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

A high temperature superconducting (HTS) magnet system is immersed in sub-cooled liquid nitrogen (LN
₂
) 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 LN
₂
for thermal and electrical safety, while cryogenic gaseous nitrogen (CGN
₂
) is present above LN
₂
. However, the method of determining the optimum level of LN
₂
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 LN
₂
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 LN
₂
and, the dielectric breakdown experiments were performed according to level of LN
₂
. The electric field intensity according to the level of LN
₂
is calculated by finite element method (FEM) and is analysed using the Weibull distribution. Moreover, the electric field utilization factor (ξ) 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 LN
₂
are proposed. Moreover, As the level of LN
₂
increased, the selection criteria increased, but the level of LN
₂
for 50% was selected for efficient design. It is found that the proposed method is helpful in selecting the level of LN
₂
of HTS magnet through the calculated selection criteria and evaluation method.