was twisted (at room temperature) to twist pitch values, L p , of 10-100 mm. Transport measurements of J c (B) and n -value were performed at 4.2 K; (ii) PIT-36 and AIMI-18 after applying bending strains up to 0.6% at room temperature. PIT-54 twisted to pitches of 100 mm down to 10 mm exhibited no degradation in J c (B) and only small changes in n -value. Both the J c (B) and n -value of PIT-36 were seen to be tolerant to bending strain of up to 0.4%. On the other hand, AIMI-18 showed ±10% changes in J c (B) and significant scatter in n -value over the bending strain range of 0-0.6%.

Among key design and operation issues for MgB 2 relevant to MRI magnets are: uniformity of current-carrying capacity over long lengths (>2 km) of wire; and reliability of a splicing technique. This paper presents experimental results of current-carrying capacities of a small test coil and joints, both made from MgB 2 round wires, multifilament and monofilament (mono), manufactured by Hyper Tech Research, Inc. The test coils were wound with 95-m long unreacted, C (carbon)-doped MgB 2 multifilament wire, sintered at 700°C for 90 min. The critical currents were measured in the 4.2 K-15 K and 0 T-5 T ranges. We have modified our original splicing technique, proven successful with unreacted, un-doped MgB 2 multifilament wire sintered at 570°C, and applied it to splice both un-doped and C-doped mono wires sintered at 700°C. Most consistently good results were obtained using the un-doped mono wires. Also presented are results of a small joint-coil-PCS assembly of mono wire, operated in persistent mode at 50 A at >10 K.

The change in the lattice parameters or the lattice disorder is claimed as a cause of the slight reduction in the transition temperature by carbon doping in MgB2. In this work, an extensive investigation on the effects of carbohydrate doping has been carried out. It is found that not only the a-axis but also the c-axis lattice parameter increases with the sintering temperature. A linear relation between the unit cell volume and the critical temperature is observed. Compared with the well known correlation between the lattice strain and the critical temperature, the X-ray peak broadening itself shows a closer correlation with the transition temperature. The residual resistivity and the critical temperature are linearly correlated with each other as well and its implication is further discussed.

Aiming to demonstrate feasibility and practicality of a low cost superconducting MRI magnet system targeted for use in small hospitals, rural communities and underdeveloped countries, MIT-Francis Bitter Magnet Laboratory has developed a 0.6 T/650 mm room temperature bore demonstration coil wound with multifilament MgB 2 conductor and cooled via an innovative cryogenic design/operation. The coil is to be maintained cold by solid nitrogen kept in the solid state by a cryocooler. In the event of a power failure the cryocooler is automatically thermally decoupled from the system. In this paper we present details of the MgB2 conductor, winding process, and preliminary theoretical analysis of the current-carrying performance of the conductively cooled coils in zero background field and over the 10-30 K temperature range.