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

Uncertain performance and long training with many unpredictable quenches and slowly increasing quench currents are characteristic of the accelerator magnets needed for high energy particle accelerators, particularly the Nb3Sn magnets planned for future high energy proton colliders. This behavior makes high-temperature superconductor (HTS) magnets a potential option, even though there are many concerns about protection of such magnets during quench. Here we describe the performance of two recent racetrack coils made with a 17-strand Rutherford cable with state-of-the-art Bi-2212 wires capable of delivering current density up to 1000 A/mm2 at 27 T. The coils carried up to 8.6 kA while generating a peak field of 3.5 T at 4.2 K, at a wire current density of 1020 A/mm2. Quite differently from Nb-Ti and Nb3Sn magnets, these Bi-2212 magnets showed no early quenching indicative of training, showed virtually no dependence of quench current on ramp rate, and entered the flux flow state in a stable manner before thermal runaway and quench occurred. These magnets show the huge potential of Bi-2212 as a high field conductor which is, like Nb-Ti and Nb3Sn, isotropic, round and multifilament and suitable for Rutherford cable use but, unlike them, much more tolerant of energy disturbances that often lead Nb-based superconducting magnets to premature quench and long training cycles. The observation of stable voltage across the coils before thermal runaway removes one of the great concerns about being able to detect the onset of quench in a timely manner in HTS magnets, a prerequisite for triggering an appropriate quench protection strategy.