glow discharge When a constant potential is applied between the cathode and the anode, held at small distance in a gas at low pressure (1 Torr) current flow is negligible, below the breakdown voltage. A small current flows between the electrodes when the cathode emits electrons as a result of heating, or irradiation with UV radiation. Increasing the spacing between the electrodes makes the current larger. This happens because an electron has to travel a minimum distance called the mean free path before it can cause ionization. When the spacing between the electrodes is increased several mean paths are available making the current larger.

The positive ions that are produced by the collision of electrons with atoms of the gas, accelerate under the influence of electric field. Their collision with cathode results in the ejection of secondary electrons from the surface of the cathode. Increasing the voltage between the electrodes progressively, we finally reach the breakdown voltage when the current rises sharply. Once the breakdown has occurred, the secondary electrons produced at the cathode is sufficient to maintain the discharge, and the glow becomes self sustained.

The self sustained glow discharge has four regions which are shown in fig. g13. In the region, called Crook’s dark space, there is an accumulation of positive ions. The thickness of this region is approximately equal to the mean free path of the electrons. At the end of Crook’s dark space, there is a glowing region, called the ‘negative glow’, where the electrons generate ion- electron pairs by colliding with gas atoms. In this region ions and electrons are in the state of plasma. At the end of negative glow, the electrons lose all their kinetic energy and cannot cause further ionization or excitation, resulting in the dark region called the Faraday dark space. After passing through the Faraday dark space by diffusion, the electrons accelerate towards the anode. There is a steady supply of electrons and the field is small. The electrons accelerate producing ionization on its way before being absorbed by the anode. This region is called the positive column.

If a very high potential difference is applied, bombardments of the cathode by positive ions eject the atoms from the cathode. These atoms may be deposited on a surface (called substrate) kept in the chamber producing a thin film of the material of the cathode. This process is called sputtering, is extensively used for production of various kinds of thin films.