Hyper Tech Research (United States)

Previous studies have shown that APC Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn strands based on the internal oxidation of Nb-Ta-Zr or Nb-Ta-Hf alloys had higher non-Cu critical current density (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i>s) than the state-of-the-art strands at high fields (e.g., ≥ 12 T) while having lower non-Cu <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i>s at low fields (e.g., ≤ 5 T), which helps to reduce the undesired persistent-current magnetization. Moreover, APC strands reacted at lower temperature tend to have flatter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i>(<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i>) curves, possibly allowing the targeting of even better <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> performance at high fields, while at the same time suppressing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i>s at low fields. However, it is required that APC strands attain large fine-grain (FG) area fractions and suppressed coarse-grain (CG) area fractions at low reaction temperature. In this work, the influence of reaction temperature on the evolution of FG and CG Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn phases in APC Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn strands was investigated. Our goal is to find more effective methods to increase the FG area fraction as well as the overall performance of APC strands.

A fine-filamentary CuZn sheathed MgB 2 wires produced by Hyper Tech Inc were used for 12 strand Rutherford cable of thickness 0.70–0.76 mm. Critical currents of short cable samples were measured at 4.2 K and external fields 4.0–8.0 T and also at temperatures 15–25 K and magnetic fields 1.0– 5.5 T. The threshold of engineering current density J e = 1 0 4 Acm −2 for the cable size of 0.72 mm × 2.55 mm was obtained at field of 4.5 T and temperature 4.2 K and at 2.3 T for 20 K, respectively. Magnetization measurements of the cable with transposed strands and twisted 20 μ m filaments have shown reduced AC losses, but, the coupling currents loss contribution was still visible. AC losses measured for the same size Rutherford cable made of single-core wires demonstrate the effect of well decoupled filaments. Consequently, the cabling of thin single-core MgB 2 wires shows a promising way for low loss conductors suitable for AC applications.

In the last few years, a new type of Nb3Sn superconducting composite, containing a high density of artificial pinning centers (APC) generated via an internal oxidation approach, has demonstrated a significantly superior performance relative to present, state-of-the-art commercial Nb3Sn conductors. This was achieved via the internal oxidation of Nb-4at.%Ta-1at.%Zr alloy. On the other hand, our recent studies have shown that internal oxidation of Nb–Ta–Hf alloys can also lead to dramatic improvements in Nb3Sn performance. In this work we follow up on this latter approach, fabricating a 61-stack APC wire based on the internal oxidation of Nb-4at.%Ta-1at.%Hf alloy, and compare its critical current density (J c) and irreversibility field with APC wires made using Nb-4at.%Ta-1at.%Zr. A second goal of this work was to improve the filamentary design of APC wires in order to improve their wire quality and electromagnetic stability. Our new modifications have led to significantly improved residual resistivity ratio and stability in the conductors, while still keeping non-Cu J c at or above the conductor J c specification required by the proposed Future Circular Collider. Further improvement via optimization of the wire recipe and design is ongoing. Finally, additional work needed to make APC conductors ready for applications in magnets is discussed.

A major problem of state-of-the-art Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn accelerator magnets is their long training due to thermo-mechanical perturbations. Increasing the specific heat, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> , of the Rutherford cable would reduce and/or eliminate training by limiting the coils temperature rise. This paper studies feasibility of increasing the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> of Rutherford-type cables by using thin composite Cu/Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Cu/Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S tapes produced by Hyper Tech Research, Inc. The tape can be either wrapped around the cable, placed on the cable wide faces under the insulation, and/or inserted as a core. Wire samples outfitted with these high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_p</i> ribbons, or tapes, were prepared and tested at FNAL for their Minimum Quench Energy ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MQE</i> ). At 90% <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> and 15 T, the average gain of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MQE</i> of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire soldered to the Cu/Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S 55 μm thick ribbon was 2.5, and further increased at larger transport currents.

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.

To develop powerful wind turbine generators using superconducting technology, high-performance superconducting racetrack coils are essential. Herein, we report an evaluation of a multifilamentary magnesium diboride (MgB2) conductor-based racetrack coil cooled and impregnated simultaneously by solid nitrogen (SN2). The coil was wound on a copper former with 13 mm winding width, an inner diameter of 124 mm at the curvature, and 130 mm length of the straight section. An in situ processed S-glass-insulated 36-filament MgB2 wire was wound on the former in two layers with 19.5 turns, and heat treated via the wind and react method without any epoxy resin. The coil was evaluated for critical temperature and transport critical current in the SN2 environment at different temperatures up to 31.3 K in self-field. The coil was able to carry 200 A transport current at 28.8 K in self-field. During coil charging and operation, SN2 effectively acted as an impregnation material. The test results demonstrate the viability to use MgB2 racetrack coil potentially with SN2 impregnation in advanced rotating machine applications.

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.

In this work we studied the influence of SnO2 doping on the critical fields and temperatures of MgB2 bulk samples. Bulk samples were made by mixing ex-situ MgB2 powder with 5 wt% SnO2 powder and then pressing the mixed powders into pellets using a pressure of 2000 psi. The bulk pellets were sintered at 900°C in a furnace under flowing Ar. The samples were quenched to room temperature after dwelling at 900°C for 5h. XRD, SEM, and magnetic measurements were made on doped and control samples. XRD showed a decomposition of the SnO2 and very slight reductions in the a-axis and c-axis lattice parameters of the MgB2 phase. M-T (Magnetization-Temperature) curves showed a decrease in Bc2 of approximately 1 T in the temperature range of 24 K - 39 K with SnO2 additions as compared to the control samples.