A direct measurement of space charge injection in strongly inhomogeneous fields

Dielectric ageing always starts at points of strong local field enhancement, such as sharp metallic points, inclusions and the like. In a separate contribution to these proceedings it is argued that the formation and growth of an electrical tree is governed by a balance between the electrostatic forces on the charges at points of local field enhancement and the local mechanical strength of the material. Space charge formation plays a crucial role in this ageing process. A conceivable mechanism of space charge formation is the creation of hot charge carriers by a local intrinsic breakdown mechanism, whenever the field exceeds a critical value. Eventually a thermalized cloud of trapped charges results in such a way that the limiting field is nowhere exceeded. (Field Limited Space Charge.) Space charge injection has two important consequences for dielectric ageing. First, the local fieldstrength is limited to an upper value, thus suppressing the tendency of tree formation and growth. On the other hand the hot charge carriers involved may cause local chemical degradation of the dielectric. In addition, local heating of the dielectric by periodic injection and extraction of electrons may lead to a local reduction of the mechanical strength of the dielectric, favouring the formation and propagation of electrical trees.