High voltage electric field and space-charge distributions in highly purified water

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Abstract
High voltage Kerr electro‐optic field mapping measurements in highly purified water show significant field distortions due to injected positive space charge over the temperature range of 8–25 °C using parallel plane stainless steel electrodes stressed to average field strengths up to 150 kV/cm. The net space charge was always positive with density of ≂2 C/m3 which is about 40% of the background value of charge density of thermally generated hydronium and hydroxyl ions. Because measurements only showed a net positive space charge, a unipolar drift dominated conduction model was developed where positive charge would migrate in an ohmic medium. This model predicts the observed propagating charge front, electric field enhancement at the noncharge injecting electrode, and field decrease at the charge injecting electrode. The time of flight of positive charge between electrodes gave an estimate of the positive ion mobility of μ≂4×10−7 m2/(V s) which is inbetween the values of the hydronium ion mobility measured at low voltages and the electrohydrodynamic mobility which accounts for fluid convection effects due to the Coulombic force putting the fluid into motion. The dielectric relaxation time at T≂8 °C (τ≂320 μsec) measured at low voltage was much less than the measured charge transport time between electrodes, τmig ≂2 msec, so that the calculated effects of space charge were much less than measured because the charge relaxes quickly as it migrates. It was found that a dielectric relaxation time of τ≂1.6 msec would provide a good fit between analysis and experiments.