## 2 Design and Fabrication of the Current Leads
### Hybrid Current Lead  
The thermal heat leakage owing to the current leads can be divided into conductive heat transfer and Joule heating. To minimize the heat leakage attributed to the current lead, a hybrid with HTS tape whose critical temperature is approximately 90 K [23; 24] can be a good option. Given that the resistance of the current lead is zero at the superconducting state [25], heat is not generated owing to electrical current.  
Figure 6a,b shows the detailed structure of the hybrid current lead and the joint between the metal and HTS current lead, respectively. As indicated in Figure 6a, the hybrid current lead is composed of metal and HTS lead. The metal and HTS were jointed using soldering; and tube made of glass fiber reinforced plastic was installed around the HTS lead for protection. The HTS with 4 mm width and 200 A critical current at 77 K from SuNAM Co. (Anseong, Korea), Ltd. was used in this study [26]. The HTS tape was composed of several layers, including substrate, buffer, protection, and superconductor layers [27]. Among various layers of the HTS tape, some conductor layers such as the stainless steel layer used as a substrate, the copper layer used as a stabilizer layer, and the silver layer used as a protecting layer, can transfer conductive heat. However, the cross-sectional areas of these layers are very small, and the conductive heat transfer is also very small as compared with the metal current lead. The stacking structure of the HTS tape is an inevitable design feature to prepare for critical current degradation by physical damage of the HTS tape and to pass by the current when damaged. Therefore, three-layer stacked HTS tape was applied to the hybrid current lead in this study.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & \begin{tabular}{c} **A (mm\({}^{2}\))** \\ **(L = 0.98 m)** \\ \end{tabular} & \begin{tabular}{c} **Conductive Heat** \\ **Leakage** \\ \end{tabular} & \
\begin{tabular}{c} **Heat** \\ **Generation** \\ \end{tabular} &
\begin{tabular}{c} **Total Heat** \\ **Leakage (W)** \\ \end{tabular} \\ \hline Brass (Cu 90: Zn 10) & 22.52 & 0.1005 & 0.06598 & 0.1665 \\ Brass (Cu 60: Zn 40) & 42.56 & 0.1098 & 0.06804 & 0.1778 \\ Copper (RRR30) & 2.38 & 0.0380 & 0.0554 & 0.0934 \\ Copper (RRR10) & 4.90 & 0.0588 & 0.0552 & 0.1140 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of experimental results and as compare with the design value.  
Figure 6: Structure of hybrid current lead. (**a**) Shape of the hybrid current lead; (**b**) Joint between metal current lead and HTS.  
The heat leakages from the vapor-cooled and hybrid current leads are compared in Figure 7. As mentioned above, the heat leakage including the conductive heat using Equation (4) and the generated heat using Equation (2) is 0.114 W. In the case of hybrid current lead, conductive heat transfer occurs from approximately 80 K to the liquid helium temperature through intermediate cooling. Accordingly, heat is not generated because the resistance of the HTS tape is zero at temperatures below 80 K. The conductive heat transfer through the HTS tape calculated using the Fourier's law conduction equation is 0.0143 W. When an electrical current of 45 A is supplied through the hybrid current lead, the total amount of heat leakage of the superconducting magnet is 0.3396 W, and the liquid helium boil-off rate is predicted to be approximately 11.23 L/day.