It uses a dimensionless potential ψ ψ = e Φ Φ k T and the lengths are measured in units of the Debye electron radius in the region of zero potential R e D = k T 4 π π e 2 n e 0 (where n e 0 denotes the number density of negative ions in the zero potential region). For the spherical case, L=2, the axial case, L=1, and the planar case, L=0.