Experiment and Analysis on Temperature-Dependent Electric Contact Resistivity of an NI HTS Coil

This paper reports experimental and numerical analysis results of a no-insulation (NI) high-temperature superconductor (HTS) coil in terms of temperature-dependent electric contact resistivity. A test coil was designed, constructed, and operated. The coil is divided into four sections according to the radial locations of inserted voltage taps to measure local voltages. A preliminary experiment was performed to evaluate the electromagnetic properties of the coil in liquid nitrogen. Then, the temperature-dependent contact resistance of the coil was evaluated from $10 \,\mathrm{K}$ to $80 \,\mathrm{K}$ at every $10 \,\mathrm{K}$ increment in a cooling conduction facility. To investigate the impact of the contact resistivity on the charge/discharge dynamics of the coil, we selected two arbitrary current ramp rates values, namely fast ( $0.5 \,\mathrm{A}\,\mathrm{s}^{-1}$ ) and slow ( $0.01 \,\mathrm{A}\,\mathrm{s}^{-1}$ ) charge/discharge. In this study, the experimental results were analyzed with equivalent circuit and finite element models. The analysis results suggest the following conclusions. First, the contact resistivity of the test coil calculated from the experiment at the coil terminals level increases by a factor 2 with respect to the temperature, ranging between $1.16 \,\mathrm{\mu }\mathrm{\Omega }\,\mathrm{c}\mathrm{m}^{-2}\,$ to $2.59 \,\mathrm{\mu }\mathrm{\Omega }\,\mathrm{c}\mathrm{m}^{-2}\,$ . Second, although a constant winding tension was applied during the coil fabrication, the contact resistivity was found to increase within different sections (inner to outer radius). Third, the impact of the ramp rate on temperature dependency is negligible.