- 86-


                    n            n
         UH   2eca   C n2 en/n2 + c
                   - -+             n 2d



                   UH = 2e-a


 x Lea + 4nc+ +.+ n2 ea,'nL            +    3     3 LJ


                                              (5.6)


The number of terms to be retained in the series (5.6) is determined by the requirement that nc 3 >   3n L This requirement also provides that nc3 >> n L  so that the partition function may now be written in a form which is convenient for computation:


           U 2e-a     n2 e a/n2+1n2(57
         uH =                 +   n           (5.7)
                    n1



       The quantum number, nc, at which the electron becomes free due to preionization of hydrogen, is a function of the charge density of the plasma. IvanovKholodnyi et aZ. have derived a relation between nc and the electron density Ne as follows:


log Ne = 21.65 - 6 log (nc + 1)


(5.8)