The application of a quadrupole mass-spectrometric probe to the study of ion extraction from a discharge plasma of nitrogen.

A versatile sampling technique is described which permits a more definitive study of ion extraction and plasma sheath formation in discharge plasmas. The ionic species from the plasma are sampled into a quadrupole mass spectrometer through a metal leak arrangement which itself acts as a Langmuir-type probe. This behaviour is substantiated by specific ion sampling probe characteristics for a microwave discharge plasma in flowing nitrogen at low pressures (9 to 27 microns). The beam of positive ions extracted from the microwave discharge source is composed of N^+, N2^+ and N3^+ ,N2^+ being the dominant ion. The kinetic energy distribution curves for individual ionic species are obtained under a variety of conditions of sampling probe potential, pressure, and electron temperature by a retarding potential applied to the entrance grid of the mass filter. The curves are discussed in terms of the currently accepted sheath model. The interpretations indicate that ion generation processes and ion-molecule collisions occurring in the sheath may strongly interfere with the sampling process