Spatial distribution of the electric field (solid line) in the z direction for x = 1.2 mm and y = 0 mm. The dashed line represents the electric field when the values of U₄ and U₅ are exchanged. Geometrical parameters used for numerical calculations are shown in Table 1, and U₃ = 10.5 kV, U₄ = 7.5 kV, and U₅ = 2.5 kV.