In order to find the cause of quenching in superconducting magnets, it is important to identify the initial normal zone. We focus on the changing tendency of the voltage across the coil just after the normal transition increases with an increase in the magnetic flux density of the initial normal zone. We develop an estimation method for the initial normal zone of the coil crosssectional direction by searching for the result closest to the measured waveforms from among the results of the transient circuit analysis that assumed the initial normal zone. Functional approximation using a nonlinear least-squares method is used for the search. Applying this technique to the low-temperature superconducting magnet system, we could identify the initial normal zone. This evaluation result is consistent with the result obtained by the interlayer voltage measurement. Since there is no need to install multiple voltage terminals inside the coil, this technique is a non-invasive evaluation method.
