Dependency of the electronegative sheath structure on the negative ion density and temperature

Some general aspects of the sheath structure in electronegative plasmas are analysed, through an extensive review of the authors´ contributions to this subject. In this work, the one dimensional collisionless sheath model of electropositive plasmas is investigated extended here to study the sheath structure of an electronegative plasma at steady state in the presence of negative ions and planar sheath configurations. The same as the electropositive plasma sheath, a net charge layer is built within the sheath. Specially, the effects of the plasma electronegativity (ratio of negative ion density to the electron density e) on the floating potential and sheath thickness for various negative ion temperature was studied. It has been observed that floating potential and, therefore, sheath thickness strongly sensitive to the plasma electronegativity. It is most interesting that the floating potential varies nonmonotonically with the plasma electronegativity of the bulk plasma when the negative ion temperature is lower than the electron temperature (T_e/T_i- < 10).