An appraisal of the mass spectrometer diagnostic technique in the study of afterglow plasmas

Abstract

An appraisal is presented of the mass spectrometer diagnostic technique as applied to the sampling of positive and negative ions diffusing towards an orifice probe located at the boundary of an afterglow plasma. The report is largely based on the results obtained from detailed studies over several years in these laboratories of time-resolved (or stationary) afterglow plasmas, together with reported observations of other workers. Several specific plasma situations are discussed and illustrated with data, including diffusion- and recombination-controlled positive-ion/electron plasmas and positive-ion/negative-ion plasmas. These examples emphasize the importance of spatial charge density distributions, space-charge sheaths and electric fields in both the vicinity of the sampling orifice and in the body of the plasma. Data are also presented which graphically illustrate the serious effects which can result from contamination of the sampling electrode in the region of the orifice. The problem of collisional break-up of weakly bound `cluster' ions in the sampling system is also briefly discussed. It is suggested that the serious discrepancies which exist between reported values of a large number of reaction rate coefficients obtained from afterglow studies are due at least in part to a lack of appreciation of some of these important physical phenomena. Throughout the paper, attempts are made to indicate the experimental conditions which must exist if valid data are to be obtained using the mass spectrometer sampling technique.