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MgB2 wires with 114, 192 and 342 filaments of size 14–19 µm manufactured by HyperTech Research, Inc. have been subjected to low-temperature DC measurements. R(T), I–V characteristics, critical currents, stress and strain tolerances of these wires differing by filament architecture and filament size sheathed by resistive CuNi alloys were measured and compared with the literature data. It was found that these fine-filamentary wires have high engineering current densities not reduced by twisting up to 10 mm, sufficient strain tolerances and therefore are promising for future applications where minimized AC losses are required due to resistive sheaths, thin MgB2 filaments and short twist pitches.

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 (ρ), 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. We found that the graphene doping has a positive impact on the above mentioned properties. In the case of the optimally doped (x = 3.0%) sample, the critical current density at 5 K corresponds to 1.4 × 105 A/cm2 for 2 T field, whereas the undoped sample showed 9.6 × 104 A/cm2 for the same field, i.e., 1.5 times improvement. Furthermore, the optimally doped sample showed a Jc of nearly 1 × 104 A/cm2 at 5 K, 8 T, which is a significantly high value. The upper critical field has been enhanced to 13 T at 20 K for the optimal doping level. The flux pinning behavior has been evaluated from the curve of flux pinning force against applied magnetic field, and it reveals that the maximum pinning has been improved by nearly 1.2 times at 20 K, due to the graphene doping.

MgB2 samples were prepared using as-supplied commercial 96% boron with strong crystalline phase and the same 96% boron (B) after ball milling. The effects of the properties of the starting B powder on the superconductivity were evaluated. We observed that samples using ball-milled 96% B, in comparison with the one made from the as-supplied 96% B, were characterized by small grain size, broadened full width at half maximum (FWHM), and enhanced magnetic critical current density (Jc). Jc reached 2 times 103 A cm-2 at 5 K and 8 T. The improved pinning of these samples seems to be caused by enhanced grain boundary pinning at high field.

Various MgB2 wires with different sheath materials provided by Hyper Tech Research Inc., have been tested in the superconducting fault current limiter (SFCL) desktop tester at 24–26 K in a self-field. Samples 1 and 2 are similarly fabricated monofilamentary MgB2 wires with a sheath of CuNi, except that sample 2 is doped with SiC and Mg addition. Sample 3 is a CuNi sheathed multifilamentary wire with Cu stabilization and Mg addition. All the samples with Nb barriers have the same diameter of 0.83 mm and superconducting fractions ranging from 15% to 27% of the total cross section. They were heat-treated at temperatures of 700 °C for a hold time of 20–40 min. Current limiting properties of MgB2 wires subjected to pulse overcurrents have been experimentally investigated in an AC environment in the self-field at 50 Hz. The quench currents extracted from the pulse measurements were in a range of 200–328 A for different samples, corresponding to an average engineering critical current density (Je) of around 4.8 × 104 A cm−2 at 25 K in the self-field, based on the 1 µV cm−1 criterion. This work is intended to compare the quench behaviour in the Nb-barrier monofilamentary and multifilamentary MgB2 wires with CuNi and Cu/CuNi sheaths. The experimental results can be applied to the design of fault current limiter applications based on MgB2 wires.

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 MgB2 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

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.

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.

AC transport current and applied magnetic field loss measurements have been made on MgB2 superconducting strands. We tested six‐filament strands made via an in‐situ route with Nb chemical barriers, a Cu‐inter‐filamentary matrix, and ODS Cu (glidcop) outer sheaths. Strands were untwisted and measured in self magnetic field at temperatures from 4.2 K to Tc. Transport ac losses agreed reasonably well with theoretical predictions for a wire of round cross‐section. AC losses in applied magnetic field perpendicular to the wire axis have been measured in fields up to 140 mT (peak) in frequency range 50–200 Hz in liquid helium bath at 4.2 K. There is a reasonable agreement of the losses with the critical state model at 50 Hz. At 200 Hz the losses are dominated by eddy currents in Cu matrix. Eddy current losses in the glidcop sheath are negligibly small.

Superconducting machines offer the significant advantage of smaller volume, lighter weight, and increased operating efficiencies compared with traditional electrical machines. To date, superconducting machines have utilized a superconducting dc field winding on the rotor of a synchronous machine. This increases the system complexity because it requires cryogenic cooling on the rotating part of the machine. The stator in these machines is generally composed of a set of conventional ac copper wire coils. Round magnesium diboride (MgB2) wire however has the potential to form low-cost ac stator coils for a superconducting machine. This could enable a stationary superconducting ac stator winding to be fabricated, reducing the complexity associated with the cryogenic cooling. This paper presents initial test results on a prototype ac solenoidal coil to represent a typical ac stator coil for a superconducting machine using MgB2 wire. The diameter of the wire with insulation was 1 mm and the coil was wound in a double-layer solenoidal arrangement. The magnetic flux density distribution, quench current level, long duration operating current level, and the ac losses of the coil were measured and discussed. This paper demonstrates the potential of MgB2 wire to develop a superconducting ac stator winding for a superconducting machine.

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.