Electron thermalization distances and free-ion yields in dielectric fluids: Effect of electron scavengers

Abstract

Free-ion yields were measured in isobutane and in solutions containing sulfur hexafluoride, at densities from the liquid at 295 K to the supercritical fluid. A modified Onsager model was used to estimate the most probable thermalization distances $b_{\mathrm{GP}}$ (GP denotes a Gaussian power-law distribution). The density-normalized distance $b_{\mathrm{GP}\mathrm{d}}$ in isobutane increased from 6.3×${10}^{\mathrm{-}6}$ kg/${\mathrm{m}}^2$ at 295 K to a maximum of 10.5×${10}^{\mathrm{-}6}$ kg/${\mathrm{m}}^2$ at 395 K, and then decreased to 5.6×${10}^{\mathrm{-}6}$ kg/${\mathrm{m}}^2$ at 408 K ($T_c$). The maximum, which is evidence of conduction-band enhancement of $b_{\mathrm{GP}}$, was suppressed in solutions containing small amounts of ${\mathrm{SF}}_6$. The ${\mathrm{SF}}_6$ captured the electrons while they were still at epithermal energies, and terminated their flight away from their sibling ions. The values of $b_{\mathrm{GP}}$ and the free-ion yields were thereby decreased.