**2. Space charge evaluation methods**

### **2.1 PEA method**

Since the 1980s, the PEA method has been used to measure the space charge of sheet or film samples in order to investigate space charge phenomena and evaluate the aging of dielectrics. A space charge distribution by converting a pressure wave generated by applying a pulse electric field to a sample under high voltage application and converting the pressure wave into an electric signal using a piezoelectric element. The thickness of the sample that can be evaluated by PEA is more than 100 μm, and its spatial resolution is about several tens of micrometers.
