We present results on the in-field critical current (I c ) performance of 4.0 µm thick REBCO film with 15% Hf addition with fields up to 31.2 T and field orientations in the B∥ab plane and B∥c axis. Unlike the behavior at B∥c, the critical current at B∥ab is only very weakly dependent on field, decreasing from self-field to 31.2 T by only 22%, i.e. from the self-field value of ∼7700 A/4 mm width to ∼6300 and 5812 A/4 mm width at 14 and 30 T, respectively. These values are remarkably 3 and 5.7x higher than the corresponding critical currents at B∥c. The in-field behavior of the present 15% Hf sample at field orientation B∥c axis is nearly identical to the previously reported record values found in 4.3 and 4.6 µm thick 15% Zr samples in terms of critical current density. In contrast to the pinning force behavior in the B∥c orientation, which saturates to a constant value of 1.7 TN m −3 above ∼5-6 T, the pinning force in the B∥ab orientation increases near-linearly, reaching a remarkable value of over 11.5 TN m −3 at 31.2 T. These results demonstrate the potential of thick REBCO conductors at 4.2 K for high field and energy density applications, in particular where the magnetic field is contained near the ab-plane.

High-temperature superconductors (HTS) make it possible to achieve magnetic fields beyond the 23.5 T limit of low-temperature superconductors (LTS). For higher energy density, high-performance HTS with Je > 1000 A/mm2 enable reduction in coil winding length and a smaller magnet size. Among HTS, REBa2Cu3O7-δ (REBCO, RE = rare earth) exhibits excellent mechanical properties and superior performance over a wide range of temperatures and magnetic fields. REBCO tapes can be converted to various formats, including round wires. The state-of-the-art REBCO superconductors for ultra-high field magnets, including cable/wire architectures, are reviewed. R&D needs to address remaining challenges with REBCO superconductors for ultra-high magnetic field applications are discussed.

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