Some of these photons have
sufficient energy to break chemical bonds, resulting in degradation
of the polymer in the space charge limited field region [6].
In addition, some electrons will gain sufficient energy in this high
field region to damage the polymer. The overall result is that the
polymer degrades relatively rapidly in this space charge limited
field region. This can be observed directly [7] using a guarded
needle [8]. Initially, the current cycling in and out of the needle
is continuous, as a result of the charge injection into the dielectric.
But after a time of such continuous charge motion and reversal,
steps start to appear in the measured charge that are the
result of microdischarges in the tip region. The initial steps are
in the range of 0.02 pC [7]. We can estimate the distance over
which these discharges extend in the direction of the field, which
turns out to be in the range of 1.5 µm.