Quench transient current and quench propagation limit in pancake wound REBCO coils as a function of contact resistance, critical current, and coil size

Abstract It is a general belief that no insulation (NI) coil technology is a path to very high field superconducting coils. Recent experience has shown that there are aspects of NI coil design that, if not addressed, can possibly lead to coil failures. One potential problem area is the large transient currents that are associated with quench propagation in NI coils. In an attempt to understand and possibly find ways to minimize the potential for damage from quench transients, a parameter study was undertaken to examine the factors that influence the magnitude of transient currents during quench in NI coils. The characteristics of the transient currents are first examined. A study is then made of a set of test coils, looking at quench propagation and the transient current magnitude as a function of contact resistance, critical current, and importantly coil size. For each coil size, it is found that as the contact resistance increases, the magnitude of quench transient currents is reduced until a condition where effective quench propagation ceases, called the quench propagation limit (QPL). As the QPL is approached, the amplitude of the transient current is decreased and may provide a regime where quench induced stress can be effectively contained in coil designs. As coil size increases, the value of contact resistance associated with the limit of quench propagation increases as well. At large coil sizes that will be characteristic of high field REBCO magnets, the QPL extends to truly large values of contact resistance compared to values observed between bare conductors. The use of methods such as resistive films on conductors and co-wind steel will be required to increase contact resistance. In recognition of this development, the use of high contact resistance achieved in this manner is appropriately called resistive insulation coil technology.