Collision frequency associated with high temperature air and scattering cross-sections of the constituents

Abstract

The plasma sheath acts as a lossy medium for electromagnetic waves, the loss being caused mainly by elastic collisions of electrons with the gas molecules, atoms and ions. In this paper, scattering cross-sections of electrons with neutral air particles are assembled from available experimental and theoretical data and are analysed. Those determinations which appear most reliable are chosen for calculating the variation of the collision cross-section with velocity for each component of air. Since the concentration of equilibrium constituents of air is dependent on temperature and density, the collision frequency, after summing over all neutral components, can be obtained as a function of three variables—temperature, density and electron velocity. One can also evaluate a collision frequency of electrons with ions as a function of these three variables. The total collision frequency is obtained by adding the collision frequency of electrons with neutral particles to the collision frequency of electrons with ions. The concept of a velocity-dependent collision frequency with ions is significant since it can be applied to partially ionized gases and to cases where a r.f. electric field exists. The effective collision frequency can be obtained as a simple function of temperature and density by averaging over a Maxwellian distribution only in certain limiting cases, viz. very high or low ratios of r.f. to collision frequency. The limitations of various averaging techniques are discussed. The above procedure is more exact than converting velocity to temperature directly through the relation 1 eV = 3 2 kT . Admittedly, the error in this direct conversion is within the spread of the various experimental results.