Space charge formation and Bohm’s criterion in the edge of thermal electronegative plasma

Abstract

The collisional electronegative plasma space charge is investigated in the presence of the thermal positive ions. The Boltzmann distribution is assumed for electrons and negative ions and fluid equations are used to treat the accelerated positive ion through the sheath region. The influence of the positive ion temperature on the profile of the space charge is obtained for different negative ion concentration and negative ion temperature for collisionless and collisional cases. It is shown that the position of the space charge peak is independent of positive ion temperature while its amplitude depends on the positive ion temperature. The presence of the negative ion leads to damping of the space charge amplitude. In addition the thermal effect of the positive ion on the kinetic energy of the ion extracted from an ion source is studied in difference of collisionality and electronegativity. It is shown that, in the presence of thermal positive ion, the influence of the negative ion temperature on the sheath characteristics disappears. It is observed that in the presence of the hot positive ion, the twofold feature of the space charge starts at higher values of negative ion temperature which is more pronounced in collisional case. Finally, the influences of the positive and negative ion temperature, as well as the electronegativity and collisionality on the net electric current are studied.