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Influence of negative ions on the plasma boundary sheath.

The collisionless boundary sheath of a plasma containing additionally negative ions and secondary electrons is investigated theoretically. Bohm's criterion is generalized for such plasmas. The following effects are considered. (i) For negative-ion densities less than a critical value, the negative ions are captured by the positive ions through Coulomb collisions in the bulk plasma and in the presheath. As a consequence, the negative ions move together with the positive ions to the cathode. The critical density is determined by the collision frequencies of the negative ions with the positive ions and with the neutral atoms. (ii) Negative ions penetrating into the collisionless boundary sheath together with the positive ions are reflected by the electric field. As a consequence a double layer appears with a maximum of the electric field within the sheath. (iii) Secondary electrons emitted by the electrode reduce the field in the sheath and can form a virtual cathode. The theoretical results are in good agreement with experimental measurements.