Notes on the effects of dust in positive columns†

Abstract

Observation of small particles in positive columns shows that considerable interaction can occur between them and the plasma. This paper suggests methods for analysing the complex situations which may arise and discusses some simplified cases, illustrated by reference to discharges in argon at about 1 torr. Ordinary positive column theory can be modified to allow for presence of cold particles by treating them as distributed insulated negative surface ionization sinks, additional to the wall sink if their effect is relatively small, and replacing it if their effect is large. Increases in electron temperature and axial field result and the radial electron concentration pattern may be altered, an off-centre maximum developing under some conditions if the particles are concentrated near the axis. Dust-laden discharges may be noisy. The particle temperature will usually exceed the gas temperature because of the surface recombination. Equilibrium temperatures are obtained by equating the recombination heat to the sum of the radiation and gas-cooling losses. The temperature lag which occurs when plasma parameters alter near a particle is also discussed. The tendency of the particles, like massive negative ions, to congregate near the axis is stagnant gas is accounted for as an effect of the radial electric field, modified substantially in some instances by the dust. When the electron concentration is large, for example in the core of a constricted column, particles may heat sufficiently to become thermionic emitters, and may evaporate and recondense farther out, or react rapidly chemically. The accompanying local heating and distortion of the plasma is one of several possible causes of gas turbulence in which the particles can participate. In strongly in-homogeneous discharges, differential ion pressure may contribute significantly to the particle dynamics.