Charge injection and transport in high voltage water/glycol capacitors

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Abstract
A water/ethylene glycol mixture to be operated at temperatures below 0 °C is being investigated for use as the high permittivity, low loss dielectric in pulse forming lines for pulsed power applications. High voltage open circuit decay curves show a dielectric relaxation time that decreases with initial voltage at room temperature, while at −10 °C, the open circuit decay has a negative second derivative with time in contrast to the expected exponential decay which always has a positive second derivative. It is hypothesized that this anomalous behavior is due to space‐charge injection which increases the ohmic conductivity σ to σ+q μ, where q is the time and space dependent net charge density with mobility μ. A drift dominated conduction model is used to solve for the electric field and charge density distributions and the terminal voltage‐current behavior of a lossy capacitor where one electrode injects charge proportional to the instantaneous local electric field. The specific case studied is of a parallel plate capacitor rapidly charged to an initial voltage and quickly open circuited. The calculated open circuit voltage decay curves show good agreement with measurements and yield experimental values for the charge injection parameter and the high field mobility of the injected charge. The value of mobility for the best fit of experimental data to the theory agrees with the value for the hydronium ion. Preliminary Kerr electro‐optic field mapping measurements in highly purified water without glycol, confirm positive charge injection.