## 4 Results and Discussions  
Figure 9 shows the cool-down curve of the superconducting magnet system, while liquid helium is supplied to the cryostat. In order to reduce the heat leakage by residual gas conduction and minimize the affection to the boil-off test results, a vacuum environment was maintained of about 0.018 Pa during the whole experimental process. To minimize the use of liquid helium, initial cooling was performed first by filling the liquid nitrogen container before liquid helium was supplied. The temperature of the superconducting magnet was measured using platinum resistance thermometer (accuracy rating \(\pm\) 0.5 K) and it was approximately 250 K. The temperature sensor (model Lakeshore Cernox(tm) 1010-AA, Lakeshore Cryotronics Inc., Westerville, OH, USA) on the current lead and baffle was installed at a position of about 590 mm from the top part of the system, and operated at approximately 180 and 215 K. It took approximately 1.2 h for the temperature of the superconducting magnet to cool to the liquid helium temperature. The amount of liquid helium was measured with a helium level sensor (model AMI liquid helium level sensor, American Magnetics Inc., Oak Ridge, TN, USA). After liquid helium was supplied, the level of liquid helium was approximately 69.72%. The quantity of liquid helium per 1% level was 0.69 L and the total amount of liquid helium was approximately 48.11 L.  
Figure 10 shows the magnetic field variation with respect to the supplied electrical current. After the liquid helium was transferred, the electrical current was supplied to the superconducting magnet to generate magnetic field. The ramping rate of electrical current was 0.75 A/min and the operating range was 0-40 A. For currents in the range from 40 to 45 A, the ramping rate of electrical current was 0.3 A/min. It took approximately 1.1 h to reach the magnetic field of 5 T, and this magnetic field was maintained subsequently during 0.8 h. After reaching the magnetic field of 5 T, when the liquid helium level was approximately 30%, the temperature of the warm end (joint with vapor-cooled metal lead) and the cold end (joint with LTS magnet) of the HTS current lead was maintained approximately at 10 and 4.2 K, respectively. Figure 8: Experimental setup of the 5 T superconducting magnet system. (**a**) Installation of the vapor-cooled current leads; (**b**) Entire experimental setup including the data acquisition system. Figure 11 shows the liquid helium level variations with respect to the magnetic field for different current leads. The cooling and magnetic field generation experiments of superconducting magnet system were conducted to assess the performances of the two types of current lead. After the superconducting magnet was cooled to 4.2 K, 45 A electrical current was supplied to generate 5 T magnetic field. Furthermore, the performance of the current lead was evaluated based on the calculation of the amount of liquid helium that decreased, while the magnetic field was maintained constant. As shown in Figure 11, when measuring the liquid helium level, a fluctuation of about \(\pm 0.6\%\) occurred by signal noise. The experimental results obtained during the boil off of liquid helium used the average value for 30 s.  
When the vapor-cooled current lead was applied, the liquid helium level decreased from 32.34% to 31.60%, while the magnetic field of the superconducting magnet was maintained at 5 T. The liquid helium level was decreased by 0.74% over the 0.8 h time span, as shown in Figure 11. The decreasing rate of liquid helium was approximately 15.21 L/day. As mentioned earlier in the design section of the vapor-cooled current lead, the calculated liquid helium boil-off rate was approximately 14.31 L/day. The results of the experiment boil-off test agreed with the analysis and were within a 6.0% margin. Figure 10: Magnetic field variation with respect to the supplied electrical current. Figure 9: Cool-down curve for the 5 tesla superconducting magnet system.