The plasma-wall boundary region in negative-ion-dominated plasmas at low pressures

This paper brings together recent work in low-pressure plasmas dominated by negative ions, examining in detail the structure of the plasma-sheath boundary. The parallels between negative ion-electron plasmas and two-electron-species plasmas are drawn, as are those between probe-plasma and plasma-wall systems. Particular attention is given to the parameter region where, for the temperature ratio T e / T n greater than a critical value and a range of values of the density ratio n n / n e , there is a structure where two `plasmas', one an ion-ion plasma and the other essentially an electron-ion plasma, are separated by a potential structure. It is shown that in neither the fluid model nor in the free-fall model is the structure a simple double layer. The results of analytical and computational treatments are brought together and it is suggested that experiment may be the only way to determine the relative merits of the two models, as well as demonstrating their general validity.