Electron Transport Coefficients and Effective Ionization Coefficients in SF₆-O₂and SF₆-Air Mixtures Using Boltzmann Analysis

Abstract

The electron drift velocity, electron energy distribution function (EEDF), densitynormalized effective ionization coefficient and density-normalized longitudinal diffusion velocity are calculated in SF6-O2 and SF6-Air mixtures. The experimental results from a pulsed Townsend discharge are plotted for comparison with the numerical results. The reduced field strength varies from 40 Td to 500 Td (1 Townsend=10 −17 V·cm 2 ) and the SF6 concentration ranges from 10% to 100%. A Boltzmann equation associated with the two-term spherical harmonic expansion approximation is utilized to gain the swarm parameters in steady-state Townsend. Results show that the accuracy of the Boltzmann solution with a two-term expansion in calculating the electron drift velocity, electron energy distribution function, and density-normalized effective ionization coefficient is acceptable. The effective ionization coefficient presents a distinct relationship with the SF6 content in the mixtures. Moreover, the E/Ncr values in SF6-Air mixtures are higher than those in SF6-O2 mixtures and the calculated value E/Ncr in SF6-O2 and SF6-Air mixtures is lower than the measured value in SF6-N2. Parametric studies conducted on these parameters using the Boltzmann analysis offer substantial insight into the plasma physics, as well as a basis to explore the ozone generation process.

Studying the Electron Energy Distribution Function (EEDF) and Electron Transport Coefficients in SF6 – He Gas Mixtures by Solving the Boltzmann Equation

“…Thus, higher EEDF profile in Figure (2) is dominated by the presence of higher percentage concentration of He gas in the mixture. The behavior of EEDF verses mean electron energy of SF 6 -He mixtures are quite analogy to that of SF 6 -CHF 3 and SF 6 -CF 4 mixtures gas [12]. Figure (3a) shows the EEDF versus the mean electron energy for SF 6 (75%) + He (25%) mixture at various values of E/N.…”

Solving of the Boltzmann transport equation using two - term approximation for pure electronegative gases (SF6, CCl2F2)

“…This activity is stimulated, firstly, by an effort to find new gases with dielectric field strength higher than of SF 6 (Devins, 1980) which is commonly used as an electrical insulator. Secondly, there are economic reasons, which determine the use of mixtures that reduce the cost of gas insulation (Malik and Qureshi, 1980), if there is a synergistic effect of gas mixtures it may be effectively exploited in practice when SF 6 and CCl 2 F 2 mixed with various percentages of different buffer gases with lower GWP than SF 6 , and CCl 2 F 2 with which to generate various mixtures, with excellent dielectric, such as air (Wei et al, 2014), N 2 (Basurto et al, 2004), CO 2 (Pinheiro and Loureiro, 2002), O 2 (Wei et al, 2014), He , CF 4 (Li et al, 2013) and CF 3 I (Dahl, 2019), (Omori et al, 2014). For instance, previous studies showed that with a mixture ratio 40%SF 6 -60%N 2 , 40%SF 6 -60%CCl 2 F 2 (Siddagangappa et al, 1983) exhibits dielectric strength of the mixture was greater than SF 6 suitable to be used as insulation in high voltage equipment.…”