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Description
Sure quench protection of HTS coils is essential for their sustainable operations. The most probable cause of quench damages is the over-heating at the highest temperature spot (hot-spot) of the coil wire during the quench protection sequence. Therefore, to avoid the damage, generated heat in the hot-spot of the coil conductor has to be decreased by quick decrease of a current flowing in the hot spot area. A common and basic method to protect an HTS coil from quench is a detect-and-dump type active method. By modifying this method, the authors propose a method to reduce hot spot temperature THS of a magnet composed of multiple pancake-coils by changing current distributions in the pancake coils at a quench event by a passive method. In a coil composed of multiple pancake-coils, amplitude of the perpendicular magnetic field components to wide faces of HTS tape wires is different by the position of a pancake coil in the whole coil. Critical current Ic of HTS wire is dominated by the perpendicular magnetic field component and Ic decreases for high perpendicular magnetic field component and a quench starts most probably in pancake coils of low Ic. When a quench is detected in a pancake coil of lower Ic, the current in the lower Ic coil is transferred to the other coils of higher Ic by resistive shorting the higher Ic coils. In the paper, current distributions of each of a pancake coil are analytically obtained and effectiveness of the method is investigated by a numerical simulation of a model coil. In the simulation study, it is shown that hot-spot temperature is decreased and that the coil can be safer from quench damages by the proposed method, comparing with the ordinal detect-and-dump method for a given value of quench detection voltage.
| Submitters Country | Japan |
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