A Versatile Modeling Technique for Predicting Dielectric Strength Improvements in Gas Mixtures for Superconducting Applications Chanyeop Park, Lukas Graber Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, GA 30332, USA Peter Cheetham, Jose G. Viquez, Chul H. Kim and Sastry Pamidi Florida State University, Center for Advanced Power Systems, Tallahassee, FL 32306, USA ABSTRACT This study presents a procedure that improves the dielectric strength estimation of gas mixtures for superconducting applications. The procedure introduces a generalization factor into the conventional formula of ionization coefficient, which provides versatility and higher accuracy in the regression process of ionization coefficients obtained by solving the Boltzmann equation of gas mixtures with various combination ratios. Generalization factors, which provide the highest accuracy in the regression process are identified. Then, versatile models are derived for each gas mixture based on the Townsend breakdown criterion. The versatile modeling procedure not only provides improved accuracy in the dielectric strength estimation of gas mixtures, but also presents an effective way of estimating variations in dielectric strength caused by changes in gas composition and temperature. The validity of the versatile modeling procedure is confirmed by breakdown measurements conducted with a number of gas mixtures at various gas pressures and temperatures. Index Terms — Gas Discharge, Paschen's Law, Dielectric Strength Estimation, Townsend Breakdown Criterion. 1 INTRODUCTION THE dielectric strength of cryogenic gas mixtures is one of major design constraints that determine the voltage ratings of high temperature superconducting (HTS) power applications. These applications operate at cryogenic temperatures with the use of liquid nitrogen ( 2 N ), the standard cryogenic medium. Nevertheless, gaseous helium ( He ) is the preferred cooling medium for shipboard HTS power applications [1, 2]. The preference in He is mainly due to the extended temperature range (as low as 4 K at 100 kPa), which permits to operate superconductors at temperatures lower than the melting point of 2 N (63 K at 100 kPa) and enables superconductors to reach higher current density. In addition, the use of gaseous cryogens is also beneficial since it reduces the risk of asphyxiation, which can occur by potential cryogen leakages in closed-space environments [3, 4]. However, one of the major drawbacks of using gaseous He as a cryogen is its weaker dielectric strength than that of liquid 2 N [5, 6]. To enhance the dielectric properties of He , the addition of various mole fractions of gaseous hydrogen ( 2 H ) was proposed in our previous studies, which showed substantial enhancements in dielectric strength [7–11]. Making practical use of the previous findings requires a development of methods that provide accurate estimations on the dielectric strength of individual gases and their mixtures. To model the dielectric strength of gas mixtures, we use the Townsend gas discharge theory. According to this theory, determining the coefficients A and B of the empirical formula of the pressure-normalized ionization coefficient ( p /  ), which describes the number of ionization collisions per unit length in the direction of electric field normalized by gas pressure, is a key step in the development of dielectric strength models. Coefficients A and B in the formula of p /  are obtained through a regression process, which utilizes the formula of p /  and p /  values obtained via experiment or numerical calculation. Conventionally, studies have used the formula of Manuscript received on 20 February 2017, in final form 16 May 2017, accepted 27 June 2017. Corresponding author: C. Park. IEEE Transactions on Dielectrics and Electrical Insulation Vol. 24, No. 5; October 2017 2755 DOI: 10.1109/TDEI.2017.006653