Presheath and current-free double layer in a two-electron-population plasma or a plasma with negative ions

An edge plasma close to the RF antenna, which has a nonthermal energetic electron population, and a plasma for the negative ion source have two populations of negative charged species. The development of the potential near the wall is treated for such a plasma with two populations. The plasma equation is analytically solved to show the possibility of steady-state potential formation in the plasma and to obtain the potential drop in the presheath. The potential drop in the presheath considerably depends on the relative densities of the two populations. It discontinuously changes from the value of the order of kTc/e to one of the order of kTh/e where Tc is the effective temperature of the cold population and Th is one of the hot population. This happens when the ratio of the particle density of the energetic population to one of the cold population nh/nc becomes greater than a certain value of the order of 0.1. It is also shown that a current-free double layer can be steadily formed in the plasma when the ratio nh/nc is of the order of 0.1. The double layer is formed so as to connect two presheath potentials with different values. The position and amplitude of the double layer are controlled by changing the ratio nh/nc and a spatial distribution of the particle source. 1 ref