Electro-optic charge injection and transport measurements in highly purified water and water/ethylene glycol mixtures

Extensive Kerr electro-optic field mapping and voltage/current measurements have been taken with highly purified water over the temperature range of T=0 to 30°C using parallel plane electrodes with average field strengths up to 160 kV/cm. The Kerr constant of water B was measured to be B ≈ 3.4 to 3.6×10⁻¹⁴ m/V² for free-space light wavelength 590 nm and varies only slightly with temperature over the measurement range. Photomultiplier tube measurements at 633 nm at ≈10°C found water to have a Kerr constant B ≈ 2.7 to 2.9×10⁻¹⁴ m/V² while ethylene glycol had a negative Kerr constant B ≈ − (.8 to.9)×10⁻¹⁴ m/V². Water/ethylene glycol mixtures had an essentially linear variation of Kerr constant between these limits as a function of weight fractions, having a zero Kerr constant at about 79% glycol/21% water by weight. With pure water, a HV step has no volume charge at t=0. For times greater than 500 µs, stainless steel and copper electrodes generally inject positive charge although under some conditions with mixed electrodes they injected negative charge, aluminum electrodes only inject negative charge, while brass electrodes can inject either positive or negative charge. Thus, by appropriate choice of electrode material combinations and voltage polarity, it is possible to have uncharged water, unipolar charged negative or positive, or bipolar charged water. Generally, the bipolar case allows a higher applied voltage without breakdown, presumably due to the lower electric field strengths at the electrodes due to the space charge shielding.