## 4 Results and Discussion
### Effects of the Copper Stabilizer Thickness on the Thermal Stability of the HTS Coil  
Copper stabilizer can improve the thermal stability of the 2G HTS tapes [30]. In this section, the effects of the copper stabilizer thickness on the thermal stability of the HTS coil during the worst short circuit fault are numerically studied.  
Figure 11 depicts the variations in the total current, the critical current and the copper layer current of turn 42 after the occurrence of the worst short circuit fault, with copper stabilizer thicknesses of 10 \(\upmu\)m and 20 \(\upmu\)m. Figure 12 depicts the ohmic loss values and the temperature values of turn 42 after the occurrence of the worst short circuit fault with copper stabilizer thicknesses of 10 \(\upmu\)m and 20 \(\upmu\)m.  
It is found that, as the copper stabilizer thickens, the temperature rise phenomenon becomes weaker. The temperature rise in the copper layer in a short period can be approximately considered to be proportional to the ohmic loss volume density (ignore the heat transfer from the copper layer to other layers to simplify the explanation). The ohmic loss volume density is proportional to the square of the current density of the copper layer, so the temperature rise in the copper layer can be approximately regarded as inversely proportional to the square of the thickness of the copper stabilizer. Thus, although the time when the total current exceeds the critical current is almost the same for the two cases, the final temperature will be lower for the thicker copper stabilizer.  
However, though the thicker copper stabilizer improves the thermal stability of the HTS coil in the event of a short circuit fault, the engineering current density also decreases. For the HTS generator designer, this requires the use of an electromagnetic-thermal coupling numerical simulation model to make trade-offs between the engineering current density and the thermal stability of the HTS tapes.  
Figure 11: Waveforms of the total current, the critical current (CC), the superconducting layer current (SLC) and the copper layer current (CLC) of turn 42 after the occurrence of the worst short circuit fault with copper stabilizer thicknesses of 10 \(\upmu\)m and 20 \(\upmu\)m.  
Figure 12: Waveforms of the ohmic loss values and the temperature values of turn 42 after the occurrence of the worst short circuit fault with copper stabilizer thicknesses of 10 \(\upmu\)m and 20 \(\upmu\)m.  
One noteworthy phenomenon is that the total current eventually becomes inconsistent after a short circuit fault occurs in both cases. The total current in both cases is the same at the beginning of the short circuit fault because the coil is initially in the superconducting state and the coil resistance is almost zero. The short circuit impedance is mainly the inductive resistance of the coil, which is the same for the two cases. Then, as the temperature rises, the coil resistance is gradually dominated by the copper layer resistance and the short circuit impedance becomes greater. For the thicker copper stabilizer case, the copper layer resistance is smaller, so the short circuit impedance is smaller, causing a greater short circuit current. Moreover, since the copper layer loss is proportional to the square of the current, finally, at t = 0.12 s, the 20-\(\upmu\)m copper stabilizer case's ohmic loss even exceeds that of the 10-\(\upmu\)m copper stabilizer case by a little.