Space charge fields in afterglow plasmas

Abstract

Measurements are described of the temporal and spatial variations of the space charge fields in afterglow plasmas together with parallel measurements of electron density (ne), positive ion density (n+), negative ion density (n-), electron temperature (Te) and mass-analysed wall currents of both positive and negative ions. In the case of the plasmas containing electrons and positive ions only, it has been shown that the spatial variation of plasma potential (Δ Va) is related to gradients in the charged particle densities and to the electron temperature via diffusion theory, and no complications in the sampling of positive ion wall currents with an orifice probe arise. For plasmas in which significant concentrations of negative ions exist, the previously observed `trapping' of negative ions within the plasma occurs, as manifest by a lack of negative ion wall current at early afterglow times followed later by a sudden onset. The probe measurements indicate a continuous diffusive loss of electrons and positive ions during the afterglow, whereas the negative ions remain within the plasma until ne has fallen to less, similar 10−2n- at which point an avalanche loss process of the remaining electrons begins which rapidly leads to the establishment of a negative-ion/positive-ion plasma. During the afterglow period, a progressive collapse of the wall potential and the ambipolar field occurs from those characteristic of an electron-dominated plasma to those of a negative-ion-dominated plasma. The sudden appearance of negative ion wall current is closely correlated with the avalanche collapse of the ambipolar field. The significance of these observations to mass-spectrometric sampling from afterglow plasma is discussed.