Packet-like space charge in polyethylene probed with a 2D-spatial resolution

Puzzling behavior of space charge into dielectrics has been revealed by the observation of charge packets that cross the insulation above a material dependent field value in the range of 70-140 kV/mm for polyethylene-based materials. Their interpretation is still unclear but all rely on the hypothesis of a uniform distribution of charge in a plane parallel to the electrodes of the metal-dielectric-metal capacitor structure because the techniques used so far to investigate charge packet and charge dynamics could not observe charge distribution in the lateral direction (perpendicular to the propagation direction). Owing to the development of the Pulsed-Electro-Acoustic (PEA) technique, it has been possible to investigate the spatial structure of a "packet" in 2D. In the first part of the paper, we introduced the newly-developed PEA system that probes the space charge in an appropriate time (every 1 sec) and with a reasonable spatial resolution in two dimensions inside a 0.1 mm-thick polyethylene film. In the second part, we report experimental observations under DC bias voltages of 5 kV, 10 kV and 15 kV (corresponding to field values of 50, 100 and 150 kV/mm respectively). In the third part, we discuss the implication of the results for our understanding of the nature of charge packets