Statistical Method to Estimate Training Quench Characteristics and Stability Designing of Superconducting Windings

Abstract

One of key issues in designing superconducting apparatuses is how to design high current density superconducting windings whose quench current surely exceed designed operation current. However, a method to quantitatively estimate a quench current of the winding has not been developed. This paper is related to this issue. MQE (Minimum Quench Energy) is a well-known criterion of the stability of superconductors but cannot be a proper measure of the stability of the superconducting windings without knowledge on the size of disturbance in the windings. Therefore, it is important to know quantitatively the size of the disturbance energy to estimate the stability.
Generally, a major cause of unexpected quenches of high current density superconducting windings is a disturbance caused by an abrupt conductor motion due to electromagnetic forces. The released energy by the conductor motion depends on many factors, such as stiffness, compliance and irregularities in dimensions of the conductors, electrical and mechanical forces to the conductors, fixing structure of the conductors etc. We developed a method to estimate the disturbance energy due to conductor motion taking into account of these factors. Once the disturbance energy is known, the stability of the windings can be estimated by knowing MQE of the conductor. The conductor motions occur at poorly supported parts of conductor in winding which are caused by irregularities in the size of conductors and structures. By assuming that the irregularities follow Gaussian process, we developed a statistical method to predict quench characteristics of superconducting windings.
In this paper, the theory to estimate quench characteristics of superconducting windings is described. We applied the theory to two different types of superconducting windings, a large bore superconducting magnet and rotor windings of a superconducting generator, and compared the estimated results with excitation test results. The estimated quench characteristics well coincided with excitation results. Details of the analysis method are presented in the paper. We also discussed about possible method to increase stability or current density keeping required stability level.