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

Abstract

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.

Figure 1

J_E(B) of an optimally processed sample of the strand used in this study in comparison to that of a Bi-2212 with the previous record performance, the LHC Nb-Ti strand, and the HL-LHC RRP Nb₃Sn strand.

Figure 2

Coil voltages (RC5) during a linear current ramp (see inset in b) that ended with a quench. (a) Voltage tap map. (b) Coil voltages V₁₃ (whole coil) and V₁₂ and V₂₃ (individual layers).

Figure 3

Ramp rate dependence of the quench current I_q of RC5 and RC6 during linear current ramps. Inset shows a 3D display of the contours of the surface magnetic flux density generated by RC6 at 8600 A.

Figure 4

I_q of RC6 for consecutive quenches before and after thermal cycling to room temperature and back to 4.2 K.

Figure 5

Voltage development of RC5 for staircase ramps of the magnet current that ended with thermal runaway and energy extraction by switching in a dump resistor. The current ramp scheme contains current holding steps during which coil inductive signals die away and noise is much reduced (a). The coil and turn-to-turn voltages are shown in (b and d), respectively. The ramp turn voltage is highlighted in (c). The ramp turn is a 14 cm long section that transitions between the two coil layers in the peak field region. In (d), L1-T1 means the turn #1 of the coil layer #1 (other turns follow the same naming method.) and it is the outermost turn in the low field region. The voltage tap length of turns decreases from 62 cm for L1-T1 and L2-T1 gradually to 52 cm for L1-T6 and L2-T6.

Figure 6

The E-I transition of RC5 and RC6 derived from tests with staircase powering schemes for the ramp turns.