Download e-book for iPad: An Introduction to Gas Discharges by A. M. Howatson (Auth.)

By A. M. Howatson (Auth.)

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From this there follow immediately a mean free path Am and collision frequency vm. All of these can be used or assumed whenever momentum transfer may be crucial. 3. Inelastic Collisions Any collision in which the internal energy of excitation of a particle is changed is inelastic. Only the excitation of electron energy levels is normally important in a gas discharge and for simplicity we shall discuss only atoms; the same principles apply 26 AN INTRODUCTION TO GAS DISCHARGES to molecules of two or more atoms.

The characteristic between the region of saturation and the point of breakdown represents the Townsend discharge, and we shall discuss Townsend's theory of this region. Consider first a low-pressure discharge in which the field E is 54 AN INTRODUCTION TO GAS DISCHARGES made uniform by having plane electrodes of dimensions large compared with their spacing, and in which constant external radiation is allowed to fall only on the cathode, there producing electrons. As E increases, the electrons leaving the cathode are accelerated more and more between collisions until their energy upon collision with neutral atoms is frequently above the ionization value.

The average kinetic energy is then constant and so also is the average directed velocity, the value of which is then known as the drift velocity of the electron. It is comparable with the terminal velocity attained by a body passing through a viscous fluid under the action of a constant force. The collisions experienced by the electron, which serve to fix this limit to the velocity it acquires from the field, have the macroscopic effect of viscous friction, which here determines the resistivity of the ionized gas.

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