Evaluation of Selection Criteria for the Level of Liquid Nitrogen in HTS Magnet System

Abstract

A high temperature superconducting (HTS) magnet system is immersed in sub-cooled liquid nitrogen (LN2) and continuously refrigerated by a cryogenic cooling system. Specifically, in the case of a side-wall cooling type, HTS magnet system is located well below the level of LN2 for thermal and electrical safety, while cryogenic gaseous nitrogen (CGN(2)) is present above LN2. However, the method of determining the optimum level of LN2 in HTS magnet system has not been deeply investigated. Therefore, in this work, the process of determining the selection criteria for the optimal level of LN2 in HTS magnet system is proposed. A sphere-to-plane electrode system made of stainless steel is used under AC and DC conditions. A sphere electrode is surrounded by CGN(2) and a plane electrode is immersed into LN2 and, the dielectric breakdown experiments were performed according to level of LN2. The electric field intensity according to the level of LN2 is calculated by finite element method (FEM) and is analysed using the Weibull distribution. Moreover, the electric field utilization factor (xi) is adopted to analyze the electric field uniformity of the electrode system. Consequently, the process of determining the selection criteria, and an efficient method for evaluating level of LN2 are proposed. Moreover, As the level of LN2 increased, the selection criteria increased, but the level of LN2 for 50% was selected for efficient design. It is found that the proposed method is helpful in selecting the level of LN2 of HTS magnet through the calculated selection criteria and evaluation method.