**5.3 Space charge distribution at 150 °C**

**Figures 8** shows the measurement results of space charge distribution at 150 °C in PI. From the measurement results of PI in **Figure 8(A-a)**, accumulation was not observed under the stress of 75 kV/mm or less. While the positive homo charge was

**Figure 8.** *Space charge distribution in PI at 150 °C.*

observed under the stress of 100 kV/mm or more, the amount of it was not so large and breakdown was not observed by the end of measurement. In this case, the electric field was not distorted by the accumulation of charge as shown in **Figure 8(A-c)**. On the other hand, in the measurement results of the treated PI sample, positive charge accumulation near the anode was observed under 50 kV/mm or more as shown in **Figure 8(B-a)**. When the applied stress increased, the positive charge distribution spread towards the cathode, then the negative homo charge accumulation was also observed near the cathode under 100 and 125 kV/mm. Furthermore, in this condition, a negative charge accumulation was also observed near the anode. Finally, the negative and the positive charges were located near the cathode and the anode, respectively, at the end of the measurement. The electric field distribution in this sample was, of course, distorted by the accumulated charges, and the electric field near the cathode and the anode were enhanced to 150 kV/mm as shown in **Figure 8(B-c)**, but the increments of them are not so large.

This process observed in the treated PI at 150 °C is similar to that in non-treated PI at 100 °C, except for the breakdown. The space charge accumulation seems to be affected by the moisture content in the sample, as mentioned above. Therefore, the measurement at 150 °C was also affected by it. In other words, the moisture in both of the non-treated and treated PI sample might be vaporized by the heating. Similar results were obtained in annealed PI sample in silicone oil [1].
