Preliminary Kerr electro-optic field measurements in melted polyethylene

Utilizing a frequency-selective amplifier, sensitive Kerr electrooptic field mapping measurements were made in melted (>110°C) low-density polyethylene (LDPE) with a small signal AC voltage superimposed on a DC voltage. Preliminary measurements show significant levels of field distortion due to bipolar homocharge injection with charge density magnitudes on the order of 0.01 C/m³ for field strengths on the order of 6 kV/cm. The experiment utilizes short parallel-plate electrodes of length 15 cm so that E _m, the electric field necessary for the first light maxima with crossed polarizers, is approximately 500 kV/cm. By normalizing the optically measured average field to the voltage divided by spacing, the Kerr constant of melted LDPE at≈140°C was found to be B≈1.3×10^-15 m/V². The high sensitivity of the experiment also makes it possible to measure the self-field, and its subsequent decay, due to the charge density alone after the electrodes have been short-circuited