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 MgB2, 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 MgB2 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 / MgB2 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.

https://iopscience.iop.org/article/10.1088/0953-2048/30/1/014007/pdf

Conductors for commercial MRI magnets beyond NbTi: requirements and challenges

Here we show that addition of Hf to Nb4Ta can significantly improve the high field performance of Nb$_{3}$Sn, making it suitable for dipole magnets for Future Circular Collider (FCC). A big challenge for the FCC is that a realistic production target for FCC Nb3Sn requires ~30% improvement over current conductor performance. Recent success with internal oxidation(IO) of Nb-Zr precursor has shown significant improvement in the layer J$_{c}$ of Nb$_{3}$Sn wires, albeit the complication of providing an internal O$_{2}$ diffusion path and avoiding degradation of irreversibility field($_{irr}$). We compare Zr and Hf additions to the standard Nb4Ta alloy of maximum H$_{c2}$ and H$_{irr}$. Nb4Ta rods with 1Zr or 1Hf were made into monofilament wires with and without SnO$_{2}$ and their properties measured over the entire superconducting range up to 31 T. We found that group IV alloying of Nb4Ta raises H$_{irr}$, though adding O$_{2}$ still degrades this slightly. As noted in Nb1Zr studies, the pinning force density F$_{p}$ is strongly enhanced and its peak value shifted to higher field by IO. A surprising result of this work is that we found better properties in Nb4Ta1Hf without SnO$_{2}$, F$_{pmax}$ achieving 2.35 times that of the standard Nb4Ta alloy, while the oxidized Nb4Ta1Zr alloy achieved 1.54 times that of the Nb4Ta alloy. The highest layer J$_{c}$ (16 T, 4.2 K) of 3700 A/mm$^{2}$ was found in the SnO$_{2}$-free wire made with Nb4Ta1Hf alloy. Using a standard A15 cross-section fraction of 60% for modern PIT and RRP wires, we estimated that a non-Cu J$_{c}$ of 2200 A/mm$^{2}$ is obtainable in modern conductors, well above the 1500A/mm$^{2}$ FCC specification. Moreover, the best properties were obtained without SnO$_{2}$, the Nb4Ta1Hf alloy appears to open a straightforward route to enhanced properties in Nb$_{3}$Sn wires.

Beneficial influence of Hf and Zr additions to Nb4at.%Ta on the vortex pinning of Nb$_{3}$Sn with and without an O source.

Beneficial influence of Hf and Zr additions to Nb4at%Ta on the vortex pinning of Nb3Sn with and without an O source

High critical current density (Jc) Nb3Sn A15 multifilamentary wires require a large volume fraction of small grain, superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In Powder-in-Tube (PIT) wires the unreacted Nb7.5wt.%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment (HT) to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this paper we investigate recent high quality PIT wires that achieve a Jc(12 T, 4.2 K) up to ~2500 A/mm-2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ~1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2-3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challengemore » of developing the highest possible Jc must be addressed by forming the maximum fraction of high Jc small-grain (SG) A15 and minimizing low Jc large-grain (LG) A15 morphologies. Finally, in one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial Jc improvement.« less

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Evaluation of critical current density and residual resistance ratio limits in powder in tube Nb3Sn conductors

Powder-in-tube (PIT) Nb3Sn wires are competing with Restacked-Rod-Process (RRP®) for the realization of the high luminosity upgrade of the Large Hadron Collider (LHC) at CERN. These two conductors have different properties and microstructures that are in both cases averages of an inhomogeneous A15 microstructure. PIT has in general a smaller fraction of A15 in the non-Cu cross-section than RRP® and a lower non-Cu Jc (12 T, 4.2 K) (2500–2700 A mm−2 versus 2900–3000 A mm−2) but it can be made in smaller filament diameters, which is an important property for LHC magnets. Another characteristic of PIT A15 is that ~25% is made up of ~1–2 μm sized grains (typically ~10 times the small grain (SG) diameter) and their contribution to transport is uncertain. Here we studied a 192 filament Ta-doped, 1 mm diameter PIT wire and combined multiple characterization techniques in order to distinguish the different wire components, to determine their individual properties and to identify which components are current-carriers. We found multiple evidence that the large A15 grains, which are also the highest-Tc grains, do not contribute to transport at high field and that the only current-carrying A15 is the SG with Tc <17.7 K. However, because of the high density of grain boundaries in the SG A15 layer, PIT has an exceptionally high SG-layer Jc and high specific grain boundary pinning force, QGB. These findings clearly show that it is essential to increase the ratio of small to large and disconnected grains in order to improve PIT performance.

https://iopscience.iop.org/article/10.1088/0953-2048/28/9/095001/pdf

Composition and connectivity variability of the A15 phase in PIT Nb3Sn wires

Improvement of small to large grain A15 ratio in Nb 3 Sn PIT wires by inverted multistage heat treatments

Future applications requiring high magnetic fields, such as the proposed Future Circular Collider, demand a substantially higher critical current density, Jc, at fields ≥16 T than is presently available in any commercial strand, so there is a strong effort to develop new routes to higher Jc Nb3Sn As a consequence, evaluating the irreversibility field (Hirr) of any new conductor to ensure reliable performance at these higher magnetic fields becomes essential To predict the irreversibility field for Nb3Sn wires, critical current measurements, Ic, are commonly performed in the 12-15 T range and the Kramer extrapolation is used to predict higher field properties The Kramer extrapolation typically models the contribution only for sparse grain boundary pinning, yet Nb3Sn wires rely on a high density of grain boundaries to provide the flux pinning that enables their high critical current density However, whole-field range VSM measurements up to 30 T recently showed for Nb3Sn RRP® wires that the field dependence of the pinning force curve significantly deviates from the typical grain boundary shape, leading to a 1-2 T overestimation of Hirr when extrapolated from the typical mid-field data taken only up to about 15 T In this work we characterized a variety of both RRP® and PIT Nb3Sn wires by transport measurements up to 29 T at the Laboratoire National des Champs Magnetiques Intenses (LNCMI), part of the European Magnetic Field Laboratory in Grenoble, to verify whether or not such overestimation is related to the measurement technique and whether or not it is a common feature across different designs Indeed we also found that when measured in transport the 12-15 T Kramer extrapolation overestimates the actual Hirr in both types of conductor with an inaccuracy of up to 16 T, confirming that high field characterization is a necessary tool to evaluate the actual high field performance of each Nb3Sn wire

Christopher Segal

Papers

Evaluation of critical current density and residual resistance ratio limits in powder in tube Nb3Sn conductors

Composition and connectivity variability of the A15 phase in PIT Nb3Sn wires

Improvement of small to large grain A15 ratio in Nb 3 Sn PIT wires by inverted multistage heat treatments

Evidence of Kramer extrapolation inaccuracy for predicting high field Nb3Sn properties