Run-aways im Neutralgas

Abstract

The motion of electrons as determined by the field acceleration and the elastic and inelastic collisions with the gas atoms is calculated from the BOLTZMANN equation. We derive the average velocity and the scattering ellipsoid as a function of time. For particles starting from rest there exists always a critical electric field Ec depending on pressure and temperature. Below this critical value electrons approach the stationary drift process. Above the critical value the electrons do not reach a stationary state, they “run away”. For a finite initial velocity ν0 and a field below the critical value Ec the particles are either accelerated to drift, or decelerated to drift, or “run away”, depending on the value ν0. From a calculation of the scattering parameters we find for E > Ec a focussing effect in the velocity space which increases with field strength. Also the relaxation time for the drift process and the stopping power for electron beams can be calculated. Applications to the glow discharge are discussed.