• Impact or collision of particles amongst each other, accelerated under electrical field, leads to the formation of charge carriers from neutral gas molecules.
• The multiplication of charge carriers in gas takes place mainly by impact of electrons with neutral molecules known as α - process (also primary process).
• The positive ions make a moderate contribution to ionization only at solid insulation surfaces, which is known as β - process (also secondary process) shown in Fig. 6.3.

Fig 6.3  Impact ionization by electron and ion

• The β - process does not play any significant role in gaseous dielectrics during the discharge leading to breakdown.
• It is the α   -process alone which plays the major role.
• When an electron gains more kinetic energy than the required ionization energy W₁of the gas molecule, it is capable of ionizing by impact, that is, ejecting an electron from a neutral molecule and thus leaving behind a positive ion.
• To cause ionization, the incoming electron must have a kinetic energy greater than or equal to the ionization energy of the molecule (eU_I).
• However, not all electrons having gained energy ≥ (eU_I) cause ionization on collision with neutral molecules. Ionization by impact is actually a probability process.
• The mean number of ionizing collisions made by a single electron per centimeter drift across the gap in a uniform field is given by α. It is known as the Townsend's first or primary 'ionization coefficient', which represents basically a probability process. For gas discharge, it is a very important coefficient strongly dependent upon the electric field intensity,