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High-temperature superconductors (HTS) could enable high-field magnets muchstronger than is possible with Nb-Ti and Nb3Sn, but two key limiting factorshave so far been the difficulty of achieving high critical current density inlong-length conductors, especially in high-current cables, and the danger ofquenches out of the superconducting into the normal state. Here we demonstratestable, reliable and training-quench-free performance of Bi-2212 racetrackcoils wound with a 17-strand Rutherford cable fabricated from wires made withnanospray Bi-2212 powder. These multifilament wires are now being delivered insingle lengths of more than 1 km with a new record whole-wire critical currentdensity up to 950 A/mm2 at 30 T at 4.2 K. These coils carried up to 8.6 kAwhile generating a peak field of 3.5 T at 4.2 K, at a wire current density of1020 A/mm2. Quite different from the unpredictable training performance ofNb-Ti and Nb3Sn magnets, these Bi-2212 magnets showed no training quenches andentered the flux flow state in a stable manner before thermal runaway andquench occurred. Also quite different from Nb-Ti, Nb3Sn, and REBCO magnets forwhich localized thermal runaways occur at unpredictable locations, the quenchesof Bi-2212 magnets consistently occurred in the high field regions over aconductor length greater than one meter. These characteristics make quenchdetection rather simple, enabling safe protection, and suggest a new paradigmof constructing quench-predictable superconducting magnets from Bi-2212, whichis, like Nb-Ti and Nb3Sn, isotropic, round, multifilament, uniform over kmlengths and suitable for Rutherford cable use but, unlike them, much moretolerant of the energy disturbances that often lead Nb-based superconductingmagnets to premature quench and long training cycles.

Stable, predictable and training-free operation of superconducting  Bi-2212 Rutherford cable racetrack coils at the very high wire current  density of more than 1000 A/mm2