Ions extracted from a plasma often have to pass a space charge existing in front of the container wall. If collisions occur within the sheath, both, the ion composition and the extraction efficiency may heavily be influenced.
The parameters of interest are the extension and the field strength of the sheath. A theory is presented by which these quantities can be calculated.
The theory holds for a positive space charge, the orifice being biased negativ with respect to the plasma potential.
Numerical results are given concerning a weekly ionized nitrogen plasma.
The mechanism of ion extraction from a weakly ionized plasma has been investigated using a quadrupole mass spectrometer. The plasma was generated in a flow system by means of a high frequency discharge.
It could be shown that the picture given by PAHL has to be extended by considering a space charge sheath spread over several mean free ion paths in front of the plane extraction probe, used. Two effects, resulting from the space charge, have been found observing the behaviour of nitrogen ions. First, the directional distribution of the extracted ions may be different for different ions. Second, ion-molecule reactions may be caused by the field strength within the sheath varying with both ion number density and probe voltage. In the case of N4+ ions the Varney reaction seems already to start in the static sheath. In order to minimize influences from the space charge, it is suitable to apply a slightly positive potential with respect to the container wall.