Resonance Characteristics and Maximum Turn Voltage of JT-60SA EF Coil

The withstand voltage of the turn insulation is one of the most essential issues for the superconducting magnet system. There is a possibility that the actual differential voltage between each turns, called turn voltage in this paper, is larger than the maximum turn voltage under the ideal condition because of the voltage fluctuations of the power supply and the inhomogeneous voltage distribution in the magnet induced by the resonance phenomenon. The actual turn voltage was evaluated by the voltage measurements and the circuit simulation. The actual voltage of the large power supply, JT-60U power supply, was measured. The results indicated that the actual maximum voltage increase the 20% from rated voltage of the power supply. The margin of the insulation voltage is generally much larger than 20%. This amplitude of the voltage fluctuations is permissible for the design of the turn insulation. The resonance characteristic test was performed using the copper dummy pancake. The test results suggested that the resonance frequency of the EF4 pancake was about 56 kHz. The resonance characteristics of the real coil was also evaluated by the numerical method based on the lumped parameter circuit model. The resonance frequencies of the EF4 coil were calculated as 180 kHz, 300 kHz and the several higher frequencies. The main frequency component of the power supply is less than 5 kHz. The influence of the resonance phenomenon on the maximum turn voltage is negligible small.