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We present magnetic, mechanical and thermal modeling results for a 3 Tesla actively shielded whole body MRI (Magnetic Resonance Imaging) magnet consisting of coils with a square cross section of their windings. The magnet design was a segmented coil type optimized to minimize conductor length while hitting the standard field quality and DSV (Diameter of Spherical Volume) specifications as well as a standard, compact size 3 T system. It had an overall magnet length and conductor length which can lead to conduction cooled designs comparable to NbTi helium bath cooled 3 T MRI magnets. The design had a magnetic field homogeneity better than 10 ppm (part-per-million) within a DSV (Diameter of Spherical Volume) of 48 cm and the total magnet winding length of 1.37 m. A new class of MgB2 strand especially designed for MRI applications was considered as a possible candidate for winding such magnets. This work represents the first magnetic, mechanical and thermal design for a whole-body 3 T MgB2 short (1.37 m length) MRI magnet based on the performance parameters of existing MgB2 wire. 3 Tesla MRI magnet can operate at 20 K at 67% of its critical current. [ABSTRACT FROM AUTHOR]

The testing of a react and wind coil with a 56 cm bore and 828 m of Mg[B.sub.2] wire is discussed. Findings reveal that the react and wind coil achieved 168 A and 0.17 T at 5 K.

The study demonstrates the influence of the ball-milled low purity boron powder on the superconductivity of [MgB.sub.2] samples. The analysis reveals that such boron powder using toluene gives better homogeneity and current density to the samples.

The oxygen concentration and distribution in the microstructure of MgB2- and YBa2Cu3O7-δ-based materials affect the formation of nanostructural defects and thus influence the critical current density, upper critical magnetic field and irreversibility field. For MgB2 oxygen containing additions (Dy-O, Ti-O) in the form of nanograins occurred not to be very effective for an increase of critical current density, jc. Sn-O additions to MgB2 can provoke even a significant decrease of jc due to a chemical interaction of Sn with MgB2. The processing pressure and temperature influence the defect density of the relevant pinning centers and the regularity of their distribution in doped and undoped MgB2 as well as in MT-YBCO. [ABSTRACT FROM AUTHOR]

High field pinning centers in MgB2 doped with 2 at. % carbon under a low and a high hot isostatic pressures have been investigated by transport measurements. The field dependence of the transport critical current density was analyzed within the different pinning mechanisms: surface pinning, point pinning, and pinning due to spatial variation in the Ginzburg-Landau parameter (Δκ pinning). Research indicates that a pressure of 1 GPa allows similar pinning centers to Δκ pinning centers to be obtained. This pinning is very important, because it makes it possible to increase the critical current density in high magnetic fields at 20K and 25 K. Our results indicate that the δTc and dl pinning mechanisms, which are due to a spatial variation in the critical temperature (Tc) and the mean free path, l, respectively, create dislocations. The high density of dislocations with inhomogeneous distribution in the structure of the superconducting material creates the δl pinning mechanism. The low density of dislocations with inhomogeneous distribution creates the δTc pinning mechanism. Research indicates that the hot isostatic pressure process makes it possible to obtain a high dislocation density with a homogeneous distribution. This allows us to obtain the δTc pinning mechanism in MgB2 wires. In addition, a high pressure increases the crossover field from the single vortex to the small vortex bundle regime (Bsb) and improves the δTc pinning mechanism. Our research has proved that a high pressure significantly increases the crossover field from the small bundle to the thermal regime (Bth), with only a modest decrease in Tc of 1.5K, decreases the thermal fluctuations, increases the irreversibility magnetic field (Birr) and the upper critical field (Bc2) in the temperature range from 4.2K to 25K, and reduces Birr and Bc2 above 25K. [ABSTRACT FROM AUTHOR]

MgB2 precursor wires were prepared using powder in tube technique by Institute of High Pressure PAS in Warsaw. All samples were annealed under isostatic pressure generated by liquid Argon in the range from 0.3 GPa to 1 GPa. In this paper, we show the effects of different processing routes, namely, cold drawing (CD), cold rolling (CR), hot isostatic pressure (HIP) and doping on critical current density (Jc), pinning force (Fp), irreversible magnetic-field (Birr), critical temperature (Tc), n value, and dominant pinning mechanism in MgB2/Fe wires with ex situ MgB2 barrier. The results show that medium pressures (∼0.35 GPa) lead to high Jc in low and medium magnetic fields (0 T - 9 T). On the other hand, higher pressures (∼1 GPa) lead to enhanced Jc in high magnetic fields (above 9 T). Transport measurements show that CD, CR, and HIP have small effects on Birr and Tc, but CD, CR, HIP, and doping enhance Jc and Fp in in situ MgB2 wires with ex situ MgB2 barrier. Transport measurements on in situ undoped MgB2 wire with ex situ MgB2 barrier yield a Jc of about 100 A/mm2 at 4.2K in 6 T, at 10K in 4 T and at 20K in 2 T. The results also show that cold drawing causes increase of n value. [ABSTRACT FROM AUTHOR]

This article discusses the influence of MgB2 wire diameter on critical parameters, dominant pinning mechanism, and n value. Studies were conducted on multifilament MgB2 wires of the 0.84 mm and 0.64 mm diameter. The wires were annealed under 1 GPa isostatic pressure, between 650 and 730 °C. Studies show that the best results (Jc, Birr, n value, and point pinning mechanism) for a 0.64-mm wire were obtained with annealing temperature of 650 °C. However, the best results for wires with a 0.84-mm diameter were obtained with annealing temperature of 730 °C. This research was conducted for exploring the use of MgB2 wires in multi-section superconducting coils, e.g., main coils generating magnetic fields and compensation coils—increasing the homogeneity of the magnetic field. Our research indicates that the main coil made with larger diameter wires should be heated at higher temperature (730 °C). In contrast, compensating coils with smaller diameter wires should be heated at a lower temperature (650 °C). [ABSTRACT FROM AUTHOR]

This article presents the impact of MgB2 wire bending and diameter on transport critical current density and irreversible magnetic field of a resultant coil. Unreacted MgB2 wires 500 mm in length and 0.63 or 0.83 mm in diameter have been used in the fabrication of small diameter (14 mm) superconducting coils. The coils were subsequently annealed under isostatic pressure of 1 GPa for 15 min at 700 °C and 725 °C. Our results indicate that larger wire diameter, higher annealing temperature, and bending lead to slight reduction of critical current density and irreversible magnetic field in the coil. [ABSTRACT FROM AUTHOR]

Field, temperature, and strain dependences of the critical current for a SiC doped multifilamentary in situ MgB2 wire have been studied. Measurement results were compared with that of the undoped wire, and the origin of the difference in the critical current is discussed. The critical current can be calculated with the percolation model considering the effect of anisotropy. The temperature dependence of the fitting parameters, the upper critical field along the ab-plane, and c-axis is compared with the dirty-limit two-gap theory. To assess the validity of the fitting parameters, resistive transition has been measured especially to extract the upper critical field directly. It is shown that even the resistive broadening can be well explained by a simple parallel path model using the fitting parameters obtained from the critical current analysis. [ABSTRACT FROM AUTHOR]

MgB2/Fe wires with 10 at. % excess Mg produced by in situ powder-in-tube processing were compared to normal stoichiometric MgB2/Fe wires prepared by the same method. It was found that the critical current density (Jc) and the irreversibility field (Hirr) were significantly enhanced for MgB2/Fe wires with excess Mg. The transport Jc for 10 at. % Mg excess samples sintered at 800 °C, measured at a field of up to 14 T, increased by a factor of 2 compared to that for the normal MgB2 wires. The best Jc results for the 10 at. % Mg excess sample were obtained by heating the sample at 600 °C for 1 h; the Jc at a field of 8 T and at temperature of 10 K reached 3×104 A/cm2. The detailed analysis of the effect of excess Mg on the microstructures, the Jc, and the Hirr, of MgB2/Fe wires are presented in this paper. [ABSTRACT FROM AUTHOR]

MgB2 samples were prepared by using 96% boron (B) powder with strong crystalline phase that had been ball milled for various times. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field critical current density (Jc) due to small grain size and better reactivity. Specifically, many grain boundaries for MgB2 could be acting as strong flux pinning centers. Based on Rowell connectivity analysis, when the ball-milling time increased, the connectivity factor, described as the active cross-sectional area fraction (AF), was decreased. This implies that the intergrain connectivity became worse. These properties could lead to poor Jc in low field. However, the pinning force strength, Jc1/2×B1/4, of samples using ball-milled 96% B is larger than that of the reference sample using commercial 96% B powder. These results accompany enhanced irreversibility (Hirr) and upper critical fields (Hc2). [ABSTRACT FROM AUTHOR]

Field, temperature and strain dependence of the critical current for a SiC doped multifilamentary in situ Mg[B.sub.2] wire are analyzed by using percolation model. The validity of the fitting parameters is assessed by measuring the resistive transition to extract the upper critical field directly.

Mg[B.sub.2]/Fe wires with excess Mg produced by in situ powder-in-tube processing are compared to normal stoichiometric Mg[B.sub.2]/Fe wires prepared by the same method. The effect of excess Mg on the microstructures, the critical current density and the irreversibility field of Mg[B.sub.2]/Fe wires is analyzed.

The superconducting properties of Mg[B.sub.2] made from low-grade 96% B powder with a strong crystalline phase as a function pf ball-milling time are examined. The pinning force strength of the samples using ball-milled 96% B is larger than that of the reference sample using commercial 96% B powder and these have accompanied enhanced irreversibility and upper critical fields.

The composition of MgB2-based materials (wires, bulks, and thin films) with high critical current densities, jc, prepared at different pressure (0.1 MPa–2 GPa)—temperature (600–1050 °C) conditions was analyzed by the X-ray and JAMP−9500F Auger spectrometer (after removing the oxidized layers at the sample surfaces by Ar ion etching performed directly in the vacuum chamber of a microscope). Similar regularly distributed inhomogeneities connected with Mg, B, and admixture O content variation on the nanolevel were observed in all types of the MgB2-based materials. The correlations between the character of the material inhomogeneities and the attained superconducting characteristics are discussed. [ABSTRACT FROM AUTHOR]

This article reports the influence of isostatic pressure (from 0.1 MPa to 1.1 GPa), low annealing temperature of 570 ∘C, and annealing time for the formation of high-field pinning centers in 2% C-doped MgB2 wires. Measurements indicate that 1.1 GPa pressure significantly increases the density of high-field pinning centers below 20 K. However, lower pressure (0.6 GPa) slightly increases the density of high-field pinning centers. Increasing the annealing time from 120 to 210 min leads to a reduction of critical temperature (Tc), irreversibility field (Birr), critical current density (Jc), and upper critical field (Bc2), suggesting that a long annealing time leads to a reduction of high-field pinning center density and the number of connections between superconducting grains. The high pressures and low annealing temperature lead to a high critical current density of 1000 A/mm2 in 7.2 T and 100 A/mm2 in 12.5 T at 4.2 K in MgB2 wires. [ABSTRACT FROM AUTHOR]

A series of MgB2 superconducting composite strands co-doped with Dy2O3 and C was prepared via an advanced internal Mg infiltration (AIMI) route. The transport properties and MgB2 layer growth were studied in terms of the Dy2O3 doping level, reaction temperature, and reaction time. Transport studies showed that both critical current densities, J cs, and irreversibility fields, B irrs, were increased with Dy2O3 doping. The highest layer Jc was 1.35 × 105 A/cm2 at 4.2 K, 10 T, 30% higher than that of the best AIMI wires without Dy2O3 doping. The highest “non-barrier" Jc reached 3.6 × 104 A/cm2 at 4.2 K, 10 T, which was among the best results reported so far. The improvements were even more pronounced at higher temperatures where the field at which the layer Jc reached 104 A/cm2 was pushed out by 0.9 T at 20 K, 1.2 T at 25 K, and 1.4 T at 30 K. While little or no enhancement in B irr was seen at 10 K and 15 K, the increases in Jc at higher temperatures were consistent with observed increases in Birr of 17% at 20 K, 44% at 25 K, and 400% at 30 K. Also, there were some indications that the reaction and layer growth of MgB2 were enhanced by Dy2O3 doping. [ABSTRACT FROM AUTHOR]

Unreacted MgB2 wires fabricated from SiC-doped precursor material by Hyper Tech Research, Inc. have been used to make small-diameter (14 mm) superconducting coils. All coils made of 500 mm length wires were subjected to hot isostatic pressure (HIP) treatment. The critical current density (Jc) parameters of coils were compared to straight samples characteristics. Both types of samples have been measured in perpendicular magnetic field configuration for Jc and pinning force density (Fp) evaluation. No significant Jc difference between the long wires on coils and straight wires was found. These results suggest that the critical current (Ic) for coils can be determined for straight samples (25 mm). SEM analysis indicated that a small diameter of the coil does not influence the structure of in situ MgB2 wire. [ABSTRACT FROM AUTHOR]

In this paper we show that an advanced internal Mg infiltration method (AIMI) is effective in producing superconducting wires containing dense MgB2 layers with high critical current densities. The in-field critical current densities of a series of AIMI-fabricated MgB2 strands were investigated in terms of C doping levels, heat treatment (HT) time and filament numbers. The highest layer Jc for our monofilamentary AIMI strands was 1.5×105 A cm-2 at 10 T, 4.2 K, when the C concentration was 3 mol% and the strand was heat-treated at 675 °C for 4 h. Transport critical currents were also measured at 4.2 K on short samples and 1 m segments of 18-filament C doped AIMI strands. The layer Jcs reached 4.3 × 105 A cm-2 at 5 T and 7.1 × 104 A cm-2 at 10 T, twice as high as those of the best powder-in-tube strands. The analysis of these results indicates that the AIMI strands, possessing both high layer Jcs and engineering Jes after further optimization, have strong potential for commercial applications. [ABSTRACT FROM AUTHOR]

Intra-wire resistance and AC loss of various multi-filamentary MgB2 wires with filaments surrounded by Nb barriers have been measured and analyzed. The intra-wire resistance is measured with a direct four-probe voltage-current method at various temperatures. The AC loss is acquired by both vibrating sample magnetometer (VSM) and magnetization measurements at 4.2 K. With the aid of finite element method simulations, the filament-to-matrix contact resistance and effective transverse resistivity are derived from direct intra-wire resistance measurements. The effective transverse resistivity values are in good agreement with those analytically derived from the AC coupling loss measurements. Surprisingly, very high values of filament-to-matrix contact resistivity are found, being 2 or 3 orders higher than commonly found for NbTi or Nb3Sn wires. [ABSTRACT FROM AUTHOR]

A prototype resistive superconducting fault current limiter (SFCL) was developed using single-strand round magnesium diboride (MgB2) wire. The MgB2 wire was wound with an interleaved arrangement to minimize coil inductance and provide adequate inter-turn voltage withstand capability. The temperature profile from 30 to 40 K and frequency profile from 10 to 100 Hz at 25 K were tested and reported. The quench properties of the prototype coil were tested using a high current test circuit. The fault current was limited by the prototype coil within the first quarter-cycle. The prototype coil demonstrated reliable and repeatable current limiting properties and was able to withstand a potential peak current of 372 A for one second without any degradation of performance. A three-strand SFCL coil was investigated and demonstrated scaled-up current capacity. An analytical model to predict the behaviour of the prototype single-strand SFCL coil was developed using an adiabatic boundary condition on the outer surface of the wire. The predicted fault current using the analytical model showed very good correlation with the experimental test results. The analytical model and a finite element thermal model were used to predict the temperature rise of the wire during a fault. [ABSTRACT FROM AUTHOR]

Raw MgB2 wires have been manufactured by Hyper Tech Inc. Each of their 18 cores contained undoped in situ powder surrounded by an Nb barrier and placed in a Cu sheath, and cores were coated in a common Monel sheath. Wires of diameters 0.63 and 0.83 mm were annealed under high argon pressure (up to 1 GPa). Parameters of such HIP process (temperature, time, and pressure) were varied in order to determine their optimal values. Superconducting properties of such samples were investigated by means of four-probe critical current j c measurement. Bitter magnet producing magnetic field up to 14 T was used. It provided place for samples 70 mm long in parallel field and 20 mm long in perpendicular field. Results include critical current j c and pinning force density F p dependencies on magnetic field as well as Kramer plots. Critical current of 104 A/cm 2 was achieved at 12 T magnetic field. SEM pictures of wire cross sections were also taken to determine quality of Nb barrier and microstructure of superconducting material. [ABSTRACT FROM PUBLISHER]

Monocore powder-in-tube MgB2 precursor strands were cold-drawn and heat-treated at 600 and 700 C for times of up to 71 h, and structure-property relationships examined. Drawing induced elongation of the Mg particles led, after heat treatment (HT), to a textured macrostructure consisting of elongated fine polycrystalline MgB2 fibers (or veins) separated by elongated pores. The superconducting transition temperature (Tc), critical current density (Jc) and bulk pinning force density (Fp) were correlated with the macrostructure and grain size. Grain size increased with HT time at both 600 and 700 C. Critical current density and hence Fp decreased monotonically but not linearly with grain size. Overall, it was observed that at 700 C the MgB2 reaction was more or less complete after as little as 30 min; at 600 C full reaction completion did not occur until 72 h into the HT. Transport, Jct.B/, was measured in a perpendicular applied field, and the magnetic critical current densities, J? cm.B/ and J cm.B/, were measured in perpendicular and parallel (axial) applied fields, respectively. Particularly noticeable was the premature drop-off of J? cm.B/ at fields well below the irreversibility field of Jct.B/. This effect is attributed to the fibrous macrostructure and its accompanying anisotropic connectivity. Magnetic measurements with the field directed along the strand axis yielded a critical density, J? cm.B/, for current flowing transversely to the strand axis that was less than, and dropped off more rapidly than, Jct.B/. In the conventional magnetic measurement, the loop currents that support the magnetization are restricted by the lower of Jct.B/ and J cm.B/: in the present case the latter, leading to the premature drop-off of the measured J? cm.B/ compared to Jct.B/ with increasing field. This result is supported by Kramer plots of the J cm.B/ and Jct.B/ data, which lead to an irreversibility field for transverse current that is very much less than the usual transport-measured longitudinal one, Birr;t. [ABSTRACT FROM AUTHOR]

we report the synthesis and characterization of \MgB2 made from nano-boron and doped with graphene in the following mole percentages, x=0, 3.0 and 12.0. The effect of graphene doping on the normal state resistivity (\rho), superconducting transition temperature (Tc), irreversibility and upper critical fields (Hirr and Hc2), and critical current density (Jc), as well as the pinning force (Fp) were evaluated. [ABSTRACT FROM AUTHOR]

A model helical undulator 250 mm in length, with a period of 14 mm, was designed, fabricated and tested. The helical coil was wound with multifilamentary, internal-Sn type Nb3Sn strand. The 0.7 mm OD strand was insulated with S-glass, wound onto the former, and reacted, after which the coil underwent vacuum epoxy impregnation. The beam aperture was 7 mm, the winding bore diameter was 8 mm, and the OD of the complete winding was 18 mm. The helical poles were made from 1016 low carbon steel, and projected slightly above the coil pack, with an OD of 19.06 mm. The coil Ic and the magnetic field in the bore were measured at 4.2 K in a liquid helium bath. The bore field was also measured as a function of position along the length of the undulator. [ABSTRACT FROM AUTHOR]

AC applied magnetic field loss measurements have been performed on MgB2 superconducting strands. We measured strands with six untwisted filaments made via an in-situ route with Nb chemical barriers, a Cu-inter-filamentary matrix, and with either non-magnetic (glidcop) or magnetic (monel) outer sheaths. AC losses in magnetic fields applied perpendicular to the wire axis have been measured in fields up to 140 mT (peak) in the frequency range 50-200 Hz in a liquid helium bath at 4.2 K. For samples with no ferromagnetic elements present in their matrix there is a reasonable agreement of the losses with the critical state model in low frequency region. At higher frequencies the losses are dominated by eddy currents in Cu matrix. The samples containing a weakly ferromagnetic matrix showed complex hysteresis loops and AC loss dependencies. [ABSTRACT FROM AUTHOR]

We have systematically studied the flux pinning behavior of MgB2 bulks synthesized by direct diffusion of Mg into pressed pellets of high purity crystalline B powder, with and without mixing with C and SiC nanoparticles, at a reaction temperature of 850°C for 10 hrs. All of the samples showed very high purity and high density, but their microstructure and flux pinning behavior showed significant differences. It was found that the pure MgB2 agrees with the δTc pinning model, nano-C doped MgB2 agrees with the 61 pinning model, while the SiC + MgB2 composite agrees with the δϵ pinning model (stress/strain field pinning), since the dominant micro-defects that influence the flux pinning in these three samples are different. [ABSTRACT FROM AUTHOR]