SHEATH POTENTIAL DISTRIBUTION IN A PLASMA INCLUDING NEGATIVE-IONS

Sheath potential distributions in electrostatic plasmas including negative ions have been studied using the Bohm equation. Since the mean velocity of the negative ions is not necessarily negligible compared with their thermal velocity, the contribution from non-thermal negative ions has been considered. The Sagdeev potential has been introduced to consider structural instabilities of the Bohm system. Wavy solutions related to ion-acoustic solitons bifurcate from the monotone asymptotic Bohm sheath solutions. The solvability conditions have also been discussed for characteristic parameter such as the concentration of negative ions and the Mach numbers of positive and negative ions.