Second Test Coil for the Development of a Compact 3 T $\hbox {MgB}_{2}$ Magnet

The authors have reported the results of low n -value from a MgB₂ test coil developed a year ago. A second test coil has been developed with wire of different structure and manufacturing process. Although the n-value related voltage of the second test coil was lower than the first test coil at designed current, it still showed low n-value. A third test coil has been wound with reduced mechanical stress. It also showed very similar n-value related voltage and n-value. Investigation of voltage distribution over the coil indicated that magnetic field was the major factor causing degradation of the n-value and resulting in n -value related voltages. Since the n-value related coil voltages were on the order of 0.1 μV/cm, the usual short sample I_c test (1 μV/cm was the definition of I_c ) might not detect the n-value related voltage and might not be able to investigate the cause of low n -value. Therefore, the medium length ( ~ 10 m) samples were tested and they showed the wire´s lengthwise nonuniformity both on n-value and I_c, which might be another potential cause of the low n-value of the coil. Along with the electrical investigation, the manufacturing process of the wire was carefully inspected for longitudinal uniformity. Some wire segment samples from the same batch exhibited nonuniformity in the particle size distribution resulting in nonuniform filaments. This might have occurred in the wire for the second and third test coils.