High-temperature superconductors (HTS) could enable high-field magnets stronger than is possible with Nb-Ti and Nb₃Sn, but two challenges have so far been the low engineering critical current density J_E, especially in high-current cables, and the danger of quenches. Most HTS magnets made so far have been made out of REBCO coated conductor. Here we demonstrate stable, reliable and training-quench-free performance of Bi-2212 racetrack coils wound with a Rutherford cable fabricated from wires made with a new precursor powder. These round multifilamentary wires exhibited a record J_E up to 950 A/mm² at 30 T at 4.2 K. These coils carried up to 8.6 kA while generating 3.5 T at 4.2 K at a J_E of 1020 A/mm². Different from the unpredictable training performance of Nb-Ti and Nb₃Sn magnets, these Bi-2212 magnets showed no training quenches and entered the flux flow state in a stable manner before thermal runaway and quench occurred. Also different from Nb-Ti, Nb₃Sn, and REBCO magnets for which localized thermal runaways occur at unpredictable locations, the quenches of Bi-2212 magnets consistently occurred in the high field regions over a long conductor length. These characteristics make quench detection simple, enabling safe protection, and suggest a new paradigm of constructing quench-predictable superconducting magnets from Bi-2212.

Multifilamentary Bi₂Sr₂CaCu₂O_x (Bi-2212) wire made by the powder-in-tube technique is the only high temperature superconductor made in the round shape preferred by magnet builders. The critical current density (J _C ) of Bi-2212 round wire was improved significantly by the development of overpressure heat treatment in the past few years. Bi-2212 wire is commercially available in multiple architectures and kilometer-long pieces and a very promising conductor for very high field NMR and accelerator magnets. We studied the effects of precursor powder and heat treatment conditions on the superconducting properties and microstructure of recent Bi-2212 wires. Short samples of recent wire with optimized overpressure processing showed J _C (4.2 K, 15 T) = 6640 A/mm² and J _C (4.2 K, 30 T) = 4670 A/mm², which correspond to engineering critical current densities J _E (4.2 K, 15 T) = 1320 A/mm² and J _E (4.2 K, 30 T) = 930 A/mm².