Effects of high magnetic field on the quench behavior of Bi₂Sr₂CaCu₂O_xcoils at 4.2 K

Abstract

The development of high field superconducting magnets using high temperature superconductors (HTSs) is progressing for high energy physics, nuclear magnetic resonance and energy storage applications. Yet the key issue of quench protection remains unresolved, primarily due to the slow normal zone propagation velocity (NZPV) in HTS magnets. High magnetic field may affect the quench behavior through two opposing effects: increased NZPV may result due to reduced critical temperature and current sharing temperature, but decreased NZPV may result due to reduced critical current density and thus operating current. At present it is unclear which effect dominates. Here, a series of quench experiments at high magnetic field on multilayer wind-and-react Bi 2 Sr 2 CaCu 2 O x (Bi2212) coils addresses this question. The two-and three-dimensional quench behavior is investigated in a magnetic field up to 20 T at 4.2 K. With increasing magnetic field, the minimum quench energy decreases significantly. The NZPV also decreases with magnetic field, but only up to about 8 T. For magnetic fields above 8 T, the NZPV is independent of magnetic field up to at least 20 T. Thus, at low field the NZPV is dominated by the decreasing critical current density, whereas at higher magnetic field the competing effects of decreasing critical current density and decreasing temperature margin offset each other.

Magnetic field dependent stability and quench behavior and degradation limits in conduction-cooled MgB2wires and coils

“…Interestingly, the NZPV(B) curve follows J c (B) very closely, decreasing rapidly with magnetic field and not showing a plateau effect like what was reported for Bi2212 [14]. The MQE(B) curve follows T c (B) closely up to 1.5 T, showing similar behavior to Bi2212, but at higher magnetic field MQE increases dramatically.…”

“…After current sharing begins, however, the temperature response is delayed relative to the voltage because the thermocouples are located on the conductor surface whereas the voltage represents the average conditions within the superconducting filaments and the metal sheath. Hence, NZPV studies focus on voltage measurements, similar to previous studies on the quench behavior of HTS conductors [14, 15, 20]. …”

“…But increasing magnetic field also decreases J c , increasing MQE and decreasing the NZPV. A recent study of the effects of high magnetic fields on the 4.2 K quench behavior of Bi2212 round wires with increasing magnetic field showed that the reduction in T c and T cs dominates the MQE behavior and that MQE decreases continuously with increasing magnetic fields up to 20 T. The NZPV, however, decreases with magnetic field only up to about 8 T. Above 8 T, the NZPV is independent of magnetic field up to at least 20 T. Thus, at low field, the NZPV is dominated by the decreasing J c , whereas at higher magnetic field the competing effects of decreasing J c and decreasing temperature margin offset [14]. …”

High-field quench behavior and dependence of hot spot temperature on quench detection voltage threshold in a Bi2Sr2CaCu2Oxcoil

“…The hot spot temperature was determined by two methods. Temperature of the quench zone was directly measured using an E--type thermocouple (Lake Shore Cryogenics, Inc., Chronmel -Constantan, 36 AWG) (as shown in Figure 3), following previous studies [15,16,19,21]. The response time of an E--type thermocouple, confirmed by our measurements, is ~100 ms so it tends to underestimate the temperature rise if the temperature rising rate is >10 K/s.…”

“…Quench protection of superconducting magnets, which must detect a non--recovery normal zone and force the magnet current to go to nearly zero within a few seconds or even some fractions of a second to prevent overheating of superconducting windings [9,10], is nontrivial even for superconducting magnet systems made from Nb--Ti and Nb 3 Sn [11,12] for which abundant coil fabrication and operation experiences have been accumulated. It is especially challenging for superconducting magnet systems based on high temperature superconductors (HTS) [13,14] because measurements in short samples of both Bi--2212 [15,16] and (RE)Ba 2 Cu 3 O 7--x (RE = rare earth) coated conductors [17] show that at 4.2 K normal zones propagate at an order of cm/s, instead of m/s for Nb--Ti and Nb 3 Sn at 4.2 K, even in strong magnetic fields ( Figure 1 summarizes propagation speed data of Ag/Bi--2212 round strands and coils available in the literature and measured for this study), limiting our ability to drive the resistive zone to occupy as large a fraction of the winding volume as possible for developing an internal resistance useful for active quench protection. During a quench, conductor temperature rises quickly.…”