The detection of a normal zone in inductively coupled superconducting coils

A method is described for the electrical detection of a normal zone in inductively coupled superconducting coils. Measurements are made with two kinds of bridges: mutual inductance bridges and self-inductance bridges. The bridge outputs are combined with other measured voltages to form a detector that can be realized with either analog circuits or a computer algorithm. The detection of a normal zone in a pair of coupled coils, each with taps, is discussed in detail. It is also shown that the method applies to a pair of coils when one has no taps and to a pair when one coil is superconducting and the other is not. The method is extended, in principle, to a number of coils. A description is given of a technique for balancing the bridges at near the operating currents of the coils.