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Conventional fusion magnet system typically discharges the stored energy by external discharge resistors as a quench protection with a relatively short time constant, such as < 15 s for ITER magnets [1]. However, the resulting coil voltage to ground can exceed several kV, increasing the risk of accidental Paschen discharge or arcing. In addition, quench protection relying on the external resistor needs numerous vacuum penetrations through the cryostat. This is particularly critical for the stellarator magnet system requiring much more coils than those of Tokamak device.
To address these issues, GFG proposes a concept of an innovative low-voltage fast discharge where the discharge time constant is long as > 40 s. This drastically reduces the coil voltage as the targeted voltage is 500 V from terminal to ground. Additionally, internal magnet structure works as the dump resistor, which drastically reduces the number of feeder lines and the penetrations of vacuum barrier. These features would not only reduce the risk of electrical failures but also relax the requirements for the insulation system and complexity of the magnet system.
In this study, we have investigated the peak voltages appearing in 40 coils of the GIGA magnet system with an electrical network model built on LTSpice. The simulation results indicate the necessity of varistor unit for surge voltage mitigation, and demonstrate that the proposed method successfully limit the coil voltage to less than +/- 500 V.
[1] I. Song, A. Roshal, V. Tanchuk, J. Thomsen, F. Milani and I. Benfatto, "The fast discharge system of ITER superconducting magnets," 2011 International Conference on Electrical Machines and Systems, Beijing, 2011, pp. 1-6, doi: 10.1109/ICEMS.2011.6073779
