Influence of magnetic hysteresis on quench-voltage detection in large superconducting magnets

We present a technical problem caused by magnetic hysteresis in quench-voltage detection systems for superconducting magnets. To detect a quench sensitively, it is necessary to use a balanced bridge circuit for canceling out inductive terminal voltages across a magnet. However, a changing magnetization due to shielding currents influences the quench-voltage detection in large superconducting magnets. In particular, the specifically designed detection system for the large helical device (LHD) cannot balance terminal voltages perfectly by adjusting a potentiometer to eliminate the inductive component. We apply a simple theoretical model and a numerical analysis to evaluate the out-of-balance voltage due to superconducting magnetization and coupling currents. The analysis indicates that the observed out-of-balance voltage in the LHD can be explained by a difference in magnetic hysteresis between two symmetric coils, caused by differences in the penetration field and coupling time constant between the conductors. Because a terminal voltage due to magnetic hysteresis is proportional to the volume of a superconductor, the influence of magnetization must be taken into account for large superconducting magnets.