The non-exponential decay characteristics of HTS NI-class coils in sudden discharge experiments

The decay of the voltage and magnetic field of the high-temperature superconductivity no-insulation-class (NI-class) coil in the sudden discharge experiment is usually considered to be exponential. However, our experiment found that it is not accurate to describe the decay characteristics of the NI-class coil with a simple exponential function. Combined with equivalent circuit models of the NI-class coil, we use the theory of ordinary differential equations to explain the non-exponential decay characteristics and point out that it is closer to the matrix exponential characteristics. Based on this finding, we believe that the non-uniform turn-to-turn contact resistivity (TTCR) is an important cause of the non-exponential decay characteristics of the NI-class coil. Reverse thinking leads us to realize that the voltage decay curve of the NI-class coil in the sudden discharge experiment contains the distribution information of its TTCR. Therefore, we propose a method for measuring the TTCR of NI-class coils based on the matrix exponential model. The co-simulation combining the finite element model and the equivalent circuit model shows that the non-uniformly distributed TTCR has a great influence on the transient process of the NI-class coil, which makes the current and current density in the transient process vary greatly in different regions of the NI-class coil. Those findings mean that the simulation study of the NI-class coils and magnets with uniform TTCR usually used in the past may be far from the actual situation.