David C. Larbalestier

Screening current in REBCO-coated conductors amplifies the Lorentz force. In high fields, screening current stress (SCS) can cause excessive magnetic stress and subsequent plastic deformation of the conductor. This is recognized as a key issue in the high-field, high-stress REBCO magnet technology at present, since such plastic damage degrades local superconductivity and eventually affects the coil performance. Although the importance of SCS has been widely recognized and extensively investigated, relevant studies are mostly limited to the calculation of peak stress and strain, rarely focusing on the damage initiation and propagation mechanism. Additionally, simulation studies of REBCO coils consider key conductor properties by referring to measured results in the literature, but rarely employ measurements obtained from the conductor used in the tested coil. We therefore performed an integrated conductor-to-coil study. First, we selected one of our previous high-field REBCO coil experiments, where the coil did not experience quench damage, although SCS-initiated wavy plastic damage and subsequent coil performance degradation were observed during the test. The conductor used in the coil was parameterized and formulated in detail based on measured electromagnetic and mechanical properties. Then, simulation work was performed to investigate plastic damage initiation and propagation, demonstrating that SCS caused local plastic damage, degrading local J c and reducing global I c , changing the effective current path, and shifting the electric field center towards the undamaged region from the physical center. One key finding is that plastic damage propagation becomes self-arresting and remains confined to a limited region of the conductor width because local J c degradation redistributes the screening current and consequently alters the stress distribution. This paper presents a detailed conductor model and SCS simulations and discusses their agreement with experimental observations.

Bi ₂ Sr ₂ CaCu ₂ O _x (Bi-2212) is an interesting high temperature superconductor (HTS) for high field applications due to its unique ability to develop high critical current density ( J c ) in polycrystalline round wires. The performance of all HTSs is highly sensitive to the doping states, particularly near the critical doping where the pseudo-gap opens and J c , the upper critical field H _{c 2} and other superconducting properties, but not T c , are maximized. However, due to Bi-2212’s complex wire microstructure, the impact of doping on the conductor performance has been poorly understood. Here we systematically investigate the superconducting behaviour across a wide doping range, separating for the first time the individual effects induced by oxygen-doping on the grain and the grain boundary (GB) properties. Our results can be interpreted as grains and GBs being affected by separate pseudo-gap openings with the grain irreversibility field H _Irr being controlled solely by the grain pseudo-gap, whereas the GB H _Irr is influenced by both. Moreover, the J c performance at 4.2 K is strongly related to the grain vortex pinning properties, but with some J _c limitation arising from the GBs. These findings demonstrate the impact of a fundamental physical phenomenon, the pseudo-gap, on the overall conductor performance, and they highlight the complexity of optimizing this material. These results also suggest that targeted adjustments to Bi-2212 processing may enable significant improvements in J c . Moreover, since the pseudo-gap is not unique to Bi-2212, similar limiting effects of different grain and GB boundary doping almost certainly operate in other HTSs, which also could gain from alternative processing strategies making them more suitable for magnets for high-field research, fusion or medical applications, and for high-power cable technologies.

REBCO coated conductors (CCs) have now been optimized for current-carrying capacity in high magnetic fields. Production-length CCs may carry more than 1500 A at 4.2 K/30 T in the B//ab configuration. Such high I_c is essential for the construction of new generations of ultra-high field (B > 40 T) magnets. However, high I_c also increases screening current stresses (SCS) and makes these contemporary tapes vulnerable to mechanical damage during magnet operation. To study this vulnerability, we constructed and tested Little Big Coil #6 (LBC6), one of a series of standardized ultra-high field insert coils operated in a 31 T resistive background field at the National High Magnetic Field Laboratory (NHMFL). LBC6 used contemporary conductor with triple the I_c of the earlier LBCs, so we hypothe sized that the coil would be destroyed by SCS during operation. Instead, LBC6 survived multiple quenches from more than 42 T and repeated cycling from 31 T to 41 T. Postmortem analysis revealed plastic deformation throughout the winding pack, but the damage to the REBCO layer was confined to one conductor edge. We propose that this coil exhibited a self-arresting damage mechanism: during the initial energization and quench, SCS damage sufficiently narrowed the superconducting filament width (thus reducing I_c) such that no further SCS damage occurred during subsequent operation. Possible implications for ultra-high field magnet design are discussed.

RE Ba ₂ Cu ₃ O _7-x ( RE BCO, RE : rare earth) coated conductors suffer from great critical current I _c differences between different manufacturers, I _c variations within individual manufacturers, and often significant lengthwise fluctuations. The understanding of such variations is complicated by the lack of a direct correlation between I _c and the critical current density J _c . In fact, although J _c is the fundamental property determined by the local vortex pinning landscape, I _c is often limited by variable current blocking mechanisms. An important practical complexity is commercial practices of performing J _c and I _c evaluations based on I _c at 77 K and self-field (sf), where connectivity variation dominates over vortex pinning variations. However, at higher fields and lower temperatures, vortex pinning becomes more complex and highly variable, making predictions of I _c and J _c at arbitrary temperature, magnetic field, and field orientation quite uncertain. To address some aspects of this problem, we conducted detailed spool-to-spool performance characterization on recently manufactured RE BCO coated conductors. Despite J _c (77 K, sf) varies by only ~11%, J _c (77 K, 1 T) of its minimum and maximum (for H // ab -plane) show variations of ~21% and ~32%, respectively. This emphasizes the shortcoming in using J _c (77 K, sf) as parameter for evaluating even the low field performance. An even more remarkable spool-to-spool J _c variation of ~68% was observed at 20 K and 15 T for H // ab -plane. To identify the origin of such lack of reproducibility we performed microstructural characterizations, which revealed, within the REBCO layer, large variation in the density of copper oxide (CuO _x ) particles ranging from 0.1 to 2 μm in size. We believe that they play a decisive role in reducing the effective cross-section of the RE BCO layer by not simply blocking current themselves, but also by nucleating off-axis RE BCO grains, whose misoriented grain boundaries adversely impact RE BCO grain-to-grain connectivity. The RE BCO growth associated with high density CuO _x particles also leads to the more disordered spatial arrangement of degrade the orientation of BaHfO ₃ precipitate arrays, which, self-aligning along the ab -planes, generate strong pinning enhancing J _c ( H // ab ) at all temperatures. In this way, we established that variations of both connectivity and vortex pinning are thus directly coupled. Our results also explain why the so-called "lift-factor", typically defined by the ratio between I _c ( T , H ) and I _c (77 K, sf), frequently turns out to be unreliable.

REBa2Cu3O7−γ coated conductors have proven themselves viable for high-field magnet applications such as compact fusion and accelerator magnets. For such applications, detailed understanding of their electromagnetic properties, namely the critical current density Jc(B, T, θ), is required to ensure safe operation and quench protection. However, this parameter space remains poorly characterized at low temperatures (T) and high magnetic fields (B), particularly near the ab-plane, where transport measurements become difficult. This study investigates the influence of BaZrO3 (BZO) nanorod morphology on Jc(B, T, θ) in commercial REBCO conductors using a combination of transport measurements and torque magnetometry. Measurements were performed up to 30 T, from 4.2 to 45 K, and over angles spanning c-axis and 30◦–150◦. Despite identical nominal pinning specifications, the conductors exhibit large variations in Jc, anisotropy and angular peak width, directly correlated with BZO nanorod diameter and density. The results reveal distinct strong and weak pinning contributions with different field and temperature scaling, which are quantitatively separated using a two-component angular model, and which highlights the need for downstream users to not assume one reel’s Ic dependence represents any other given reel. Torque magnetometry is shown to be a powerful tool to access these regimes rapidly and reliably, allowing full characterization of large numbers of reels required for HTS magnets.

Bi₂Sr₂CaCu₂O _{8− x} (Bi-2212) multi-filament round wire is a high-temperature superconductor (HTS) capable of carrying high transport currents, which makes it suitable for high-field magnet applications. However, its weak Ag–Mg sheath leaves it vulnerable to mechanical stress, posing challenges for high-field magnet design. To better understand and improve mechanical stress management in Bi-2212 winding packs, we conducted an experimental study evaluating the axial stress–strain behavior of five winding pack configurations with varying insulation materials, reinforcement strategies, and construction quality. Using uniaxial tensile testing at 77 K, we measured Young’s modulus and Poisson’s ratio for each composition. Our results show that pure alumina braid insulation and co-wind reinforcements significantly enhance stiffness compared to aluminosilicate braids, with more than 2.5 times increased winding pack Young’s modulus. Rule of mixtures analysis further quantified the contribution of non-wire composite components to overall stiffness. These findings highlight the critical role of insulation material selection and reinforcement design in optimizing Bi-2212 coil performance under stress, providing a foundation for improved mechanical models and more reliable high-field HTS magnet designs.

In recent years there have been growing concerns about the proper evaluation of physical properties of superconductors, in particular for quantities extracted from magnetic characterizations. Errors can and often do occur due to the following issues: i) several measurement instruments still use Gaussian & cgs-emu units instead of the preferable International System (SI) units; ii) there are decades of valuable publications where, however, equations were expressed in Gaussian & cgs-emu or other unit systems or where constants were normalized to unity, which requires proper understanding and unit conversion in order to correctly evaluate the measured physical quantities; iii) the conversion between unit systems sometimes appears challenging and may not be properly performed. In this paper we will describe how to properly convert physical quantities relevant for the evaluation of magnetic and other properties focusing on the still most used unit systems, SI and Gaussian & cgs-emu. We will provide examples of how to properly verify and understand the physical formulae. We will include examples for the correct method to determine the critical current density J _c of a superconductor from the measurement of its magnetic hysteresis loop through the Bean model, and the correct conversion to SI of the equations for H _c2 ( T ) according to the Werthamer-Helfand-Hohenberg (WHH) formulation. The goals of this paper are to make the readers aware of the unit conversion issue, to provide useful hands-on tools for proper conversion and to strongly encourage future exclusive use of the SI units and formulae.

To explore AC loss characteristics of commercial Bi-2212 conductors made in the USA, a small ohmic heating (OH) prototype coil is being developed for compact tokamaks using high performance Bi-2212 wires newly manufactured by Bruker EST. The goal is to build and test Bi-2212 prototype coils for fast ramp central solenoid (CS) operation of spherical tokamaks (ST). For a ST pilot plant, one of the key challenges is the tight space available for CS in a center column where in-board legs of the toroidal field (TF) and OH coils are assembled into a cryostat with a few millimeters clearance. A high current density Rutherford cable consisting of 17 legacy Bi-2212 wires, a contribution of the US Magnet Development Program (MDP), was fabricated at the Lawrence Berkeley National Laboratory (LBNL) for the development of fast ramp CS magnets in the ST Advanced Reactor (STAR) designed by PPPL. Characterization of the new and legacy HTS wires is presented, along with the coil design wound directly with new wire, and a Rutherford cable-wound solenoid using the legacy wire. A comparison of AC losses is made based on expected performance for a fast ramp ST operation. The HTS cable and cabled coil fabrication issues such as the heat treatment, coil mandrel design, assembly and 2212 quench characteristics are discussed, along with a test plan for CS design validation.

This paper reports screening current simulations of two REBCO coils with different winding approaches considered: one with quasi-layer winding and the other with pancake winding. In-house simulation models were used to calculate the screening current, Lorentz force, and mechanical stress of each coil. The comparison showed that the maximum hoop stress of a quasi-layer-wound coil was lower than that of a pancake-wound coil by more than 10%. Detailed analysis confirmed that such mitigation resulted from the benefit of a quasi-layer-wound coil. The key was that the direction of screening current induction can be manipulated, thus offsetting the superposition of the Lorentz force amplified by the screening current. This result suggests that a quasi-layer winding approach could be a competitive option in the insert module of a high-field superconducting magnet in terms of stress/strain margin. This paper provides a quasi-layer-wound coil simulation model, simulations of quasi-layer-wound and pancake-wound coils, and comparison results. We then discuss quasi-layer winding and cabling approaches to reduce excessive electromagnetic stresses in high-field superconducting magnets.