Gas Discharge Fundamentals

Abstract

We consider first the collisions of ions and electrons with the neutral atoms in a partially ionized plasma; collisions between charged particles are more complicated and will be treated later. Since neutral atoms have no external electric field, ions and electrons do not feel the presence of a neutral until they come within an atomic radius of it. When an electron, say, collides with a neutral, it will bounce off it most of the time as if it were a billiard ball. We can then assign to the atom an effective cross sectional area, or momentum transfer cross section, which means that, on the average, an electron hitting such an area around the center of an atom would have its (vector) momentum changed by a lot; a lot being a change comparable to the size of the original momentum. The cross section that an electron sees depends on its energy, so in general a cross section depends on the energy, or, on average, the temperature of the bombarding particles. Atoms are about 10-8 cm (1 Angstrom) in radius, so atomic cross sections tend to be around 10-16 cm2 (1 A2) in magnitude.