Effects of high magnetic field on the quench behavior of Bi ₂Sr₂CaCu₂O_x coils at 4.2 K

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 ₂Sr₂CaCu₂O_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.