Gas discharge phenomenon is an essential characteristic of gas discharge lamps used in various lighting applications. When electric power is applied across the electrodes present inside a discharge tube, the electrodes give out electrons which bombard with the gas atoms introduced inside the discharge tube. Due to these collisions, a considerable part of the applied electrical energy to visible electromagnetic radiation (when the collisions are inelastic) and the remaining electrical energy is converted to heat energy (when the collisions are elastic).
Hence a gas discharge for successful completion requires the following things:
- A discharge tube of transparent or translucent material
- Sealed-in electrodes
- Inert gas and/or metal vapour
The discharge tube contains sealed-in electrodes inside it and is filled with inert gas/or metal vapour. The inert gas is used as auxiliary or starting gas as it can reduce the starting voltage needed for gas discharge. Generally, a mixture of inert gases (Neon with 1% Argon) known as ‘penning mixture’ is used. When we apply electric power to the electrodes, electrons are emitted. The emitted electrons are accelerated by the applied electric field and collide with the gas atoms or molecules. The electron velocity depends on the mean free path, gas density and strength of the applied electric field. The different velocities of the emitted electrons cause different types of collision with the gas atoms. There are mainly three different processes that occur during gas discharge due to this difference in velocity. They are as follows:
When the speed of the emitted electron is such that elastic collisions take place between the electron and gas atom, means electron hits the gas atom and bounces back, then only a small part of the low kinetic energy is transferred to the gas atom. This is because the mass of an electron is tiny compared to the gas atom. However, there are large numbers of these types of collisions which results in the transfer of a considerable amount of energy which results in the rise in gas temperature.
Gas Atom Excitation
In some cases, the speed of the electron, colliding with the gas atom may be so high that it transfers one of the electrons of the gas atom to an orbit further away from the nucleus. This gas electron absorbs most of the energy of the free electron while moving to the higher energy orbit. But, the strong electrostatic attraction force of the nucleus pulls the electron back to its original orbit. This movement of the gas electron from the higher energy level to its ground level is completed via intermediate orbits and accompanied with the release of absorbed energy during the collision in the form of electromagnetic radiation. The gas electron can move only in certain discretely defined orbits around the nucleus, and this causes the emitted electromagnetic radiation to be made of discrete spectral lines. This phenomenon of transition of an electron from a higher energy level back to its ground level, while emitting electromagnetic radiation, takes only about 10-8 seconds. However, there are some energy levels presented from which a direct transition to a lower energy level is not possible. So in such cases, the gas electron is moved to further higher energy level by collision with other electrons from where the excited electron can return to its ground state via intermediate orbits. This process requires extra time than the earlier process. The electromagnetic radiation emitted during gas discharge may be in the visible radiation range (380nm-760nm) or maybe in Ultra-violet range. If the electromagnetic radiation is in the UV range, then it is converted to visible radiation with the help of different phosphors through phosphorescence phenomenon.
Gas Atom Ionisation
If the velocity of the emitted electron is so high that during collision it knocks out an electron completely from the atom of the gas, then as a result only a positively charged ion is left. This free electron, in turn, will collide with other gas atoms and can generate heat or excite the gas atoms.This process is repeated many times depending upon the kinetic energy of the electron. This causes the gas to ionize and an electric current is established in the discharge tube. However, continuing ionization can lead to a current runaway which should be avoided by introducing an impedance in the circuit.
The gas discharge may occur at low pressure and high pressure and therefore is of two types:
Low-Pressure Gas Discharge
The low-pressure gas discharge takes place at a pressure of about 0.4 Pa which is very less than atmospheric pressure. The low-pressure discharge is utilized in a low-pressure sodium vapor lamp and fluorescent lamp.
High-Pressure Gas Discharge
The high-pressure gas discharge takes place at pressures more than 104 Pa, which is about 20 % of atmospheric pressure. The high-pressure discharge is utilized in a high-pressure sodium vapor lamp, the high-pressure mercury vapor lamp, and the metal halide lamp.