Current Distribution and Stability of a Hybrid Superconducting Conductors Made of LTS/HTS

Abstract

Although having made great progress in many applications, such as high magnetic field inserts in magnets at helium temperature and electrical engineering application in low magnetic fields at nitrogen temperature, the high temperature superconductor (HTS) is less commercially viable in midand largescale magnets because of its high cost, low engineering critical current density, mechanical brittleness and low n value compared with conventional low temperature superconductors (LTS). The superconductor with a high n value transfers quicker from superconducting state to the normal conducting state. From the standpoint of application, the transient characteristics strongly affect its stability. With a high current, in the low n value area, flux flow voltage becomes lower than in the high n value area. Generally, it is considered that quenching occurs at a weak point, which is defined as a low Ic and low n value area. However, when such transition is observed, it is predicted that the limit current of quenching will be reached sooner for the high n value than for the lower n value (Torii et al., 2001, Dutoit et al, 1999). In general, the traditional superconductor has a higher n value than the Bi2223/Ag tape. In order to improve its stability, a LTS is always connected to a conventional conductor with low resistivity and high thermal conductivity, such as copper and aluminum, which then reduces its engineering critical current. To enhance the performance of conventional composite NbTi superconductors with large current capacity (several tens of kA) utilized in large helical devices (LHD), a new LTS/HTS hybrid in which HTS is used as a part stabilizer in place of low-resistivity metals, was proposed (Wang et al, 2004; Gourab et al, 2006; Nagato et al, 2007). Thus its cryogenic stability against thermal disturbance, steady-state cold-end recovery currents and the minimum propagation currents (MPC) can be greatly improved because the HTS has low resistance and current diffusion which is faster than that in a pure conventional conductor matrix.