Experimental Determination Of The Development And Evolution Of The Spatial Distribution Of Charge Injected In Polyethylene By A Voltage-biased Needle

We have used the pressure-wave-propagation method [l] on a microscale to measure the spatial distribution of charge in polyethylene between the point of an imbedded, voltage-biased needle electrode and a grounded, planar, counter electrode on one surface. The pulsed laser beam that produces the pressure pulses was focused on a small diameter aperture as a means of maintaining energy while reducing the diameter of the laser-induced pressure wave front. The capacitance that is modulated by the pressure wave and contributes to the response, thus is confined to that small volume swept-out by the pressure wave within which the injected charge is expected to reside. Reducing unwanted capacitance modulations from regions outside this volume is the dynamic equivalent of electrostatically guarding the needle point when measuring injection currents from the point [2]. In the present work and in [2], the charge injected from the point is rendered observable. Guarding is of limited usefulness when studying injection near breakdown because, if the guard is grounded, the distance from the needle point to the guard cannot be made much smaller than the distance between the point and the ground plane, while, if the guard is at the needle potential, it too may be a source of injected charge.