Influence of Resonance Phenomenon on Voltage Distribution in Central Solenoid of JT-60SA

In the large-scale superconducting magnet, a nonuniform voltage distribution is present due to the resonance phenomenon resulting from inductance and capacitance. To investigate this phenomenon, a central solenoid (CS) model coil (four-layer) was prepared and the frequency dependence of the voltage in different coil turns was measured. Simulation was also performed for a CS model coil in order to determine the effect of factors such as the relative permittivity and the electrical conductivity of the insulation between the conductors. The results indicated that both the maximum voltage in the turns and the frequency at which resonance occurs strongly depend on the relative permittivity, whereas only the maximum voltage depends on the conductivity. Based on this simulation, we investigated the resonance phenomenon of JT-60SA CS. Consequently, the maximum voltage between layers at the resonance frequency is about 2 times higher than that at 5 kHz, and the maximum voltage between turns is about 20 times higher, so there is a possibility that the insulation between the turns is damaged at the resonance frequency. These results, therefore, represent important information for the safe operation of the JT-60SA.