Ion Sampling Considerations for a Discharge Plasma of Nitrogen

A quadrupole mass-spectrometric sampling probe is employed for the study of ion extraction and plasma sheath formation in a discharge plasma of nitrogen at low pressures (9 to 27 μ). The dependence of the specific ion current ratio i(N+)/i(N2+) on the potential of the sampling probe is presented for various pressures and a fixed electron temperature. Kinetic energy distribution curves for individual ionic species are obtained under a variety of conditions of sampling probe potential, pressure, and electron temperature using a retarding potential field applied at the entrance grid of a quadrupole mass filter. Interpretations of the results indicate that ion generation processes and ion-molecule collisions occurring in the sheath may strongly influence the sampling process.