The expression ( ε ε i + 1 − − ε ε i ) is the energy required to remove the ( i + 1 ) th electron. In the case where only one level of ionization is important, we have n 1 = n e and defining the total density n as n = n 0 + n 1 , the Saha equation simplifies to: n e 2 n − − n e = 2 λ λ th 3 g 1 g 0 exp ⁡ ⁡ [ − − ε ε k B T ] where ε ε is the energy of ionization. We can define the degree of ionization x = n 1 / n and find x 2 1 − − x = A = 2 n λ λ th 3 g 1 g 0 exp ⁡ ⁡ [ − − ε ε k B T ]