Effect of space charge on the electric-field intensity in polymer dielectrics

The effect of a negative space charge (NSC) on the stationary distribution of the electric field in a polymer dielectric is discussed. The NSC accumulates as a result of injection of electrons from microprotuberances on the cathode. A microprotuberance is modeled by a concentric spherical capacitor, in which the cathode is the electrode with the smaller radius. The facts that the NSC distribution depends on the rate of trapping and detrapping of electrons and that the activation energy of the process can depend on the intensity of the field are taken into account in the calculations. It is shown that the NSC limits the field near a microprotuberance on the cathode. The dependence of the maximum attainable field strength in the sample Emax on the magnitude of the voltage V applied to it can be roughly represented as a broken straight line consisting of two segments. The slope of the first segment is equal to the slope of the straight line characterizing the dependence Emax(V) in the absence of NSC, while the slope of the second line is close to that of the straight line giving Eav(V)