2. Basic Physics of Electrical Discharges

The main constituents of air in the Earth’s atmosphere are nitrogen (78 %), oxygen (20 %), noble gases (1 %), carbon dioxide (0.97 %), water vapor (0.03 %), and other trace gases. Because of the ionization of air by the high-energy radiation of cosmic rays and radioactive gases generated from the Earth, each cubic centimeter of air at ground level contains approximately ten free electrons. In general, air is a good insulator, and it can retain its insulating properties until the applied electric field exceeds approximately 3 × 10⁶ V/m at standard atmospheric conditions (i.e., T = 293 K and P = 1 atm). When the background electric field exceeds this critical value, air is converted very rapidly into a conducting medium, making it possible for electrical currents to flow through it in the form of sparks. Let us now consider the basic processes that make possible the conversion of air from an insulator into a conductor and the different types of discharge that take place in air under various conditions.