**5.2 Space charge distribution at 100°C**

**Figures 7(A)** and **(B)** show the measurement results of space charge distributions at 100 °C in non-treated and treated PI films respectively. In these figures, (a), (b) and (c) show a time dependence of space charge distribution, a charge density distribution and an electric field distribution profiles, respectively. In figure (a), the charge density is described using a color scale put above each result. In figures(b) and (c) show the profiles at 30 min under each applied electric stress.

From the results of PI shown in **Figure 7(A**-**a)**, accumulation of positive and negative homo charges was observed in the vicinity of both anode and cathode electrodes, respectively, under the applied stress of 100 kV/mm or less in the untreated PI sample. When the stress became 125 kV/mm or more, accumulation of positive hetero charge was observed on the cathode side. Finally, a dielectric breakdown was observed at about 10 min later after 150 kV/mm was applied to this sample. In PI sample, the breakdown occurred after such positive hetero charge accumulation was also observed in other reports [1, 3]. It can be considered that the positive hetero charge accumulation may be a kind of "sign" for the breakdown in PI sample under dc high stress. On the other hand, in treated PI, as shown in **Figure 7(B-a)**, an accumulation of positive homo charge was observed on the anode side from under 25 kV/mm, and it spread towards the cathode side with increase of the applied stress. Then, finally in this sample, a breakdown was observed at about a few minutes later after stress of 125 kV/mm was applied. Judging from the electric field distribution in non-treated PI shown in **Figure 7(A-c)**, while the electric field near electrode seem to be enhanced slightly by the hetero charge accumulation, it can hardly expect to be a main reason for the breakdown. On the other hand, as shown in **Figure 7(B-c)**, a relatively large electric field enhancement was observed
