**5.1 Samples and measurement procedure**

The Kapton®500H (Dupont-Toray Co.Ltd., nominal thickness is 125 μm) was used as measurement sample. Further, heating and humidifying treatment was previously applied to some samples. In this treatment, the sample were kept in a thermostatic chamber IH 400 set at 80 °C with humid of 80% for 1 hour.

Time dependence of the space charge distribution in the sample was measured using the apparatus (up to 150 °C), which is described at the chapter 3, for 100–150 °C. In the measurement, at fast, a dc voltage corresponding to 25 kV/ mm was applied to the sample for 30 min at the designated temperature, and the distribution was measured with interval of 5 s by applying a pulse voltage of 200 V with a pulse width of 5 ns. The repetition frequency of the pulse voltage application was 400 Hz and the averaged signal was obtained from 700 signals. Then, the circuit was shorted for 5 min, and the measurement was also carried out using above procedure. After the measurement, next measurement was started under elevated electric stress. The increment of the voltage was corresponding to 25 kV/mm, then the applied stresses were 25, 50, 75, 100, 125, and 150 kV/mm.
